Accounting for sap flow from different parts of the root system improves the prediction of xylem ABA concentration in plants grown with heterogeneous soil moisture

When soil moisture is heterogeneous, sap flow from, and ABA status of, different parts of the root system impact on leaf xylem ABA concentration ([X-ABA]leaf). The robustness of a model for predicting [X-ABA]leaf was assessed. 'Two root-one shoot' grafted sunflower (Helianthus annuus L.) plants received either deficit irrigation (DI, each root system received the same irrigation volumes) or partial rootzone drying (PRD, only one root system was watered and the other dried the soil). Irrespective of whether relative sap flow was assessed using sap flow sensors in vivo or by pressurization of de-topped roots, each root system contributed similarly to total sap flow during DI, while sap flow from roots in drying soil declined linearly with soil water potential (Psisoil) during PRD. Although Psisoil of the irrigated pot determined the threshold Psisoil at which sap flow from roots in drying soil decreased, the slope of this decrease was independent of the wet pot Psisoil. Irrespective of whether sap was collected from the wet or dry root system of PRD plants, or a DI plant, root xylem ABA concentration increased as Psisoil declined. The model, which weighted ABA contributions of each root system according to the sap flow from each, almost perfectly explained [X-ABA] immediately above the graft union. That the model overestimated measured [X-ABA]leaf may result from changes in [X-ABA] along the transport pathway or an artefact of collecting xylem sap from detached leaves. The implications of declining sap flow through partially dry roots during PRD for the control of stomatal behaviour and irrigation scheduling are discussed.

Stomatal movements and long-distance signaling in plants

As the nerve-mediated signaling in animals, long-distance signaling in plants is a prerequisite for plants to be able to perceive environmental stimuli and initiate adaptive responses. While intracellular signal transduction has been attracting considerable attentions, studies on long-distance signaling in plants has been relatively overlooked. Stomatal movements are well recognized as a model system for studies on cellular signal transduction. It has been demonstrated that the stomatal movements may be frequently tuned by long-distance signaling under various environmental stimuli. Stomatal movements can not only respond to persistent stress stimuli but also respond to shock stress stimuli. Stomatal responses to drought stress situations may be best characterized in terms of interwoven networks of chemical signaling pathways playing predominant roles in these adaptive processes. In cases of shock stress stimuli, stomatal movements can be more sensitively regulated through the long-distance signaling but with distinctive patterns not observed for drought or other persistent stresses. Here, the fundamental characteristics of stomatal movements and associated long-distance signaling are reviewed and the implications for plant responses to environmental stresses are discussed.

NaCl-induced changes in cytosolic free Ca2+ in Arabidopsis thaliana are heterogeneous and modified by external ionic composition

Increases in cytosolic free Ca(2+) ([Ca(2+)](cyt)) are common to many stress-activated signalling pathways, including the response to saline environments. We have investigated the nature of NaCl-induced [Ca(2+)](cyt) signals in whole Arabidopsis thaliana seedlings using aequorin. We found that NaCl-induced increases in [Ca(2+)](cyt) are heterogeneous and mainly restricted to the root. Both the concentration of NaCl and the composition of the solution bathing the root have profound effects on the magnitude and dynamics of NaCl-induced increases in [Ca(2+)](cyt). Alteration of external K(+) concentration caused changes in the temporal and spatial pattern of [Ca(2+)](cyt) increase, providing evidence for Na(+)-induced Ca(2+) influx across the plasma membrane. The effects of various pharmacological agents on NaCl-induced increases in [Ca(2+)](cyt) indicate that NaCl may induce influx of Ca(2+) through both plasma membrane and intracellular Ca(2+)-permeable channels. Analysis of spatiotemporal [Ca(2+)](cyt) dynamics using photon-counting imaging revealed additional levels of complexity in the [Ca(2+)](cyt) signal that may reflect the oscillatory nature of NaCl-induced changes in single cells.

Root to leaf electrical signaling in avocado in response to light and soil water content

Phytomonitoring techniques for irrigation of avocado orchards indicate that plants respond very rapidly to fluctuations in soil water content. Root to leaf abscicic acid transport cannot fully explain the almost immediate response of stomata to either irrigation and/or sudden changes in climatic conditions. Therefore, we studied the existence of a fast conducting signal between roots and leaves, and the possible involvement of such a signal in the regulation of stomatal behavior. Two-year-old avocado trees were subjected to drying and re-watering cycles or changes in incident radiation (light or darkness). The difference in extracellular electrical potential between the leaf petiole and the base of stem (DeltaV(L-S)) was continuously recorded. Stomatal conductance (gs) was also recorded for the same leaves that were used for voltage difference measurements. A sudden change in soil water content induced by root drying and re-watering was accompanied by a slow, significant change in the recorded DeltaV(L-S) signal, which was fully developed at 52 and 32min for root drying and re-watering, respectively. We found an inverse correlation (r=-0.56) between the change of DeltaV(L-S) and the gs difference measured before and after each soil-drying treatment. Plants that were girdled to disrupt the phloem and then irrigated tended to have lower DeltaV(L-S) differences over time than non-girdled irrigated plants, suggesting that the electrical signal was transmitted in the phloem. The existence of a fast signal transmitted from the root to the leaf that can be measured and correlated with stomatal control opens the possibility of developing a new phytomonitoring technique and/or artificially modifying plant responses by imposing agronomic management strategies aimed at rapid stomatal adaptation to changes in soil water content.

Molecular cloning of peroxidase cDNAs from dehydration-treated fibrous roots of sweetpotato and their differential expression in response to stress

Three peroxidase (POD) cDNAs were isolated from dehydration-treated fibrous roots of sweetpotato (Ipomoea batatas) plant via the screening of a cDNA library, and their expressions were assessed to characterize functions of each POD in relation to environmental stress. Three PODs were divided into two groups, designated the basic PODs (swpb4, swpb5) and the anionic PODs (swpa7), on the basis of the pI values of mature proteins. Fluorescence microscope analysis indicated that three PODs are secreted into the extracellular space. RTPCR analysis revealed that POD genes have diverse expression patterns in a variety of plant tissues. Swpb4 was abundantly expressed in stem tissues, whereas the expression levels of swpb5 and swpa7 transcripts were high in fibrous and thick pigmented roots. Swpb4 and swpa7 showed abundant expression levels in suspension cultured cells. Three POD genes responded differently in the leaf and fibrous roots in response to a variety of stresses including dehydration, temperature stress, stress-associated chemicals, and pathogenic bacteria.

K(+) starvation inhibits water-stress-induced stomatal closure

The effect of potassium starvation on stomatal conductance was studied in olive trees and sunflower plants, two major crops with greatly differing botanical characteristics. In both species, K(+) starvation inhibited water-stress-induced stomatal closure. In olive trees, potassium starvation favoured stomatal conductance and transpiration, as well as inhibiting shoot growth, in the three cultivars studied: 'Lechín de Granada', 'Arbequina' and 'Chetoui'. However, 'Lechín de Granada' - generally considered more drought-tolerant than 'Arbequina' and 'Chetoui' - proved less susceptible to potassium starvation. Results for olive trees also suggest genetic variability in olive cultivars in relation to potassium requirements for stem growth and the regulation of water transpiration. The results obtained suggest that inhibition of the stomatal closure mechanism produced by moderate potassium starvation is a widespread plant physiological disorder, and may be the cause of tissue dehydration in many water-stressed crops.

Discovery of oxidative burst in the field of plant immunity: Looking back at the early pioneering works and towards the future development

This article is introductory to the series of works presented in this special issue on the homeostasis and the signaling roles of reactive oxygen species (ROS) in plants. Upper half of this article briefly describes the history of the ROS study in the field of plant immunity research initiated by the observation that the attacks by pathogenic microorganisms possibly stimulate the burst of ROS production in the plant tissues. The topics covered in the series of works presented here include the plants' responses to abiotic oxidative stress (atmospheric ozone), regulation of seed germination, chemical interaction between parasitic and host plants and the draught tolerance, all controlled through homeostasis of ROS at biochemical and molecular biological levels. Lastly a discussion forum was proposed to further deepen our understanding of ROS behaviors in plants.

Molecular characterization of a cDNA encoding DRE-binding transcription factor from dehydration-treated fibrous roots of sweetpotato

A new dehydration responsive element-binding (DREB) protein gene encoding for an AP2/EREBP-type transcription factor was isolated by screening of the cDNA library for dehydration-treated fibrous roots of sweetpotato (Ipomoea batatas). Its cDNA (referred to as swDREB1) fragment of 1206bp was sequenced from, which a 257 amino acid residue protein was deduced with a predicted molecular weight of 28.17kDa. A search of the protein BLAST database revealed that this protein can be classified as a typical member of a DREB subfamily. RT-PCR and northern analyses revealed diverse expression patterns of the swDREB1 gene in various tissues of intact sweetpotato plant, and in leaves and fibrous roots exposed to different stresses. The swDREB1 gene was highly expressed in stems and tuberous roots. In fibrous roots, its mRNA accumulation profiles clearly showed strong expression under various abiotic stress conditions such as dehydration, chilling, salt, methyl viologen (MV), and cadmium (Cd) treatment, whereas it did not respond to abscisic acid (ABA) or copper (Cu) treatment. The above results indicate that swDREB1 may be involved in the process of the plant response to diverse abiotic stresses through an ABA-independent pathway.

High crop productivity with high water use in winter and summer on the Liverpool Plains, eastern Australia

We report exceptional productivity and associated water-use efficiency across seasons for commercial crops of rainfed spring wheat and grain sorghum growing on stored soil water in Vertosols on the Liverpool Plains, central-eastern Australia. Agreement between the independently measured terms of evapotranspiration (ET) and the soil water balance (in-crop rainfall + δsoil water) was achieved within acceptable uncertainty across almost all measurement intervals, to provide a reliable dataset for the analysis of growth and water-use relationships without the confounding influence of water outflow either overland or within the soil. Post-anthesis intrinsic transpiration efficiency (kc ) values of 4.7 and 7.2 Pa for wheat and sorghum, respectively, and grain yields of 8 and 7 t/ha from ET of 450 and 442 mm (1.8 and 1.6 g/m2.mm), clearly demonstrate the levels of productivity and water-use efficiency possible for well-managed crops within an intensive and productive response cropping sequence. The Vertosols in which the crops were grown enabled rapid and apparently unconstrained delivery of significant quantities of subsoil water (34% and 51% of total available) after anthesis, which enabled a doubling of pre-anthesis standing biomass and harvest indices of almost 50%. Durum wheat planted into only 0.30 m of moist soil and enduring lower than average seasonal rainfall, yielded less biomass and grain (2.3 t/ha) with lower water-use efficiency (0.95 g/m2.mm) but larger transpiration efficiency, probably due to reduced stomatal conductance. We argue that crop planting in response to stored soil water and management for high water-use efficiency to achieve high levels of average productivity of crop sequences over time can have a significant effect on both increased productivity and enhanced hydrological stability across alluvial landscapes.

Xylem tension affects growth-induced water potential and daily elongation of maize leaves

Diurnal rates of leaf elongation vary in maize (Zea mays L.) and are characterized by a decline each afternoon. The cause of the afternoon decline was investigated. When the atmospheric environment was held constant in a controlled environment, and water and nutrients were adequately supplied to the soil or the roots in solution, the decline persisted and indicated that the cause was internal. Inside the plants, xylem fluxes of water and solutes were essentially constant during the day. However, the forces moving these components changed. Tensions rose in the xylem, and gradients of growth-induced water potentials decreased in the surrounding growing tissues of the leaf. These potentials, measured with isopiestic thermocouple psychrometry, changed because the roots became less conductive to water as the day progressed. The increased tensions were reversed by applying pressure to the soil/root system, which rehydrated the leaf. Afternoon elongation immediately recovered to rapid morning rates. The rapid morning rates did not respond to soil/root pressurization. It was concluded that increased xylem tension in the afternoon diminished the gradients in growth-induced water potential and thus inhibited elongation. Because increased tensions cause a similar but larger inhibition of elongation if maize dehydrates, these hydraulics are crucial for shaping the growth-induced water potential and thus the rates of leaf elongation in maize over the entire spectrum of water availability.

Soil temperature and intermittent frost modulate the rate of recovery of photosynthesis in Scots pine under simulated spring conditions

An earlier onset of photosynthesis in spring for boreal forest trees is predicted as the climate warms, yet the importance of soil vs air temperatures for spring recovery remains to be determined. Effects of various soil- and air-temperature conditions on spring recovery of photosynthesis in Scots pine (Pinus sylvestris) seedlings were assessed under controlled environmental conditions. Using winter-acclimated seedlings, photosynthetic responses were followed after transfer to different simulated spring conditions. Recovery rates for photosynthetic electron transport and net CO(2) uptake were slower in plants from cold or frozen soil compared with controls. In addition, a greater fraction of light absorbed was not used photochemically, but was dissipated thermally via xanthophyll cycle pigments. Intermittent frost events decreased photosynthetic capacity and increased thermal energy dissipation. Within a few days after frost events, photosynthetic capacity recovered to prefrost levels. After 18 d under spring conditions, no difference in the optimum quantum yield of photosynthesis was observed between seedlings that had been exposed to intermittent frost and control plants. These results show that, if air temperatures remain favourable and spells of subfreezing air temperatures are only of short duration, intermittent frost events delay but do not severely inhibit photosynthetic recovery in evergreen conifers during spring. Cold and/or frozen soils exert much stronger inhibitory effects on the recovery process, but they do not totally inhibit it.

Manipulation of the apoplastic pH of intact plants mimics stomatal and growth responses to water availability and microclimatic variation

The apoplastic pH of intact Forsythiaxintermedia (cv. Lynwood) and tomato (Solanum lycopersicum) plants has been manipulated using buffered foliar sprays, and thereby stomatal conductance (g(s)), leaf growth rate, and plant water loss have been controlled. The more alkaline the pH of the foliar spray, the lower the g(s) and/or leaf growth rate subsequently measured. The most alkaline pH that was applied corresponds to that measured in sap extracted from shoots of tomato and Forsythia plants experiencing, respectively, soil drying or a relatively high photon flux density (PFD), vapour pressure deficit (VPD), and temperature in the leaf microclimate. The negative correlation between PFD/VPD/temperature and g(s) determined in well-watered Forsythia plants exposed to a naturally varying summer microclimate was eliminated by spraying the plants with relatively alkaline but not acidic buffers, providing evidence for a novel pH-based signalling mechanism linking the aerial microclimate with stomatal aperture. Increasing the pH of the foliar spray only reduced g(s) in plants of the abscisic acid (ABA)-deficient flacca mutant of tomato when ABA was simultaneously sprayed onto leaves or injected into stems. In well-watered Forsythia plants exposed to a naturally varying summer microclimate (variable PFD, VPD, and temperature), xylem pH and leaf ABA concentration fluctuated but were positively correlated. Manipulation of foliar apoplastic pH also affected the response of g(s) and leaf growth to ABA injected into stems of intact Forsythia plants. The techniques used here to control physiology and water use in intact growing plants could easily be applied in a horticultural context.

Transpiration, CO2 assimilation, WUE, and stomatal aperture in leaves of Viscum album (L.): Effect of abscisic acid (ABA) in the xylem sap of its host (Populus x euamericana)

Leaves of the mistletoe Viscum album (L.) show a high rate of transpiration, even when the host is under severe drought stress. The hypothesis that a strong control of ABA influx from the xylem sap of the host into the mistletoe prevents stomatal closure in mistletoe leaves was tested under the following conditions: sections of poplar twigs carrying a mistletoe were perfused with artificial xylem sap that contained different ABA concentrations and both transpiration and ABA levels were analysed in mistletoe leaves. Despite variation by a factor of 10(4), the ABA content of the host xylem did not affect ABA levels, leaf transpiration, CO(2) assimilation, WUE, or the degree of stomatal aperture in mistletoe leaves. These observations support the hypothesis of a strong control of ABA influx from the host of the xylem into the mistletoe, although degradation of ABA before it enters the mistletoe leaves cannot be excluded. This mechanism may ensure a water and nutritional status favourable for the mistletoe, even if the water status of the host is impaired. Despite the lack of short-term sensitivity of ABA levels in mistletoe leaves to even strong changes of ABA levels in the xylem sap of the host, ABA levels in mistletoe leaves were relatively high compared to ABA levels in the leaves of several tree species including poplar. Since significant transpiration of the mistletoe leaves was observed despite high ABA levels, a diminished sensitivity of the stomata of mistletoe leaves to ABA has to be concluded. The stomatal density of adaxial Viscum leaves of 89+/-23 stomata per mm is lower than those reported in a study performed at the end of the 19th century.

Ecophysiology of tea

Tea [Camellia sinensis (L.) O. Kuntze] is one of the most important beverage crops in the world. The major tea-growing regions of the world are South-East Asia and Eastern Africa where it is grown across a wide range of altitudes up to 2200 m a.s.l.. This paper reviews the key physiological processes responsible for yield determination of tea and discusses how these processes are influenced by genotypic and environmental factors. Yield formation of tea is discussed in terms of assimilate supply through photosynthesis and formation of harvestable sinks (i.e. shoots). The photosynthetic apparatus and partial processes (i.e. light capture, electron transport and carboxylation) of tea show specific adaptations to shade. Consequently, maximum light-saturated photosynthetic rates of tea are below the average for C3 plants and photoinhibition occurs at high light intensities. These processes restrict the source capacity of tea. Tea yields are sink-limited as well because shoots are harvested before their maximum biomass is reached in order to maintain quality characters of made tea. In the absence of water deficits, rates of shoot initiation and extension are determined by air temperature and saturation vapour pressure deficit, with the former having positive and the latter having negative relationships with the above rates. During dry periods, when the soil water deficit exceeds a genotypically- and environmentally-determined threshold, rates of shoot initiation and extension are reduced with decreasing shoot water potential. Tea yields respond significantly to irrigation, a promising option to increase productivity during dry periods, which are experienced in many tea-growing regions.

Some aspects of citrus ecophysiology in subtropical climates: re-visiting photosynthesis under natural conditions

In this review we re-visit and discuss the current knowledge on ecophysiology of citrus trees, addressing the influence of environmental conditions on citrus photosynthesis. Knowledge of physiological responses of citrus trees to their surrounding environment is essential in order to improve crop production and plant development, both being consequences of appropriate horticultural management in citrus orchards. In this context, citrus photosynthesis is addressed as the primary source of carbon and energy for plant growth and development. The photosynthetic activity on both a daily and a seasonal scale is reviewed, taking into consideration the physiological aspects related to seasonal variation of photochemical and biochemical activities, stomatal conductance and leaf water potential. These aspects are treated for citrus plants growing in subtropical climates with varying environmental conditions, such as moderate to severe drought during the winter season. In addition, the possible inhibitory/stimulatory effects of carbohydrate metabolism on citrus photosynthesis are discussed with regard to the source-sink relationship. Field experimentation that enhances knowledge concerning citrus ecophysiology in subtropical climates is highlighted. Among interesting subjects to be unraveled by future research, we may point out the effects of low temperatures on citrus photosynthesis and water relations, the nature of the relationship between leaf carbohydrate content and photosynthesis, and the significance of photosynthesis in different canopy layers and positions in relation to the total carbon gain in mature citrus trees.

Chill-induced decrease in capacity of RuBP carboxylation and associated H2O2 accumulation in cucumber leaves are alleviated by grafting onto figleaf gourd

BACKGROUND AND AIMS

Chilling results in a significant decrease in Rubisco content and increased generation of reactive oxygen species (ROS) in cucumber (Cucumis sativus), a chilling-sensitive species. The role of roots in the regulation of the tolerance is unknown. Here, cucumber plants grafted onto figleaf gourd (Cucurbita ficifolia), a chilling-tolerant species were used to study the role of roots in the regulation of shoot functioning and the associated root-to-shoot communication.

METHODS

Gas exchange and chlorophyll fluorescence were measured using an infrared gas analyser combined with a pulse amplitude fluorimeter during chilling at 14 degrees C or 7 degrees C and subsequent recovery. At the same time, Rubisco content and activity and ROS generation were spectrophotometrically assayed. Abscisic acid and cytokinin concentrations in xylem sap were also determined by enzyme-linked immunosorbent assay.

KEY RESULTS AND CONCLUSIONS

Grafted plants showed a significantly higher light-saturated rate of CO(2) assimilation (A(sat)) than own-rooted plants when roots were gradually cooled, but no differences were detected when shoots were cooled. Chill at 7 degrees C irreversibly reduced A(sat), and significantly decreased maximum carboxylation activity, Rubisco content and initial Rubisco activity. However, grafted plants showed weaker inhibition, together with decreased electron flux in the water-water cycle. Higher activity of antioxidant enzymes with less ROS production was found in grafted plants. In addition, ABA concentration increased by 48.4-fold whilst cytokinin concentration decreased by 91.5% in the xylem sap of own-rooted plants after exposure to a 7 degrees C chill. In comparison, ABA and cytokinin concentrations increased by 10.5-fold and 36.9%, respectively, for the grafted plants. Improved plant growth was also observed in grafted plants after the chill. These results suggest that some signals coming from chilling-resistant roots (i.e. ABA and cytokinins) protect leaf photosynthesis in shoots of chilling-sensitive plants.

Cloning of a 9-cis-epoxycarotenoid dioxygenase gene (SgNCED1) from Stylosanthes guianensis and its expression in response to abiotic stresses

Abscisic acid (ABA) regulates plant adaptive responses to various environmental stresses. Oxidative cleavage of cis-epoxycarotenoids catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED) is the main regulatory step in the biosynthesis of ABA in higher plants. A NCED gene, SgNCED1, was cloned from the dehydrated leaves of Stylosanthes guianensis. The 2,241-bp full-length SgNCED1 had a 1,809-bp ORF, which encodes a peptide of 602 amino acids. The deduced amino acid sequence of SgNCED1 protein shared high identity with other NCEDs. At the N-terminus of the SgNCED1 located a chloroplast transit peptide sequence. DNA blot analysis revealed that SgNCED1 was a single copy gene in the genome of S. guianensis. The relationship between expression of SgNCED1 and endogenous ABA level was investigated. The expression of SgNCED1 was induced in both leaves and roots of S. guianensis under drought stress. Dehydration and salt stress induced the expression of SgNCED1 strongly and rapidly. The ABA accumulation was coincidently induced with the SgNCED1 mRNA under drought, dehydration and salt stress. The expression of SgNCED1 and ABA accumulation were also induced under chilling condition.

Soil moisture heterogeneity during deficit irrigation alters root-to-shoot signalling of abscisic acid

The effects of different irrigation techniques on leaf xylem ABA concentration ([X-ABA]_leaf) were compared in tomato (Lycopersicon esculentum Mill.). During partial rootzone drying (PRD), water was distributed unevenly to the root system such that part was irrigated while the remainder was allowed to dry the soil. During conventional deficit irrigation (DI), plants received the same volume of water as PRD plants, but water was distributed evenly to the entire root system. When the plant root system was allowed to explore two separate soil compartments, DI plants had a higher [X-ABA]_leaf than PRD plants with moderate soil drying, but PRD plants had a higher [X-ABA]_leaf than DI plants as the soil dried further. The difference in [X-ABA]_leaf between the two sets of plants was not because of differences in either whole pot soil water content (θ_pot) or leaf water potential (Ψ_leaf). To investigate the contribution of different parts of the root system to [X-ABA]_leaf, individual shoots were grafted onto the root systems of two plants grown in two separate pots, so that the graft union had the appearance of an inverted 'Y'. After sap collection from detached leaves, removal of the shoot below the graft union allowed sap collection from each root system. Again, DI plants had a higher [X-ABA]_leaf than PRD plants when the soil was relatively wet, but the opposite occurred as the soil dried. Root xylem ABA concentration ([X-ABA]_root) increased exponentially as soil water content (θ) declined. In DI plants, [X-ABA]_root from either pot (or the arithmetic mean of [X-ABA]_root) accounted for a similar amount of the variation in [X-ABA]_leaf. In PRD plants, [X-ABA]_root from the watered side underestimated [X-ABA]_leaf, whereas [X-ABA]_root from the dry side overestimated [X-ABA]_leaf. The arithmetic mean of [X-ABA]_root best explained the variation in [X-ABA]_leaf, implying continued sap flow from the dry part of the root system (J_dry) at soil water potentials (Ψ_soil) at which J_dry had ceased in previous studies of PRD plants (Yao et al. 2001). Evaluating the relationship between J_dry and Ψ_soil may assist in maintaining export of ABA (and other growth regulators) from the drying part of the root system, to achieve desirable horticultural outcomes during PRD.

Stomatal regulation by microclimate and tree water relations: interpreting ecophysiological field data with a hydraulic plant model

Dynamics in microclimate and physiological plant traits were studied for Pubescent oak and Scots pine in a dry inner-alpine valley in Switzerland, at a 10 min resolution for three consecutive years (2001-2003). As expected, stomata tended to close with increasing drought in air and soil. However, stomatal aperture in oak was smaller than in pine under relatively wet conditions, but larger under dry conditions. To explore underlying mechanisms, a model was applied that (i) quantifies water relations within trees from physical principles (mechanistic part) and (ii) assumes that signals from light, stomatal aperture, crown water potential, and tree water deficit in storage pools control stomata (systemic part). The stomata of pine showed a more sensitive response to increasing drought because both factors, the slowly changing tree water deficit and the rapidly changing crown water potential, closed the stomata. By contrast, the stomata of oak became less drought-sensitive as the closing signal of crown water potential was opposed by the opening signal of tree water deficit. Moreover, parameter optimization suggests that oak withdrew more water from the storage pools and reduced leaf water potentials to lower levels, without risking serious damage by cavitation. The new model thus suggests how the hydraulic water flow and storage system determines the responses in stomatal aperture and transpiration to drought at time scales ranging from hours to multiple years, and why pine and oak might differ in such responses. These differences explain why oaks are more efficient competitors during drought periods, although this was not the case in the extremely dry year 2003, which provoked massive leaf loss and, from July onwards, physiological activity almost ceased.

Intracellular localization of integrin-like protein and its roles in osmotic stress-induced abscisic acid biosynthesis in Zea mays

Plants have evolved many mechanisms to cope with adverse environmental stresses. Abscisic acid (ABA) accumulates significantly in plant cells in response to drought conditions, and this is believed to be a major mechanism through which plants enhance drought tolerance. In this study, we explore the possible mechanisms of osmotic stress perception by plant cells and the consequent induction of ABA biosynthesis. Immunoblotting and immunofluorescence localization experiments, using a polyclonal antibody against human integrin beta1, revealed the presence of a protein in Zea mays roots that is similar to the integrin proteins of animals and mainly localized in the plasma membrane. Treatment with GRGDS, a synthetic pentapeptide containing an RGD domain, which interacted specifically with the integrin protein and thus blocked the cell wall-plasma membrane interaction, significantly inhibited osmotic stress-induced ABA biosynthesis in cells, and the GRGDS analog which does not contain the RGD domain had no effect. Our results show that a strong interaction exists between the cell wall and plasma membrane and that this interaction is largely mediated by integrin-like proteins. They also imply that the cell wall and/or cell wall-plasma membrane interaction plays important roles in the perception of osmotic stress. Accordingly, we conclude that the cell wall and/or cell wall-plasma membrane interaction mediated by the integrin-like protein plays important roles in osmotic stress-induced ABA biosynthesis in Zea mays.

Alternation of wet and dry sides during partial rootzone drying irrigation alters root-to-shoot signalling of abscisic acid

Partial rootzone drying (PRD) is an irrigation technique where water is distributed unevenly to the root system such that part is irrigated while the remainder is allowed to dry the soil. Tomato (Lycopersicon esculentum Mill.) plants were grown with their roots in two soil columns to compare the physiological consequences of alternation of wet and dry columns during PRD irrigation (alternate PRD, PRD-A) with retention of the same wet and dry columns (fixed PRD, PRD-F). When PRD plants received 50% less water than well-watered (WW) plants, xylem ABA concentration ([X-ABA]) increased and stomatal conductance decreased relative to WW plants. Although both sets of PRD plants received the same amount of water, [X-ABA] of PRD-A plants increased up to 2-fold above that of PRD-F plants, which further decreased stomatal conductance. Differences in [X-ABA] were detected within an hour of alternation, but did not persist beyond the photoperiod of alternation. [X-ABA] increased linearly as whole-pot soil water content (θ_pot) and leaf water potential (Ψ_leaf) declined, but the difference in [X-ABA] between the two sets of PRD plants was not due to differences in either θ_pot or Ψ_leaf. In PRD-F plants, the unwatered part of the root system contributes proportionally less to the transpiration stream as the soil progressively dries (Yao et al. 2001, Plant, Cell & Environment 24, 227-235). In PRD-A plants, we hypothesise that re-watering the dry part of the root system allows these roots to contribute proportionally more to total sap flux, thus liberating a pulse of ABA to the transpiration stream as the root ABA pool accumulated during soil drying is depleted. Since the enhancement of [X-ABA] caused by PRD-A increased as θ_pot and Ψ_leaf declined, an optimal frequency of alternation to maximise the cumulative physiological effects of this ABA pulse must consider possible negative impacts of leaf water deficit as soil water status declines.

Identification of novel drought-related mRNAs in common bean roots by differential display RT-PCR

Drought is a major constraint for the production of common bean (Phaseolus vulgaris L.). To identify molecular responses to water deficit, we performed a differential display RT-PCR (DDRT) analysis using roots of bean plants grown aeroponically and submitted to dehydration. This allowed us to visualise 1200 DDRT bands, 8.7% of which showed a clear regulation by dehydration, and to clone 42 cDNAs, called PvD1 to PvD42. Among them, 20 early-dehydration-responsive cDNAs were selected by reverse northern that were induced or repressed before detectable water status changes and induction of ABA-regulated genes. Northern analysis for 16 PvD clones confirmed these early regulations and allowed us to identify four late dehydration-responsive genes. Their putative involvement in signalling, protein turn-over and translocation, chaperones as well as root growth modulations in response to water stress is discussed.

STABILIZED1, a stress-upregulated nuclear protein, is required for pre-mRNA splicing, mRNA turnover, and stress tolerance in Arabidopsis

In plants, many gene transcripts are very unstable, which is important for the tight control of their temporal and spatial expression patterns. To identify cellular factors controlling the stability of unstable mRNAs in plants, we used luciferase imaging in Arabidopsis thaliana to isolate a recessive mutant, stabilized1-1 (sta1-1), with enhanced stability of the normally unstable luciferase transcript. The sta1-1 mutation also causes the stabilization of some endogenous gene transcripts and has a range of developmental and stress response phenotypes. STA1 encodes a nuclear protein similar to the human U5 small ribonucleoprotein-associated 102-kD protein and to the yeast pre-mRNA splicing factors Prp1p and Prp6p. STA1 expression is upregulated by cold stress, and the sta1-1 mutant is defective in the splicing of the cold-induced COR15A gene. Our results show that STA1 is a pre-mRNA splicing factor required not only for splicing but also for the turnover of unstable transcripts and that it has an important role in plant responses to abiotic stresses.

Abscisic acid and ethylene interact in wheat grains in response to soil drying during grain filling

Grain filling is an intensive transportation process regulated by soil drying and plant hormones. This study investigated how the interaction between abscisic acid (ABA) and ethylene is involved in mediating the effects of soil drying on grain filling in wheat (Triticum aestivum). Two wheat cultivars, cv. Yangmai 6 and cv. Yangmai 11, were field-grown, and three irrigation treatments, well-watered, moderately soil-dried (MD) and severely soil-dried (SD), were imposed from 9 d post anthesis until maturity. A higher ABA concentration and lower concentrations of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike, those grains that were filled earlier and reached a greater size) than in inferior grains (within a spike, those grains that were filled later and were smaller), and were associated with a higher filling rate in the superior grains. An increase in ABA concentration and reductions in ethylene and ACC concentrations in grains under MD conditions increased the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under SD conditions reduced the grain-filling rate. Application of chemical regulators gave similar results. The results did not differ between the two cultivars. The grain-filling rate in wheat is mediated by the balance between ABA and ethylene in the grains, and an increase in the ratio of ABA to ethylene increases the grain-filling rate.

Differential adaptation of two varieties of common bean to abiotic stress: II. Acclimation of photosynthesis

The photosynthetic characteristics of two contrasting varieties of common bean (Phaseolus vulgaris) have been determined. These varieties, Arroz and Orfeo, differ in their productivity under stress conditions, resistance to drought stress, and have distinctly different stomatal behaviour. When grown under conditions of high irradiance and high temperature, both varieties displayed evidence of photosynthetic acclimation at the chloroplast level-there was an increase in chlorophyll a/b ratio, a decreased content of Lhcb proteins, and an increased xanthophyll cycle pool size. Both varieties also showed reduced chlorophyll content on a leaf area basis and a decrease in leaf area. Both varieties showed an increase in leaf thickness but only Arroz showed the characteristic elongated palisade cells in the high light-grown plants; Orfeo instead had a larger number of smaller, rounded cells. Differences were found in stomatal development: whereas Arroz showed very little change in stomatal density, Orfeo exhibited a large increase, particularly on the upper leaf surface. It is suggested that these differences in leaf cell structure and stomatal density give rise to altered rates of photosynthesis and stomatal conductance. Whereas, Arroz had the same photosynthetic rate in plants grown at both low and high irradiance, Orfeo showed a higher photosynthetic capacity at high irradiance. It is suggested that the higher yield of Orfeo compared with Arroz under stress conditions can be explained, in part, by these cellular differences.

Approaches to increasing the salt tolerance of wheat and other cereals

This review describes physiological mechanisms and selectable indicators of gene action, with the aim of promoting new screening methods to identify genetic variation for increasing the salt tolerance of cereal crops. Physiological mechanisms that underlie traits for salt tolerance could be used to identify new genetic sources of salt tolerance. Important mechanisms of tolerance involve Na+ exclusion from the transpiration stream, sequestration of Na+ and Cl- in the vacuoles of root and leaf cells, and other processes that promote fast growth despite the osmotic stress of the salt outside the roots. Screening methods for these traits are discussed in relation to their use in breeding, particularly with respect to wheat. Precise phenotyping is the key to finding and introducing new genes for salt tolerance into crop plants.

The short-term growth response to salt of the developing barley leaf

Recent results concerning the short-term growth response to salinity of the developing barley leaf are reviewed. Plants were grown hydroponically and the growth response of leaf 3 was studied between 10 min and 5 d following addition of 100 mM NaCl to the root medium. The aim of the experiments was to relate changes in variables that are likely to affect cell elongation to changes in leaf growth. Changes in hormone content (ABA, cytokinins), water and solute relationships (osmolality, turgor, water potential, solute concentrations), gene expression (water channel), cuticle deposition, membrane potential, and transpiration were followed, while leaf elongation velocity was monitored. Leaf elongation decreased close to zero within seconds following addition of NaCl. Between 20 and 30 min after exposure to salt, elongation velocity recovered rather abruptly, to about 46% of the pre-stress level, and remained at the reduced rate for the following 5 d, when it reached about 70% of the level in non-stressed plants. Biophysical and physiological analyses led to three major conclusions. (i) The immediate reduction and sudden recovery in elongation velocity is due to changes in the water potential gradient between leaf xylem and peripheral elongating cells. Changes in transpiration, ABA and cytokinin content, water channel expression, and plasma membrane potential are involved in this response. (ii) Significant solute accumulation, which aids growth recovery, is detectable from 1 h onwards; growing and non-growing leaf regions and mesophyll and epidermis differ in their solute response. (iii) Cuticular wax density is not affected by short-term exposure to salt; transpirational changes are due to stomatal control.

Stomatal development in new leaves is related to the stomatal conductance of mature leaves in poplar (Populus trichocarpaxP. deltoides)

In general, stomatal density (SD) decreases when plants are grown at high CO2 concentrations. Recent studies suggest that signals produced from mature leaves regulate the SD of expanding leaves. To determine the underlying driver of these signals in poplar (Populus trichocarpaxP. deltoides) saplings, a cuvette system was used whereby the environment around mature (lower) leaves could be controlled independently of that around developing (upper) leaves. A series of experiments were performed in which the CO2 concentration, vapour pressure deficit (D), and irradiance (Q) around the lower leaves were varied while the (ambient) conditions around the upper leaves were unchanged. The overall objective was to break the nexus between leaf stomatal conductance and transpiration and photosynthesis rates of lower leaves and determine which, if any, of these parameters regulate stomatal development in the upper expanding leaves. SD, stomatal index (SI), and epidermal cell density (ED) were measured on the adaxial and abaxial surfaces of fully expanded upper leaves. SD and SI decreased with increasing lower leaf CO2 concentration (150-780 ppm) at both ambient (1.3-1.6 kPa) and low (0.7-1.0 kPa) D. SD and SI at low D were generally higher than at ambient D. By contrast, ED was relatively insensitive to both vapour pressure and CO2 concentration. When lower leaves were shaded, upper leaf SD, SI, and ED decreased but did not change with varying CO2 concentration. These results suggest that epidermal cell development and stomatal development are regulated by different physiological mechanisms. SI of the upper leaves was positively and highly correlated (r2>0.84) with the stomatal conductance of the lower leaves independent of their net photosynthesis and transpiration rates, suggesting that the stomatal conductance of mature leaves has a regulatory effect on the stomatal development of expanding leaves.

Stomatal movement in response to long distance-communicated signals initiated by heat shock in partial roots of Commelina communis L

The systematic or long-distance signal transmission plays crucial roles in animal lives. Compared with animals, however, much less is known about the roles of long-distance signal communication in plant lives. Using the model plant Commelina communis L., we have probed the root to shoot communication mediated by heat-shock signals. The results showed that a heat shock of 5 min at 40 degrees C in partial roots, i.e. half or even 1/4 root system, could lead to a significant decrease in stomatal conductance. The regulation capability depends on both heat shock temperature and the amount of root system, i.e. with higher temperature and more roots stressed, the leaf conductance would decrease more significantly. Interestingly, the stomatal regulation by heat shock signal is in a manner of oscillation: when stomata conductance decreased to the lowest level within about 30 min, it would increase rapidly and sometimes even exceed the initial level, and after several cycles the stomata conductance would be finally stabilized at a lower level. Feeding xylem sap collected from heat-shocked plants could lead to a decrease in stomata conductance, suggesting that the heat shock-initiated signal is basically a positive signal. Further studies showed that heat shock was not able to affect ABA content in xylem sap, and also, not able to lead to a decrease in leaf water status, which suggested that the stomatal regulation was neither mediated by ABA nor by a hydraulic signal. Heat shock could lead to an increase in xylem sap H2O2 content, and moreover, the removal of H2O2 by catalase could partially recover the stomatal inhibition by xylem sap collected from heat-shocked plants, suggesting that H2O2 might be able to act as one of the root signals to control the stomatal movement. Due to the fact that heat-shock and drought are usually two concomitant stresses, the stomatal regulation by heat-shock signal should be of significance for plant response to stresses. The observation for the stomatal regulation in an oscillation manner by presently identified new signals should contribute to further understanding of the mystery for the pant systematic signaling in response to stresses.

Nitric oxide is involved in abscisic acid-induced antioxidant activities in Stylosanthes guianensis

Previous studies suggest that abscisic acid (ABA) stimulates the activities of antioxidant enzymes under normal and chilling temperature and enhanced chilling resistance in Stylosanthes guianensis. The objective of this study was to test whether nitric oxide (NO) is involved in the ABA-induced activities of the antioxidant enzymes in Stylosanthes guianensis due to its nature as a second messenger in stress responses. Plants were treated with NO donors, ABA, ABA in combination with NO scavengers or the nitric oxide synthase (NOS) inhibitor and their effects on the activity of antioxidant enzymes and NO production were compared. The results showed that ABA increased the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The effect of ABA on antioxidant enzyme activities was suppressed by the NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA), and the NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (PTIO). NO content increased after 5 h of ABA treatment. The NO-scavenger, PTIO, and the NOS-inhibitor, L-NNA, inhibited the accumulation of NO in ABA-treated Stylosanthes guianensis. NO donor treatment enhanced the activities of SOD, CAT, and APX. The results suggested that NO was involved in the ABA-induced activities of SOD, CAT, and APX in Stylosanthes guianensis. ABA triggered NO production that may lead to the stimulation of antioxidant enzyme activities.

SKS6, a multicopper oxidase-like gene, participates in cotyledon vascular patterning during Arabidopsis thaliana development

SKU5-Similar 6 (SKS6) is a one of a large gene family of 19 members in Arabidopsis thaliana (L.) Heynh that encode multicopper oxidase-like proteins that are related to ferroxidases, ascorbate oxidases and laccases. Only one member of the family has been previously studied; Skewed5 (SKU5) is involved in the control of root growth. The encoded SKS6 protein, like SKU5 appears to lack a functional copper-binding site and is most closely related to Bp10 from Brassica napus and Ntp303 from Nicotiana tobacum. The SKS6 promoter contains many putative regulatory sites and differential expression of an SKS6::GUS reporter gene revealed selective induction in several seedling tissues including guard cells, root cortex cells, and leaf margin hydathodes. It was also expressed later in flower development in flower primordia, ovules, and the abscission zones of seeds and siliques. Furthermore, SKS6 was upregulated in roots in response to treatment of seedlings with the hormones abscisic acid, indole-3 acetic acid, 2,4-dichlorophenoxyacetic acid and aminocyclopropane-1-carboxylate. A loss-of function sks6-1 T-DNA insertion allele revealed that cotyledon vascular patterning is affected in the mutant, suggesting a role for the protein in metabolism of nutrients or hormones in the hydathodes, the sites of auxin synthesis and chemical recycling.

Relationships between xylem sap constituents and leaf conductance of well-watered and water-stressed maize across three xylem sap sampling techniques

Many different techniques have been used for xylem sap collection, but few direct comparisons of techniques have been conducted and few comparisons have been based on comprehensive analyses of xylem sap. Moreover, the suitability of extraction techniques for use on plants grown under water-stress conditions has not been addressed. Xylem sap was extracted from both well-watered and water-stressed Zea mays plants using three different techniques. The main aim was to determine how the extraction method altered the correlations between sap constituents and stomatal conductance in order to determine which relationships change with extraction technique. A 'root pressure' technique was the simplest method of extracting large volumes of sap, but the low sap delivery rates altered the composition of sap. Two pressurization techniques that varied in the position from which sap was collected were tested. The pressurization techniques allowed for the control of delivery rates that influence sap constituent concentrations. The position from which xylem sap was collected on the plant was also found to be important. All three techniques produced consistent correlations between ABA and chloride delivery rates and changes in stomatal conductance, suggesting that each technique could be applied to identify certain putative xylem-borne signals.

Protein phosphorylation is a prerequisite for intracellular Ca2+ release and ion channel control by nitric oxide and abscisic acid in guard cells

Recent work has indicated that nitric oxide (NO) and its synthesis are important elements of signal cascades in plant-pathogen defence, and are a prerequisite for drought and abscisic acid (ABA) responses in Arabidopsis thaliana and Vicia faba guard cells. NO regulates inward-rectifying K+ channels and Cl- channels of Vicia guard cells via intracellular Ca2+ release. However, its integration with related signals, including the actions of serine-threonine protein kinases, is less well defined. We report here that the elevation of cytosolic-free [Ca2+] ([Ca2+]i) mediated by NO in guard cells is reversibly inhibited by the broad-range protein kinase antagonists staurosporine and K252A, but not by the tyrosine kinase antagonist genistein. The effects of kinase antagonism translate directly to a loss of NO-sensitivity of the inward-rectifying K+ channels and background (Cl- channel) current, and to a parallel loss in sensitivity of the K+ channels to ABA. These results demonstrate that NO-dependent signals can be modulated through protein phosphorylation upstream of intracellular Ca2+ release, and they implicate a target for protein kinase control in ABA signalling that feeds into NO-dependent Ca2+ release.

Impact of deficit irrigation on water use efficiency and carbon isotope composition (delta13C) of field-grown grapevines under Mediterranean climate

The objective of this study was to evaluate the effect of deficit irrigation on intrinsic water use efficiency (A/g(s)) and carbon isotope composition (delta13C) of two grapevine cultivars (Moscatel and Castelão), growing in a commercial vineyard in SW Portugal. The study was done in two consecutive years (2001 and 2002). The treatments were full irrigation (FI), corresponding to 100% of crop evapotranspiration (ETc), rain-fed (no irrigation, NI), and two types of deficit irrigation (50% ETc): (i) by supplying the water either to one side of the root system or to the other, which is partial rootzone drying (PRD), or (ii) dividing the same amount of water by the two sides of the root system, the normal deficit irrigation (DI). The water supplied to the PRD treatment alternated sides approximately every 15 d. The values of predawn leaf water potential (Psi(pd)) and the cumulative integral of Psi(pd) (S(Psi)) during the season were lower in 2001 than in the 2002 growing season. Whereas differences in Psi(pd) and S(Psi) between PRD and DI were not significantly different in 2001, in 2002 (a dryer year) both cultivars showed lower values of S(Psi) in the PRD treatment as compared with the DI treatment. This suggests that partial rootzone drying may have a positive effect on water use under dryer conditions, either as a result of better stomatal control and/or reduced vigour. The effects of the water treatments on delta13C were more pronounced in whole grape berries and pulp than in leaves. The delta13C of pulp showed the best correlation with intrinsic water use efficiency (A/g(s)) as well as with S(Psi). In spite of the better water status observed in PRD compared with DI in the two cultivars in 2002, no statistical differences between the two treatments were observed in A/g(s) and delta13C. On the other hand, they showed a higher delta13C compared with FI. In conclusion, it is apparent that the response to deficit irrigation varies with the environmental conditions of the particular year, the driest conditions exacerbating the differences among treatments. The highest values of delta13C found in the pulp of NI vines in Castelão compared with Moscatel suggest different sensitivities to water deficits in the two cultivars, as was empirically observed.

The regulation of anion loading to the maize root xylem

The regulation of anion loading to the shoot in maize (Zea mays) was investigated via an electrophysiological characterization of ion conductances in protoplasts isolated from the root stele. Two distinct anion conductances were identified. In protoplasts from well-watered plants, Z. mays xylem-parenchyma quickly-activating anion conductance (Zm-X-QUAC) was the most prevalent conductance and is likely to load the majority of NO(3)(-) and Cl(-) ions to the xylem in nonstressed conditions. Z. mays xylem-parenchyma inwardly-rectifying anion conductance was found at a lower frequency in protoplasts from well-watered plants than Zm-X-QUAC, was much smaller in magnitude in all observed conditions, and is unlikely to be such a major pathway for anion loading into the xylem. Activity of Z. mays xylem-parenchyma inwardly-rectifying anion conductance increased following a water stress prior to protoplast isolation, but the activity of the putative major anion-loading pathway, Zm-X-QUAC, decreased. Addition of abscisic acid (ABA) to protoplasts from well-watered plants also inhibited Zm-X-QUAC activity within minutes, as did a high free Ca(2+)concentration in the pipette. ABA was also seen to activate a Ca(2+)-permeable conductance (Z. mays xylem-parenchyma hyperpolarization activated cation conductance) in protoplasts from well-watered plants. It is postulated that the inhibition of anion loading into the xylem (an important response to a water stress) due to down-regulation of Zm-X-QUAC activity is mediated by an ABA-mediated rise in free cytosolic Ca(2+).

A review of drought adaptation in crop plants: changes in vegetative and reproductive physiology induced by ABA-based chemical signals

This review discusses the role of abscisic acid (ABA)-based drought stress chemical signalling in regulating crop vegetative and reproductive development and its contributions to crop drought adaptation. Increased concentrations of ABA in the root induced by soil drying may maintain root growth and increase root hydraulic conductivity; both lead to an increase in water uptake and thereby postpone the development of water deficit in the shoot. Root ABA is also transported in the xylem to the shoot and is perceived at the acting sites, where it causes stomatal closure and reduced leaf expansion, thereby preventing dehydration of leaf tissues and enhancing the chance for survival under prolonged drought. ABA-based chemical signalling can be amplified by several factors, particularly increased pH in the xylem/apoplast, which retains anionic ABA. Such an increase in xylem pH detected in field-grown maize might have been brought about by reduced nitrate uptake by plants during soil drying. In contrast, xylem sap alkalinisation was not found in soybeans, which depend on fixing nitrogen through their association with Rhizobium japonicum. Evidence has also shown that the xylem-borne ABA can be transported to plant reproductive structures and influence their development, presumably by regulating gene expression that controls cell division and carbohydrate metabolic enzyme activity under drought conditions. The possible involvement of ABA in the up- and down-regulation of acid invertase in crop source (adult leaves) v. sink (young ovaries) organs indicates a crucial role of the hormone in balancing source and sink relationship in plants according to the availability of water in the soil. A novel irrigation technique named partial root-zone drying (PRD), has been developed to allow exploitation of ABA-based drought stress signalling to improve water-use efficiency (WUE) based on its roles in regulating stomatal aperture and leaf expansion. However, little is known about how crop reproductive development is regulated when irrigated under PRD. We suggest that more attention should be paid to the latter aspect as it directly relates to crop yield and quality.

Activities of fructan- and sucrose-metabolizing enzymes in wheat stems subjected to water stress during grain filling

This study investigated if a controlled water deficit during grain filling of wheat (Triticum aestivum L.) could accelerate grain filling by facilitating the remobilization of carbon reserves in the stem through regulating the enzymes involved in fructan and sucrose metabolism. Two high lodging-resistant wheat cultivars were grown in pots and treated with either a normal (NN) or high amount of nitrogen (HN) at heading time. Plants were either well-watered (WW) or water-stressed (WS) from 9 days post anthesis until maturity. Leaf water potentials markedly decreased at midday as a result of water stress but completely recovered by early morning. Photosynthetic rate and zeatin + zeatin riboside concentrations in the flag leaves declined faster in WS plants than in WW plants, and they decreased more slowly with HN than with NN when soil water potential was the same, indicating that the water deficit enhanced, whereas HN delayed, senescence. Water stress, both at NN and HN, facilitated the reduction in concentration of total nonstructural carbohydrates (NSC) and fructans in the stems but increased the sucrose level there, promoted the re-allocation of pre-fixed (14)C from the stems to grains, shortened the grain-filling period, and accelerated the grain-filling rate. Grain weight and grain yield were increased under the controlled water deficit when HN was applied. Fructan exohydrolase (FEH; EC 3.2.1.80) and sucrose phosphate synthase (SPS; EC 2.4.1.14) activities were substantially enhanced by water stress and positively correlated with the total NSC and fructan remobilization from the stems. Acid invertase (EC 3.2.1.26) activity was also enhanced by the water stress and associated with the change in fructan concentration, but not correlated with the total NSC remobilization and (14)C increase in the grains. Sucrose:sucrose fructosyltransferase (EC 2.4.1.99) activity was inhibited by the water stress and negatively correlated with the remobilization of carbon reserves. Sucrose synthase (EC 2.4.1.13) activity in the stems decreased sharply during grain filling and showed no significant difference between WW and WS treatments. Abscisic acid (ABA) concentration in the stem was remarkably enhanced by water stress and significantly correlated with SPS and FEH activities. Application of ABA to WW plants yielded similar results to those for WS plants. The results suggest that the increased remobilization of carbon reserves by water stress is attributable to the enhanced FEH and SPS activities in wheat stems, and that ABA plays a vital role in the regulation of the key enzymes involved in fructan and sucrose metabolism.

Hydraulics of plant growth

Multicellular plants rely on growth in localised regions that contain small, undifferentiated cells and may be many millimetres from the nearest differentiated xylem and phloem. Water and solutes must move to these small cells for their growth. Increasing evidence indicates that after exiting the xylem and phloem, water and solutes are driven to the growing cells by gradients in water potential and solute potential or concentration. The gradients are much steeper than in the vascular transport system and can change in magnitude or suffer local disruption with immediate consequences for growth. Their dynamics often obscure how turgor drives wall extension for growth, and different flow paths for roots and shoots have different dynamics. In this review, the origins of the gradients, their mode of action and their consequences are discussed, with emphasis on how their dynamics affect growth processes.

Activities of key enzymes in sucrose-to-starch conversion in wheat grains subjected to water deficit during grain filling

This study tested the hypothesis that a controlled water deficit during grain filling of wheat (Triticum aestivum) could accelerate grain-filling rate through regulating the key enzymes involved in Suc-to-starch pathway in the grains. Two high lodging-resistant wheat cultivars were field grown. Well-watered and water-deficit (WD) treatments were imposed from 9 DPA until maturity. The WD promoted the reallocation of prefixed 14C from the stems to grains, shortened the grain-filling period, and increased grain-filling rate or starch accumulation rate (SAR) in the grains. Activities of Suc synthase (SuSase), soluble starch synthase (SSS), and starch branching enzyme (SBE) in the grains were substantially enhanced by WD and positively correlated with the SAR. ADP Glc pyrophosphorylase activity was also enhanced in WD grains initially and correlated with SAR with a smaller coefficient. Activities of granule-bound starch synthase and soluble and insoluble acid invertase in the grains were less affected by WD. Abscisic acid (ABA) content in the grains was remarkably enhanced by WD and very significantly correlated with activities of SuSase, SSS, and SBE. Application of ABA on well-watered plants showed similar results as those by WD. Spraying with fluridone, an ABA synthesis inhibitor, had the opposite effect. The results suggest that increased grain-filling rate is mainly attributed to the enhanced sink activity by regulating key enzymes involved in Suc-to-starch conversion, especially SuSase, SSS, and SBE, in wheat grains when subjected to a mild water deficit during grain filling, and ABA plays a vital role in the regulation of this process.

Abscisic acid is involved in the water stress-induced betaine accumulation in pear leaves

ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.

Seeing 'cool' and 'hot'--infrared thermography as a tool for non-invasive, high-throughput screening of Arabidopsis guard cell signalling mutants

The use of Arabidopsis mutants defective in abscisic acid (ABA) perception has been instrumental in the understanding of stomatal function, in particular, ABA signalling in guard cells. The considerable attention devoted to ABA signalling in guard cells is due in part to (1) the fundamental role of ABA in drought stress and (2) the use of a screening protocol based on the sensitivity of seed germination to ABA. Such a screen has facilitated the isolation of ABA signalling mutants with genetic lesions that exert pleiotropic effects at the whole plant level. As such, there is a requirement for new approaches to complement the seed germination screen. The recent advances made in the use of infrared thermography as a non-invasive, high-throughput tool are reviewed here and the versatility of this technique for screening Arabidopsis defective in stomatal regulation is highlighted.

Optimization modeling of plant root architecture for water and phosphorus acquisition

An optimization model is presented that examines the relationship between root architecture and multiple resource acquisition, specifically water and phosphorus in spatially heterogeneous environments. The basal root growth angle of an individual common bean plant, which determines the orientation and localization of the bulk of the root system, was modeled as the decision variable. The total payoff to the plant, the benefit obtained from water and phosphorus acquisition, minus the costs of spatial competition between roots, is given as a function of the (x,y) coordinates of the basal root in two-dimensional Cartesian space. We obtained a general solution and applied it to four unique environmental cases which are as follows: (1) the case of uniformly distributed water and phosphorus; (2) the case of localized shallow phosphorus; (3) the case of localized deep water; and (4) the case of shallow phosphorus and deep water. The general solution states that the optimal basal root growth angle will occur at the point where the total rate of change in the value of the resources acquired equals the total rate of change in cost that results from locating the root deeper in the soil. An optimizing plant locates its roots deeper in the soil profile until the marginal benefit exactly equals the marginal cost. The model predicts that the basal root angle of an optimizing plant will be shallower for Case 2 and deeper for Case 3, relative to the basal root angle obtained in the case of uniformly distributed water and phosphorus. The optimal basal root angle for Case 4 will depend on the marginal rate of substitution of water availability for phosphorus availability that occurs with depth. Empirical observations of bean root architecture in the greenhouse and in the field confirm model results and are discussed. In addition, the potential importance of phenotypic plasticity and phenotypic variation are discussed in relation to optimization of traits and adaptation to spatially heterogeneous environments.