Tansley Review No. 27 The control of carbon partitioning in plants

Sugars and flowering in the grapevine (Vitis vinifera L.)

Sugars play an important role in grapevine flowering. This complex process from inflorescence initiation to fruit maturity takes two growing seasons. Currently, most of the available data concern the involvement of sugars as energy sources during the formation of reproductive structures from initiation of inflorescences during the summer of the first year, until flower opening during the following spring. Sugars devoted to the development of reproductive structures are supplied either by wood reserves or by photosynthesis in leaves or inflorescences, depending on the stage of development. Female meiosis appears to be a key point in the success of flower formation because (i) flowers are vulnerable at this stage and (ii) it corresponds in the whole plant to the transition between reserve mobilization from perennial organs (roots, trunk, and canes) towards efficient leaf photosynthesis. The perturbation of reserve replenishment during the previous year provokes perturbation in the development of inflorescences, whereas altering the photosynthetic sources affects the formation of flowers during the same year. In particular, a lack of sugar availability in flowers at female meiosis caused by various environmental or physiological fluctuations may lead to drastic flower abortion. Apart from energy, sugars also play roles as regulators of gene expression and as signal molecules that may be involved in stress responses. In the future, these two topics should be further investigated in the grapevine considering the sensitivity of flowers to environmental stresses at meiosis.

Cloning and expression analysis of rice sucrose transporter genes OsSUT2M and OsSUT5Z

Two sucrose transporter (SUT) cDNAs, OsSUT2M and OsSUT5Z, were isolated from rice (Oryza sativa L.) by reverse transcription polymerase chain reaction (RT-PCR). Sequencing results indicate they are 1,531 bp and 1,635 bp in length including complete open reading frame 1,506 bp and 1,608 bp, which encode 502 amino acids and 536 amino acids, respectively. The TopPred program suggested that both sucrose transporter proteins, OsSUT2M and OsSUT5Z, consist of potentially 12 transmembrane domains. Semi-quantitative RT-PCR was carried out to investigate the gene expression patterns of OsSUT2M and OsSUT5Z. In vegetative organs, transcripts of OsSUT2M were higher in source leaf blades than in other organs at the same development stage, whereas transcripts of OsSUT5Z were less traceable in all organs investigated. In reproductive organs, both transcripts of these two genes were high in panicles from the booting stage to 7 days after flowering (DAF) and then sharply declined. The potential physiology functions of these two sucrose transporters are discussed.

Effects of partial rootzone drying and rootstock vigour on growth and fruit quality of 'Pink Lady' apple trees in Mediterranean environments

We investigated the effects of partial rootzone drying (PRD) and rootstock vigour on water relations, and vegetative and productive performance of ‘Pink Lady’ apple (Malus domestica Borkh.) trees in central Sicily. In a first field trial, trees on MM.106 rootstock were subjected to: Conventional irrigation (CI), maintaining soil moisture above 80% of field capacity; PRD irrigation, where only one alternated side of the rootzone received 50% of the CI irrigation water; and continuous deficit irrigation (DI), where 50% of the CI water was equally applied to both sides of the rootzone. In a second trial, trees on M.9 or MM.106 were subjected to CI and PRD irrigation. PRD reduced stomatal conductance (gs) more consistently in trees on MM.106 than in trees on M.9, but maintained relative water content (RWC) to the levels of CI. DI induced greater gs reductions than PRD and lower RWC than CI and PRD. Rootstock vigour did not influence plant response to irrigation strategy. PRD induced some reduction in fruit number but no change in yields and fruit quality compared with CI, whereas DI reduced fruit size and marketable yields. Significant reductions in shoot and leaf growth were induced by DI, whereas only leaf growth was affected by PRD. Our observations indicate that responses induced by PRD are due to a combination of the amount and way of applying water, and not just to reductions in irrigation volumes, suggesting a possible use of PRD for increasing apple water-use efficiency in Mediterranean environments.

Physiology and biochemistry of source-regulated protein accumulation in the wheat grain

Wheat is unique among cereals for the baking qualities of its flour, which are dependent upon the type and concentration of its proteins. As a consequence, the grain protein concentration (GPC) is one of the main determinants of wheat international market price. More than 50-70% of the final grain N is accumulated before flowering and later remobilized to the grain, N fertilization being the common practice used to produce high GPC. However, after incremental additions of N fertilizer, GPC reaches a maximum and then remains constant, without any increase in N uptake or remobilization by the crop, thus decreasing the efficiency of N fertilizer. Although, the genetic and molecular mechanisms that regulate N uptake by the roots are being clarified quickly, the regulation and physiology of N transport from the leaves to the grain remains less clear. In this review, the possible regulatory points involved in N transport to the grain and the difficulties for increasing GPC are discussed. It has been demonstrated that protein synthesis in the grain is source-limited, and that the grain can accumulate protein limited only by the amino acids provided by the phloem. It has also been shown that there is no limitation in the amino acid/sugar ratios that can be exported to the phloem. On the other hand, NO(3)(-) uptake transporters are depressed when the plant concentration of some amino acids, such as glutamine, is high. It has also been shown that a high N supply increases cytokinins concentration, preventing leaf senescence and proteolysis. Based on this information, it is postulated that there are two main regulatory points during grain filling when plant N status is ample. On the one hand, the N uptake transporters in the roots are depressed due to the high amino acids concentration in the tissues, and N uptake is low. On the other, a high amino acids concentration keeps the cytokinins level high, repressing leaf protein degradation and decreasing amino acid export to the phloem. As a consequence, GPC cannot be increased despite the ample N supply.

Contribution of new photosynthetic assimilates to respiration by perennial grasses and shrubs: residence times and allocation patterns

Quantification of the fate of carbon (C) used by plant metabolism is necessary to improve predictions of terrestrial ecosystem respiration and its sources. Here, a dual isotope ((13)C and (14)C) pulse-label was used to determine the allocation of new C to different respiratory pathways in the early and late growing seasons for two plant functional types, perennial grasses and shrubs, in the Owens Valley, CA, USA. Allocation differences between plant types exceeded seasonal allocation variation. Grasses respired 71 and 64% and shrubs respired 22 and 17% of the label below-ground in the early and late growing seasons, respectively. Across seasons and plant types, approximately 48-61% of the label recovered was respired in 24 h, approximately 68-84% in 6 d, and approximately 16-33% in 6-36 d after labeling. Three C pools were identified for plant metabolism: a fast pool with mean residence times (MRTs) of approximately 0.5 and approximately 1 d below- and above-ground, respectively; an intermediate pool with MRTs of 19.9 and 18.9 d; and a storage pool detected in new leaf early growing season respiration > 9 months after assimilation. Differences in allocation to fast vs intermediate C pools resulted in the mean age of C respired by shrubs being shorter (3.8-4.5 d) than that of the grasses (4.8-8.2 d).

Sucrose deficiency delays lycopene accumulation in tomato fruit pericarp discs

Tomato (Solanum lycopersicum) fruit ripening is characterized by a massive accumulation of carotenoids (mainly lycopene) as chloroplasts change to chromoplasts. To address the question of the role of sugars in controlling carotenoid accumulation, fruit pericarp discs (mature green fruits) were cultured in vitro in the presence of various sucrose concentrations. A significant difference in soluble sugar content was achieved depending on external sucrose availability. Sucrose limitation delayed and reduced lycopene and phytoene accumulation, with no significant effect on other carotenoids. Chlorophyll degradation and starch catabolism were not affected by variations of sucrose availability. The reduction of lycopene synthesis observed in sucrose-limited conditions was mediated through metabolic changes illustrated by reduced hexose accumulation levels. In addition, variations of sucrose availability modulated PSY1 gene expression. Taken together our results suggest that the modulation of carotenoid accumulation by sucrose availability occurs at the metabolic level and involves the differential regulation of genes involved in carotenoid biosynthesis.

Resource translocation within seagrass clones: allometric scaling to plant size and productivity

The allometric scaling of resource demand and translocation within seagrass clones to plant size (i.e. shoot mass and rhizome diameter), shoot production and leaf turnover was examined in situ in eight seagrass species (Cymodocea nodosa, Cymodocea serrulata, Halophila stipulacea, Halodule uninervis, Posidonia oceanica, Thalassodendron ciliatum, Thalassia hemprichii and Zostera noltii), encompassing most of the size range present in seagrass flora. One fully developed shoot on each experimental rhizome was incubated for 2-3 h with a pulse of NaH(13)CO(3) (235 micromol) and (15)NH(4)Cl (40 micromol). The mobilisation of incorporated tracers across the clone was examined 4 days later. Carbon and nitrogen demand for shoot production across seagrass species scaled at half of the shoot mass, whereas seagrass leaves incorporated tracers ((13)C and (15)N) at rates proportional to the shoot mass. The shoots of all seagrass species shared resources with neighbours, particularly with younger ones. The time scales of physiological integration and the absolute amount of resources shared by seagrass ramets scaled at 2.5 power of the rhizome diameter. Hence, the ramets of larger species were physiologically connected for longer time scales and share larger absolute amounts of resources with neighbours than those of smaller species. The different pattern of resource translocation exhibited by seagrasses helps explain the ecological role displayed by these species and the success of large seagrasses colonising nutrient-poor coastal areas, where they often dominate.

FOLIAR DAMAGE TO PARENTAL PLANTS INTERACTS TO INFLUENCE MATING SUCCESS OFIPOMOEA PURPUREA

Studies of how herbivory affects plant fitness often determine whether damage to one parent alters reproductive output (i.e., seed set or paternity) but ignore the possibility that the outcome may be different if both parents were damaged (i.e., the presence of maternal x paternal damage interactions). Using inbred lines of the common morning glory, Ipomoea purpurea, I conducted a series of greenhouse experiments to test whether foliar damage from a generalist insect herbivore, Trichoplusia ni, alters male and female fitness components when neither parent, one parent, or both I. purpurea parents had been damaged. In a single-donor experiment, flowers on both damaged and undamaged maternal plants received pollen from either damaged or undamaged paternal plants. I. purpurea flowers were more likely to be aborted when they received pollen from damaged paternal plants, or when maternal plants were both damaged and grown under low-resource conditions. Foliar damaged plants also produced less seed and pollen than undamaged plants, although seed mass and pollen viability were not affected by damage. In a multiple-donor experiment, flowers on damaged and undamaged maternal plants simultaneously received pollen from damaged and undamaged paternal plants, and F1 seeds were analyzed for paternity. Damaged paternal plants had reduced siring success compared to undamaged paternal plants, and this discrepancy was most pronounced when competition occurred on damaged maternal plants. Thus, damaged maternal plants were more "selective" than undamaged maternal plants. Although previous studies have demonstrated that herbivory can alter fruit and seed production and paternity patterns, this is the first study to show that the magnitude of herbivore damage experienced by both parents can interact to influence maternal and paternal mating success.

Branch development controls leaf area dynamics in grapevine (Vitis vinifera) growing in drying soil

BACKGROUND AND AIMS

Soil water deficit is a major abiotic stress with severe consequences for the development, productivity and quality of crops. However, it is considered a positive factor in grapevine management (Vitis vinifera), as it has been shown to increase grape quality. The effects of soil water deficit on organogenesis, morphogenesis and gas exchange in the shoot were investigated.

METHODS

Shoot organogenesis was analysed by distinguishing between the various steps in the development of the main axis and branches. Several experiments were carried out in pots, placed in a greenhouse or outside, in southern France. Soil water deficits of various intensities were imposed during vegetative development of the shoots of two cultivars ('Syrah' and 'Grenache N').

KEY RESULTS

All developmental processes were inhibited by soil water deficit, in an intensity-dependent manner, and sensitivity to water stress was process-dependent. Quantitative relationships with soil water were established for all processes. No difference was observed between the two cultivars for any criterion. The number of leaves on branches was particularly sensitive to soil water deficit, which rapidly and strongly reduced the rate of leaf appearance on developing branches. This response was not related to carbon availability, photosynthetic activity or the soluble sugar content of young expanding leaves. The potential number of branches was not a limiting factor for shoot development.

CONCLUSIONS

The particularly high sensitivity to soil water deficit of leaf appearance on branches indicates that this process is a major determinant of the adaptation of plant leaf area to soil water deficit. The origin of this particular developmental response to soil water deficit is unclear, but it seems to be related to constitutive characteristics of branches rather than to competition for assimilates between axes differing in sink strength.

Changes in carbohydrate and nutrient contents throughout a reproductive cycle indicate that phosphorus is a limiting nutrient in the epiphytic bromeliad, Werauhia sanguinolenta

BACKGROUND AND AIMS

This study examined the physiological basis of the cost of reproduction in the epiphytic bromeliad Werauhia sanguinolenta, growing in situ in a tropical lowland forest in Panama.

METHODS

Entire mature plants were sampled repeatedly over the course of 2 years, which represents the common interval between reproductive events. Due to the uncertainty concerning the appropriate currency of resource allocation to reproduction, the temporal changes of the contents of total non-structural carbohydrates (TNC) and of all major nutrient elements in different plant parts were studied (stems, green leaves, non-green leaf bases, roots and reproductive structures when present).

KEY RESULTS

Although TNC varied with time in all compartments, this variation was more related to seasonal fluctuations than to reproductive status. The contents of the nutrient elements, N, P, K, Mg and S, on the other hand, showed significant differences between reproductive and non-reproductive individuals, while Ca did not change with reproductive status. Differences in nutrient contents were most pronounced in stems. Seeds were particularly enriched in P, much less so in N and the other nutrient elements. Model calculations of nutrient fluxes indicate that a plant needs about 2 years to accumulate the amount of P invested in a fruit crop, while the estimated uptake rates for N were much faster.

CONCLUSIONS

Since most mature individuals of this species fruit every other year, it is hypothesized that P is the prime limiting factor for reproduction. These findings therefore add to an increasing body of evidence that P rather than N is limiting growth and reproduction in vascular epiphytes.

Scaling phloem transport: elasticity and pressure-concentration waves

Earlier theoretical analyses of the rate of propagation of pressure-concentration waves in the phloem were performed without adequate attention to the elastic expansion of sieve tube walls. Here, it is shown that the rate of propagation of pressure-concentration waves in phloem sieve tubes is not significantly impeded by wall elasticity, but rather, as previously implicated, by the ratio of sap osmotic pressure to the axial drop in sap hydrostatic pressure. It is also shown that pressure-concentration waves move equally well in both the upstream and downstream directions. These results permit future models to ignore elastic effects, and lend additional theoretical support to the "osmoregulatory flow" hypothesis, which argues that efficient molecular control of the phloem is permitted by maintaining sieve sap hydrostatic pressure at a value that is spatially nearly constant, which in turn permits changes in sieve tube state to be rapidly transmitted throughout the sieve tube via pressure-concentration waves.

Mast fruiting of large ectomycorrhizal African rain forest trees: importance of dry season intensity, and the resource-limitation hypothesis

Mast fruiting is a distinctive reproductive trait in trees. This rain forest study, at a nutrient-poor site with a seasonal climate in tropical Africa, provides new insights into the causes of this mode of phenological patterning. At Korup, Cameroon, 150 trees of the large, ectomycorrhizal caesalp, Microberlinia bisulcata, were recorded almost monthly for leafing, flowering and fruiting during 1995-2000. The series was extended to 1988-2004 with less detailed data. Individual transitions in phenology were analysed. Masting occurred when the dry season before fruiting was drier, and the one before that was wetter, than average. Intervals between events were usually 2 or 3 yr. Masting was associated with early leaf exchange, followed by mass flowering, and was highly synchronous in the population. Trees at higher elevation showed more fruiting. Output declined between 1995 and 2000. Mast fruiting in M. bisulcata appears to be driven by climate variation and is regulated by internal tree processes. The resource-limitation hypothesis was supported. An 'alternative bearing' system seems to underlie masting. That ectomycorrhizal habit facilitates masting in trees is strongly implied.

Costs of reproduction as related to the timing of phenological phases in the dioecious Shrub Pistacia lentiscus L

Females of woody dioecious species usually devote more resources to reproduction than males. This may lead to a decrease in female survival and growth. The costs of reproduction, however, can be lightened through a number of mechanisms, as for example avoiding the temporal coincidence of reproduction and vegetative growth. The aim of this study was to evaluate whether males and females of P. lentiscus differ in the timing of their vegetative growth, and to assess whether the sequencing of vegetative growth and reproduction reduces reproductive costs. We monitored phenology in males and females. We also compared male and female allocation of nutrients and biomass in the branch, and the developmental stability of the growing shoots. We did this both prior to and at the end of the fruiting period. Males and females showed similar vegetative and flowering phenologies. Males invested more biomass in flowering, but the sexes showed equal vegetative biomass and nutrient content prior to the fruiting period. In female branches, no trade-off was found between fruit load and current-year vegetative growth. In P. lentiscus, avoiding the overlap of flowering, vegetative growth and fruiting probably contributes to reduce the immediate costs of reproductive efforts, both in males and females.

Sink strength regulates photosynthesis in sugarcane

The relationship in sugarcane (Saccharum spp.) between photosynthetic source tissue and sink material was examined through manipulation of the sink:source ratio of field-grown Saccharum spp. hybrid cv. N19 (N19). To enhance sink strength, all leaves, except for the third fully expanded leaf, were enclosed in 90% shade cloth for varying periods of time. Variations in sucrose, glucose and fructose concentrations were measured and the effects of shading on the leaf gas exchange and fluorescence characteristics recorded. Changes in carbon partitioning caused by shading were examined based on the uptake and translocation of fixed 14CO2. Following a decline in sucrose concentrations in young internodal tissue and shaded leaves, significant increases in the CO2-saturated photosynthetic rate (Jmax), carboxylation efficiency (CE) and electron transport rate were observed in unshaded leaves after 8 d of shading treatment. It was concluded that up-regulation of source-leaf photosynthetic capacity is correlated with a decrease in assimilate availability to acropetal culm sink tissue. Furthermore, a significant relationship was revealed between source hexose concentration and photosynthetic activity.

Plant CO2 responses: an issue of definition, time and resource supply

In this review I am drawing attention to some constraints and biases in CO2 enrichment experiments and the analysis of data in the literature. Conclusions drawn from experimental works differ when the data are grouped in a way such that the relative frequency of test conditions does not determine the emerging trends, for instance unrealistically strong CO2-'fertilization' effects, which are in conflict with some basic ecological principles. I suggest separating three test conditions: uncoupled systems (plants not depending in a natural nutrient cycle) (I); expanding systems, in which plants are given ample space and time to explore otherwise limited resources (II); and fully coupled systems in which the natural nutrient cycling governs growth at steady-state leaf area index (LAI) and fine root renewal (III). Data for 10 type III experiments yield rather moderate effects of elevated CO2 on plant biomass production, if any. In steady-state grassland, the effects are water-related; in closed tree stands, initial effects decline rapidly with time. Plant-soil coupling (soil conditions) deserves far greater attention than plant-atmosphere coupling (CO2 enrichment technology).

Effects of drought stress and nutrient availability on dry matter allocation, phenolic glycosides, and rapid induced resistance of poplar to two lymantriid defoliators

The growth-differentiation balance hypothesis (GDBH) postulates that variation in resource availability can increase or decrease allocation to secondary metabolism, depending on how growth is affected relative to carbon assimilation. Growth and leaf area of black poplar (Populus nigra) increased substantially in response to increased nutrient availability, while net assimilation rate and photosynthesis were less strongly affected. In response, total phenolic glycoside concentrations declined, which is consistent with GDBH. Drought stress decreased net assimilation rate and photosynthesis as well as growth, while increasing total phenolic glycoside concentrations. This pattern does not follow GDBH, which predicts lower secondary metabolism when resource limitation decreases both growth and carbon assimilation. However, there was a strong negative correlation between growth and total phenolic glycoside concentration consistent with a trade-off between primary and secondary metabolism, a key premise of GDBH. Drought decreased the growth of gypsy moth (Lymantria dispar) larvae but had no effect on whitemarked tussock moth (Orgyia leucostigma). Increased nutrient availability had a positive linear effect on growth of whitemarked tussock moth, but no effect on gypsy moth. Treatment effects on gypsy moth corresponded closely with effects on total phenolic glycosides, whereas effects on whitemarked tussock moth more closely tracked changes in nutritional quality. Localized gypsy moth herbivory elicited rapid induced resistance to gypsy moth, with the effect being independent of water and nutrient availability, but did not affect whitemarked tussock moth, indicating that the effects of biotic and abiotic stress on insect resistance of trees can be species-specific.

New understanding on phloem physiology and possible consequences for modelling long-distance carbon transport

Most current models of assimilate carbohydrate partitioning are based on growth patterns observed under a range of experimental conditions, from which a set of empirical rules are derived to simulate partitioning. As a result, they are not good at extrapolating to other conditions; this requires a mechanistic approach, which only transport-resistance (TR) models currently provide. We examine an approach to incorporating recent progress in phloem physiology into the TR approach, which leads to a 'minimalist' Munch model of a branched system with competing sinks. In vivo whole-plant measurements have demonstrated that C-flow rates are dependent not only on the properties of the sink, but also on the properties of the whole transport system, and the detailed dynamics of this behaviour is mimicked by the proposed model. This model provides a sound theoretical framework for an unambiguous definition of sink and source strengths, with sink priority being an emergent property of the model. Further developments are proposed, some of which have already had limited application, to cope with the complexity of plants; the emphasis is on a modular approach, together with the importance of choosing the appropriate scale level for both structure and function. Whole-plant experiments with in vivo measurement of the phloem dynamics will be needed to help with this choice.

Modelling the fate of sulphur-35 in crops. 2. Development and validation of the CROPS-35 model

Gas-cooled nuclear power plants in the UK release sulphur-35 during their routine operation, which can be readily assimilated by vegetation. It is therefore necessary to be able to model the uptake of such releases in order to quantify any potential contamination of the food chain. A model is described which predicts the concentration of (35)S in crop components following an aerial gaseous release. Following deposition the allocation to crop components is determined by an export function from a labile pool, the leaves, to those components growing most actively post exposure. The growth rates are determined by crop growth data, which is also used to determine the concentration. The loss of activity is controlled by radioactive decay only. The paper describes the calibration and the validation of the model. To improve the model, further experimental work is required particularly on the export kinetics of (35)S. It may be possible to adapt such a modelling approach to the prediction of crop content for gaseous releases of (3)H and (14)C from nuclear facilities.

Gas exchange and yield response to foliar phosphorus application in Phaseolus vulgaris L. under drought

This study was conducted to evaluate the effect of foliar Pi application on gas exchange and yield of bean genotypes submitted to a mild water deficit at the pre-flowering stage. In the first experiment, when extra Pi (10 g.L-1) was sprayed on leaves during water stress or during recovery, there was no effect on gas exchange or yield in the A320, Carioca and Ouro Negro genotypes. However, net CO2 assimilation (A) of A320 and Ouro Negro was less affected, but not significantly, than Carioca at the end of the stress, when Pi was supplied five days before water deficit. In the second experiment, two different doses of Pi (10 and 20 g.Pi L-1) were sprayed five days before water deficit on the Carioca genotype. During the last three days of the mild water deficit, A values were significantly higher for the Pi20 treatment when compared to the control plants without extra Pi supply. The intrinsic water use efficiency for plants receiving Pi20 was significantly higher than for the other treatments. In addition, seed dry weight per plant was higher for plants receiving Pi20 dose than for plants with Pi10 and its control.

Non-structural carbohydrate pools in a tropical forest

The pool size of mobile, i.e. non-structural carbohydrates (NSC) in trees reflects the balance between net photosynthetic carbon uptake (source) and irreversible investments in structures or loss of carbon (sink). The seasonal variation of NSC concentration should reflect the sink/source relationship, provided all tissues from root to crown tops are considered. Using the Smithsonian canopy crane in Panama we studied NSC concentrations in a semi-deciduous tropical forest over 22 months. In the 9 most intensively studied species (out of the 17 investigated), we found higher NSC concentrations (starch, glucose, fructose, sucrose) across all species and organs in the dry season than in the wet season (NSC 7.2% vs 5.8% of dry matter in leaves, 8.8/6.0 in branches, 9.7/8.5 in stems, 8.3/6.4 in coarse and 3.9/2.2 in fine roots). Since this increase was due to starch only, we attribute this to drought-constrained growth (photosynthesis less affected by drought than sink activity). Species-specific phenological rhythms (leafing or fruiting) did not overturn these seasonal trends. Most of the stem volume (diameter at breast height around 40 cm) stores NSC. We present the first whole forest estimate of NSC pool size, assuming a 200 t ha(-1) forest biomass: 8% of this i.e. ca. 16 t ha(-1) is NSC, with ca. 13 t ha(-1) in stems and branches, ca. 0.5 and 2.8 t ha(-1) in leaves and roots. Starch alone (ca. 10.5 t ha(-1)) accounts for far more C than would be needed to replace the total leaf canopy without additional photosynthesis. NSC never passed through a period of significant depletion. Leaf flushing did not draw heavily upon NSC pools. Overall, the data imply a high carbon supply status of this forest and that growth during the dry season is not carbon limited. Rather, water shortage seems to limit carbon investment (new tissue formation) directly, leaving little leeway for a direct CO2 fertilization effects.

Herbivore effects on developmental instability and fecundity of holm oaks

Plants are able to compensate for loss of tissue due to herbivores at a variety of spatial and temporal scales, masking detrimental effects of herbivory on plant fitness at these scales. The stressing effect of herbivory could also produce instability in the development of plant modules, and measures of such instability may reflect the fitness consequences of herbivory if instability is related to components of plant fitness. We analyse the relationships between herbivory, developmental instability and production of female flowers and fruits of holm oak Quercus ilex trees by means of herbivore removal experiments. Removal of leaf herbivores reduced herbivory rates at the tree level, but had no effect on mean production of female flowers or mature fruits, whereas herbivory tended to enhance flower production and had no effect on fruit abortion at the shoot level. Differences in herbivory levels between shoots of the same branch did not affect the size and fluctuating asymmetry of intact leaves. These results indicate compensation for herbivory at the tree level and over-compensation at the shoot level in terms of allocation of resources to female flower production. Removal of insect herbivores produced an increase in the mean developmental instability of leaves at the tree level in the year following the insecticide treatment, and there was a direct relationship between herbivory rates in the current year and leaf fluctuating asymmetry the following year irrespective of herbivore removal treatment. Finally, the production of pistillate flowers and fruits by trees was inversely related to the mean fluctuating asymmetry of leaves growing the same year. Leaf fluctuating asymmetry was thus an estimator of the stressing effects of herbivory on adult trees, an effect that was delayed to the following year. As leaf fluctuating asymmetry was also related to tree fecundity, asymmetry levels provided a sensitive measure of plant performance under conditions of compensatory responses to herbivory.

Daily photosynthetic and C-export patterns in winter wheat leaves during cold stress and acclimation

Diurnal patterns of whole-plant and leaf gas exchange and 14C-export of winter wheat acclimated at 20 and 5 degrees C were determined. The 5 degrees C-acclimated plants had lower relative growth rates, smaller biomass and leaf area, but larger specific leaf weight than 20 degrees C plants. Photosynthetic rates in 20 degrees C and 5 degrees C-acclimated leaves were similar; however, daytime export from 5 degrees C-acclimated leaves was 45% lower. Photosynthesis and export remained steady in 20 degrees C and 5 degrees C-acclimated leaves during the daytime. By comparison, photosynthesis in 5 degrees C-stressed leaves (20 degrees C-acclimated plants exposed to 5 degrees C 12 h before and during measurements) declined from 70 to 50% of the 20 degrees C-acclimated leaves during the daytime, while export remained constant at 35% of the 20 degrees C-acclimated and 60% of the 5 degrees C-acclimated leaves. At high light and CO2, photosynthesis and export increased in both 20 degrees C and 5 degrees C-acclimated leaves, but rates in 5 degrees C-stressed leaves remained unchanged. At all conditions daytime export was greater than nighttime export. Taken together, during cold acclimation photosynthesis was upregulated, whereas export was only partially increased. We suggest that this reflects a requirement of cold-acclimated plants to both sustain an increased leaf metabolic demand while concomitantly supporting translocation of photoassimilates to overwintering sinks.

Physiological, biochemical and molecular analysis of sugar-starvation responses in tomato roots

Two-month-old tomato plants were submitted to day/night cycles and to prolonged darkness in order to investigate the physiological and biochemical response to sugar starvation in sink organs. Roots appeared particularly sensitive to the cessation of photosynthesis, as revealed by the reduction of the growth rate and the decline of the carbohydrate and protein content. Therefore, excised tomato roots were used as a model to deepen the characterization of sugar starvation symptoms. In excised roots, the endogenous sugars were rapidly exhausted and significant degradation of protein was observed. Glutamine and asparagine accounted for most of the nitrogen released by protein breakdown. Respiration declined and proliferation- and growth-associated genes were repressed soon after the beginning of the sugar depletion. Among the genes studied, only the gene encoding asparagine synthetase was strongly induced. All the starvation symptoms were reversible when the roots were resupplied with sugar. When the culture conditions deteriorated, the metabolic and molecular changes led to the triggering of apoptosis of the root cells.

Extracellular invertase: key metabolic enzyme and PR protein

Extracellular invertase is the key enzyme of an apoplasmic phloem unloading pathway and catalyses the hydrolytic cleavage of the transport sugar sucrose released into the apoplast. This mechanism contributes to long-distance assimilate transport, provides the substrate to sustain heterotrophic growth and generates metabolic signals known to effect various processes of primary metabolism and defence responses. The essential function of extracellular invertase for supplying carbohydrates to sink organs was demonstrated by the finding that antisense repression of an anther-specific isoenzyme provides an efficient method for metabolic engineering of male sterility. The regulation of extracellular invertase by all classes of phytohormones indicates an essential link between the molecular mechanism of phytohormone action and primary metabolism. The up-regulation of extracellular invertase appears to be a common response to various biotic and abiotic stress-related stimuli such as pathogen infection and salt stress, in addition to specific stress-related reactions. Based on the observed co-ordinated regulation of source/sink relations and defence responses by sugars and stress-related stimuli, the identified activation of distinct subsets of MAP kinases provides a mechanism for signal integration and distribution within such complex networks. Sucrose derivatives not synthesized by higher plants, such as turanose, were shown to elicit responses distinctly different from metabolizable sugars and are rather perceived as stress-related stimuli.

[Hydrous and photosynthetic adaptations of common and durum wheat to saline stress]

Seven varieties of bred wheat and seven varieties of durum wheat were cultivated in three different sites from the area of Errachidia (southeastern Morocco). These sites differ by the degree of salinity in the irrigation water. Results obtained showed that the reduction in leaf area is the principal strategy that makes it possible to attenuate the effects of the reduction in the availability of water under saline stress. Bread wheat, which limited the reduction in the leaf area, with the risk to undergo some hydrous problems, seems to better preserve its photosynthetic potentialities and grain productivity.

The costs of reproduction in plants

This review reports on the processes associated with costs of reproduction, including some theoretical considerations, definitions and methodological aspects, followed by a list of the situations where costs are difficult to find. Despite some exceptions, case studies, examined by trade-offs between reproduction and other life-history traits, generally support the predictions of the cost of reproduction hypothesis. The cost of reproduction as an evolutionary determinant of sexual dimorphism in life history traits in dioecious species was specifically tested, considering that the higher cost of reproduction in females has driven the life history traits related to sexual dimorphism. Females of woody dioecious species were consistently smaller than males supporting the costs of reproduction hypothesis. By contrast, females of herbaceous perennials were generally the larger sex, which did not fit the expectations of the hypothesis. Finally, the mechanisms that enable the compensation of the reproductive costs are detailed, including the plastic responses of photosynthesis and growth, the effects of the timing of investment, plant architecture and plant physiological integration. Contents Summary 321 I. Introduction 321 II. Theory on costs of reproduction 322 III. Methodological aspects 324 IV. Empirical evidence 328 V. Plant size and costs of reproduction 330 VI. Costs of reproduction in sexually dimorphic plants 331 VII. Compensation of the costs 333 VIII. Concluding comments and future perspectives 336 Acknowledgements 337 References 337.

Carbohydrate differences in strawberry crowns and fruit (Fragaria x ananassa) during plant development

Carbohydrates accumulation and mobilization are highly relevant in plants because they have been related to yield and quality. Therefore, the aims of this work were to determine soluble carbohydrates and starch in strawberry (Fragaria x ananassa cv. Camarosa) crown sections (basal, middle, and upper) at three different plant growth stages (vegetative, blooming, and fruiting), and in fruit varieties (cv. Camarosa, Seascape, and Oso Grande) grown on the same field and in a different geoclimate. The main soluble carbohydrates found were glucose, sucrose, and fructose. Concentration differences were found among crown sections and time. The lowest levels of glucose, fructose, and sucrose were present at the beginning of fruit formation (6.2, 1.8 mg/g, and trace, respectively). Starch increased in basal and middle sections at the same time (8.6 to 109.6 and 6.6 to 93.5 mg/g, respectively). There appears to be a relationship between crown and fruit soluble carbohydrates. The most abundant fruit monosaccharides in all varieties were glucose (160-190 mg/g), fructose (90-180 mg/g), and sucrose (30-120 mg/g), followed by myo-inositol (10-23 mg/g). Strawberry crowns are an important source of carbohydrates and they might play a role during plant development specifically related to fruit sweetness. Fruit quality is highly influenced by a combination of several factors such as genotype, geoclimate, and probably carbon partitioning.

Seed-specific overexpression of a potato sucrose transporter increases sucrose uptake and growth rates of developing pea cotyledons

During the storage phase, cotyledons of developing pea seeds are nourished by nutrients released to the seed apoplasm by their maternal seed coats. Sucrose is transported into pea cotyledons by sucrose/H+ symport mediated by PsSUT1 and possibly other sucrose symporters. PsSUT1 is principally localised to plasma membranes of cotyledon epidermal and subepidermal transfer cells abutting the seed coat. We tested the hypothesis that endogenous sucrose/H+ symporter(s) regulate sucrose import into developing pea cotyledons. This was done by supplementing their transport activity with a potato sucrose symporter (StSUT1), selectively expressed in cotyledon storage parenchyma cells under control of a vicilin promoter. In segregating transgenic lines, enhanced [(14)C]sucrose influx into cotyledons above wild-type levels was found to be dependent on StSUT1 expression. The transgene significantly increased (approximately 2-fold) transport activity of cotyledon storage parenchyma tissues where it was selectively expressed. In contrast, sucrose influx into whole cotyledons through the endogenous epidermal transfer cell pathway was increased by only 23% in cotyledons expressing the transgene. A similar response was found for rates of biomass gain by intact cotyledons and by excised cotyledons cultured on a sucrose medium. These observations demonstrate that transport activities of sucrose symporters influence cotyledon growth rates. The attenuated effect of StSUT1 overexpression on sucrose and dry matter fluxes by whole cotyledons is consistent with a large proportion of sucrose being taken up at the cotyledonary surface. This indicates that the cellular location of sucrose transporter activity plays a key role in determining rates of sucrose import into cotyledons.

Global allocation rules for patterns of biomass partitioning in seed plants

A general allometric model has been derived to predict intraspecific and interspecific scaling relationships among seed plant leaf, stem, and root biomass. Analysis of a large compendium of standing organ biomass sampled across a broad sampling of taxa inhabiting diverse ecological habitats supports the relations predicted by the model and defines the boundary conditions for above- and below-ground biomass partitioning. These canonical biomass relations are insensitive to phyletic affiliation (conifers versus angiosperms) and variation in averaged local environmental conditions. The model thus identifies and defines the limits that have guided the diversification of seed plant biomass allocation strategies.

Sink feedback regulation of photosynthesis in vines: measurements and a model

An experimental and modelling study of source-sink interactions in Vitis vinifera L., cv. Cabernet Sauvignon, rooted cuttings under non-limiting environmental conditions with a 12 h photoperiod is presented here. After 4 h, measured photosynthesis, stomatal conductance and leaf carbohydrate content reached maximum values. Over the remainder of the photoperiod, photosynthesis and stomatal conductance decreased continuously, whereas leaf carbohydrate content remained relatively constant. Because the experiment took place in a non-limiting environment, the results suggest that stomatal regulation of photosynthesis was mediated by an internal factor, possibly related to sink activity. A simple 1-source, 2-sink model was developed to examine the extent to which the data could be explained by a hypothetical sink-to-source feedback mechanism mediated by carbohydrate levels in either the mesophyll, the source phloem or the phloem of one of the two sinks. Model simulations reproduced the data well under the hypothesis of a phloem-based feedback signal, although the data were insufficient to elucidate the detailed nature of such a signal. In a sensitivity analysis, the steady-state response of photosynthesis to sink activity was explored and predictions made for the partitioning of photosynthate between the two sinks. The analysis highlights the effectiveness of a phloem-based feedback signal in regulating the balance between source and sink activities. However, other mechanisms for the observed decline in photosynthesis, such as photoinhibition, endogenous circadian rhythms or hydraulic signals in the leaf cannot be excluded. Nevertheless, it is concluded that the phloem-based feedback model developed here may provide a useful working hypothesis for incorporation into plant growth models and for further development and testing.

Interactive effects of soil temperature, atmospheric carbon dioxide and soil N on root development, biomass and nutrient uptake of winter wheat during vegetative growth

Nutrient requirements for plant growth are expected to rise in response to the predicted changes in CO(2) and temperature. In this context, little attention has been paid to the effects of soil temperature, which limits plant growth at early stages in temperate regions. A factorial growth-room experiment was conducted with winter wheat, varying soil temperature (10 degrees C and 15 degrees C), atmospheric CO(2) concentration (360 and 700 ppm), and N supply (low and high). The hypothesis was that soil temperature would modify root development, biomass allocation and nutrient uptake during vegetative growth and that its effects would interact with atmospheric CO(2) and N availability. Soil temperature effects were confirmed for most of the variables measured and 3-factor interactions were observed for root development, plant biomass components, N-use efficiency, and shoot P content. Importantly, the soil temperature effects were manifest in the absence of any change in air temperature. Changes in root development, nutrient uptake and nutrient-use efficiencies were interpreted as counterbalancing mechanisms for meeting nutrient requirements for plant growth in each situation. Most variables responded to an increase in resource availability in the order: N supply >soil temperature >CO(2).

Limited carbohydrate availability as a potential cause of fruit abortion in Rubus chamaemorus

Fruit abortion can be caused by a range of abiotic and biotic factors. To gain a better understanding of the causes of the high fruit abortion frequency in cloudberry (Rubus chamaemorus L.), we manipulated different sources of carbon, that is, leaves and rhizome. We also manipulated flower number to see if competition between floral ramets explained fruit abortion in cloudberry. Reducing the number of flowers had no impact on fruit abortion frequency. In fact, the species forms an extensive rhizome network with only a few ramets per clone and competition between floral ramets is unlikely. Ramet defoliation had limited impact on fruit abortion, but successful fruit development was affected by rhizome length. The longer the rhizome, the higher the chances to mature a fruit. These results suggest that current photoassimilate production by the reproductive ramet alone is insufficient to insure fruit development. Carbon can come from other ramets but distances are usually high between ramets. Fruit production might thus depend on the use of stored carbohydrates in the rhizome to balance insufficient photosynthetic contribution during fruit production.

Carbon isotope discrimination and mineral composition of three organs in durum wheat genotypes grown under Mediterranean conditions

Carbon isotope discrimination (delta) has been proposed as a good criterion for transpiration efficiency and grain yield improvement. Its measurement, however, remains very expensive. Ash content (ma) has been proposed as an alternative criterion for delta in bread wheat and barley. The aims of this study were (i) to analyse the relationships between delta and mineral composition in different durum wheat plant parts and (ii) to compare the variation of these traits between landraces and improved varieties from different geographic origins. For this purpose, delta, ma, and composition in four minerals (K, Mg, P and Si) were assessed in flag leaves and awns at anthesis, and in mature grains of ten durum wheat genotypes grown under rainfed Mediterranean conditions. The three plant parts differed significantly for the measured traits. Significant correlations were noted between delta and ma in the flag leaf and in the grain. Silicon content in flag leaves and potassium content in awns were also positively related to delta of the considered plant part. The coefficient of correlation between delta and ma was generally higher than that observed between individual mineral content and delta, suggesting that ma is the better alternative criterion for delta. In addition, grain yield was related to grain delta and both ma and potassium content in awns. Harvest index was correlated with delta and ma of grain and flag leaf. These results emphasised that ma values in flag leaf and grain represent the efficiency of carbon partitioning to the grain. Improved varieties showed higher delta and ma values than landraces. Differences between Middle-East and West Mediterranean genotypes for the measured traits were also presented and discussed.

Sink-limitation and the size-number trade-off of organs: production of organs using a fixed amount of reserves

To analyze the nature of size-number trade-off of organs, we develop models in which the effects of sink-limitation in the growth of organs and the loss of resources by maintenance respiration are taken into consideration. In these models, the resource absorption rate of an organ is proportional to either its absolute size or its surface area and either the initial size of an organ or the total initial size of the organs produced is fixed. In all models, organs are produced using a fixed amount of reserved resources and no additional resources become newly available for their growth. We theoretically show that size-number trade-offs are nonlinear if the resource absorption rate of an organ is proportional to the absolute size of the organ and the initial size of the individual organs is fixed or if the resource absorption rate of an organ is proportional to the surface area of the organ. In these nonlinear size-number trade-offs, the size of individual organs increases less rapidly than in linear trade-offs with a decrease in the number of organs and the total size of organs is an increasing function of the number of organs produced. This implies that increasing the number of organs produced is advantageous in terms of resource-use efficiency. In contrast, size-number trade-off is linear if the resource absorption rate of an organ is proportional to the absolute size of the organ and there is a linear trade-off between the initial size of organs and their number. To exemplify the effects of those size-number trade-offs on the life-history evolution, we calculate the optimal offspring sizes that maximize the number of offspring successfully being established. In the case of nonlinear size-number trade-offs, the optimal offspring sizes are smaller than the optimal offspring size in the case of linear size-number trade-offs, namely, that in the model of Smith and Fretwell (1974). Our optimal offspring size depends on the metabolism of organ development; the optimal offspring size decreases with an increase in maintenance respiration rate relative to the growth coefficient of organs.

Elevated CO2 and ozone reduce nitrogen acquisition by Pinus halepensis from its mycorrhizal symbiont

The effects of 700 µmol mol-1 CO2 and 200 nmol mol-1 ozone on photosynthesis in Pinus halepensis seedlings and on N translocation from its mycorrhizal symbiont, Paxillus involutus, were studied under nutrient-poor conditions. After 79 days of exposure, ozone reduced and elevated CO2 increased net assimilation rate. However, the effect was dependent on daily accumulated exposure. No statistically significant differences in total plant mass accumulation were observed, although ozone-treated plants tended to be smaller. Changes in atmospheric gas concentrations induced changes in allocation of resources: under elevated ozone, shoots showed high priority over roots and had significantly elevated N concentrations. As a result of different shoot N concentration and net carbon assimilation rates, photosynthetic N use efficiency was significantly increased under elevated CO2 and decreased under ozone. The differences in photosynthesis were mirrored in the growth of the fungus in symbiosis with the pine seedlings. However, exposure to CO2 and ozone both reduced the symbiosis-mediated N uptake. The results suggest an increased carbon cost of symbiosis-mediated N uptake under elevated CO2, while under ozone, plant N acquisition is preferentially shifted towards increased root uptake.

Ash content might predict carbon isotope discrimination and grain yield in durum wheat

•  Dry mass per unit of leaf area (LDM) and ash content were evaluated as alternative criteria for carbon isotope discrimination (Δ) in durum wheat (Triticum durum) flag leaves and grains. •  Using correlation analysis the relationships between the three parameters (LDM, Δ, ash content) and productivity were determined over three consecutive years in 37 field-grown durum wheat genotypes under contrasting drought conditions. •  Highly significant differences were found between years and among genotypes for all measured traits. Grain Δ and ash content, and LDM and flag leaf Δ were negatively correlated under nondroughted conditions. Positive correlations were found between grain yield, harvest index and both Δ and ash content of the flag leaf under drought. No significant correlations were found between LDM and both Δ and grain yield. •  Differences in LDM do not predict variations in Δ, whereas ash content of grain and flag leaf (under droughted conditions) might be useful in predicting Δ and grain yield. Ash content might provide an alternative screening method in the improvement of drought tolerance and yield stability in durum wheat.

Deposition of gaseous radionuclides to fruit

14C, 35S and 3H are released to the environment during the operation of gas-cooled reactors and were identified as radionuclides of interest by the BIOMASS Fruits Working Group. This paper provides a review of the deposition, uptake, allocation and loss of these radionuclides with respect to fruit and conceptual models for gaseous radionuclides. It is concluded that the mechanisms for the uptake of CO35S, HTO and 14CO2 are well understood and that their deposition velocities have been quantified. There is also a reasonable body of work on the translocation of 14C once in the crop, but much less for 35S and 3H, which are considered to follow source-sink relationships. The loss rates of the three radionuclides show large differences, with tritium lost rapidly in the form of HTO but retained longer when converted to OBT. The losses of 14C are less and those of sulphur are minimal post fixation. When fruit crops alone are considered, the quantity of information is further reduced but predictions on possible behaviour of these radionuclide species can be made from the current knowledge.

Productivity and carbon isotope discrimination in durum wheat organs under a Mediterranean climate

For durum wheat, we promote the use of carbon isotope discrimination (delta) as an indirect selection criterion for transpiration efficiency and grain yield (GY), and we identify the most effective organ for characterising delta genotypic variation. A field experiment was conducted in the South of France on 144 accessions, with a drought period occurring from February to June. Harvest index (HI), GY and delta (delta L, flag leaf; delta A, awn, delta G grain) were measured. Significant positive genetic correlations were noted between delta and both GY and HI. A larger genotypic variation and a higher broad-sense heritability were noted for delta G compared to delta L and delta A. delta G correlated better with GY and HI than delta L and delta A, showing that delta G could provide a better assessment of genotypic behaviours under drought during grain filling. Moreover, the indirect selection based on delta G (even when evaluated with one replicate) appeared more efficient than the direct selection for grain yield. This result emphasised the potential value of grain carbon isotope discrimination as a criterion for grain yield improvement under stressed Mediterranean conditions.

Effect of Nutrients on Growth Rate and Carbohydrate Storage in Oxytropis sericea: A Test of the Carbon Accumulation Hypothesis

The pool of storage carbohydrates in plants may be formed in competition with allocation to growth ("true reserves") or, where growth is inhibited by external factors, with no cost to growth ("accumulated reserves"). The latter hypothesis was addressed using Oxytropis sericea, a perennial herb from the Rocky Mountains. To generate variation in growth rate, greenhouse-cultivated plants were assigned to low or high nutrient levels in 2 yr. Age of plants was also varied either by conducting consecutive harvests (1996) or using plants of different initial age (1997). In both years, plants from the high-nutrient group attained larger vegetative biomass than plants in the low-nutrient group and also displayed an overall lower carbon allocation to storage, consistent with the accumulation hypothesis.