Dynamic stomatal behavior and its role in carbon gain during lightflecks of a gap phase and an understory Piper species acclimated to high and low light

Citrus photosynthesis and morphology acclimate to phloem-affecting huanglongbing disease at the leaf and shoot levels

Huanglongbing (HLB) is a phloem-affecting disease in citrus that reduces growth and impacts global citrus production. HLB is caused by a phloem-limited bacterium (Candidatus Liberibacter asiaticus). By inhibiting phloem function, HLB stunts sink growth, including the production of new shoots and leaves, and induces hyperaccumulation of foliar starch. HLB induces feedback inhibition of photosynthesis by reducing foliar carbohydrate export. Here, we assessed the relationship of bacterial distribution within the foliage, foliar starch accumulation, and net CO₂ assimilation (A_net ). Because HLB impacts canopy morphology, we developed a chamber to measure whole-shoot A_net to test the effects of HLB at both the leaf and shoot level. Whole-shoot level A_net saturated at high irradiance, and green stems had high photosynthetic rates compared to leaves. Starch accumulation was correlated with bacterial population, and starch was negatively correlated with A_net at the leaf but not at the shoot level. Starch increased initially after infection, then decreased progressively with increasing length of infection. HLB infection reduced A_net at the leaf level but increased it at the whole-shoot level, in association with reduced leaf size and greater relative contribution of stems to the photosynthetic surface area. Although HLB-increased photosynthetic efficiency, total carbon fixed per shoot decreased because photosynthetic surface area was reduced. We conclude that the localized effects of infection on photosynthesis are mitigated by whole-shoot morphological acclimation over time. Stems contribute important proportions of whole-shoot A_net , and these contributions are likely increased by the morphological acclimation induced by HLB.

Specialized stomatal humidity responses underpin ecological diversity in C3 bromeliads

The Neotropical Bromeliaceae display an extraordinary level of ecological variety, with species differing widely in habit, photosynthetic pathway and growth form. Divergences in stomatal structure and function, hitherto understudied in treatments of bromeliad evolutionary physiology, could have been critical to the generation of variety in ecophysiological strategies among the bromeliads. Because humidity is a key factor in bromeliad niches, we focussed on stomatal responses to vapour pressure deficit (VPD). We measured the sensitivity of stomatal conductance and assimilation rate to VPD in eight C₃ bromeliad species of contrasting growth forms and ecophysiological strategies and parameterised the kinetics of stomatal responses to a step change in VPD. Notably, three tank-epiphyte species displayed low conductance, high sensitivity and fast kinetics relative to the lithophytes, while three xeromorphic terrestrial species showed high conductance and sensitivity but slow stomatal kinetics. An apparent feedforward response of transpiration to VPD occurred in the tank epiphytes, while water-use efficiency was differentially impacted by stomatal closure depending on photosynthetic responses. Differences in stomatal responses to VPD between species of different ecophysiological strategies are closely linked to modifications of stomatal morphology, which we argue has been a pivotal component of the evolution of high diversity in this important plant family.

The acclimation of Tilia cordata stomatal opening in response to light, and stomatal anatomy to vegetational shade and its components

Stomatal anatomical traits and rapid responses to several components of visible light were measured in Tilia cordata Mill. seedlings grown in an open, fully sunlit field (C-set), or under different kinds of shade. The main questions were: (i) stomatal responses to which visible light spectrum regions are modified by growth-environment shade and (ii) which separate component of vegetational shade is most effective in eliciting the acclimation effects of the full vegetational shade. We found that stomatal opening in response to red or green light did not differ between the plants grown in the different environments. Stomatal response to blue light was increased (in comparison with that of C-set) in the leaves grown in full vegetational shade (IABW-set), in attenuated UVAB irradiance (AB-set) or in decreased light intensity (neutral shade) plus attenuated UVAB irradiance (IAB-set). In all sets, the addition of green light-two or four times stronger-into induction light barely changed the rate of the blue-light-stimulated stomatal opening. In the AB-set, stomatal response to blue light equalled the strong IABW-set response. In attenuated UVB-grown leaves, stomatal response fell midway between IABW- and C-set results. Blue light response by neutral shade-grown leaves did not differ from that of the C-set, and the response by the IAB-set did not differ from that of the AB-set. Stomatal size was not modified by growth environments. Stomatal density and index were remarkably decreased only in the IABW- and IAB-sets. It was concluded that differences in white light responses between T. cordata leaves grown in different light environments are caused only by their different blue light response. Differences in stomatal sensitivity are not dependent on altered stomatal anatomy. Attenuated UVAB irradiance is the most efficient component of vegetational shade in stimulating acclimation of stomata, whereas decreased light intensity plays a minor role.

Coping with low light under high atmospheric dryness: shade acclimation in a Mediterranean conifer (Abies pinsapo Boiss.)

Plant species living in the understory increase carbon (C) allocation toward leaf production for maximizing light capture at the expense of roots and stems, with negative consequences for the whole-plant hydraulic conductance. Moreover, under some conditions, the high atmospheric evaporative demand occurring in Mediterranean areas may be not well buffered by the canopy, which might be the case for relict conifer Abies pinsapo Boiss. growing in the forest understory. We hypothesized that acclimation to combined understory shade and high atmospheric dryness can be achieved through the adjustment of water losses to cope with the restriction in water transport. The results reveal high structural plasticity in A. pinsapo that allows light harvesting of this species to maximize light capture in the forest understory, and maintain a positive C balance under low light conditions. However, growth in the understory resulted in reduced leaf-specific conductivity, up to approximately four to five times, implying decreased plant capacity to supply water to the leaves. In order to cope with the high atmospheric evaporative demand in the understory, there is an adjustment of the stomatal conductance to the hydraulic conductivity by means of a reduction in the stomatal density in understory individuals, which is due to the almost complete lack of stomata in the adaxial side of the needles. To the extent of our knowledge, such a drastic phenotypic response found in a conifer when growing under shaded conditions had not been previously reported.

Effect of elevated carbon dioxide concentration on carbon assimilation under fluctuating light

Natural fluctuations in light intensity may significantly affect the amount of CO assimilated by plants and ecosystems. Little is known, however, about the interactive effect of dynamic light conditions and atmospheric CO concentrations. The hypothesis that elevated CO concentration (EC; 700 μmol CO mol) increases photosynthetic efficiency in dynamic light environments as compared to ambient CO concentration (AC; 385 μmol CO mol) was tested. Sun leaves of European beech ( L.) and current-year shoots of Norway spruce [ (L). Karst.] were exposed to five dynamic light regimes (LRs) occurring within forest canopies due to variable cloud cover or self-shading of leaves and to a steady-state LR. The LRs differed in the time course of incident irradiance, whereas the overall duration (600 s) and total amount of radiation (35.88 mmol photons m) were the same in all LRs. The EC treatment enhanced the amount of CO assimilated under all LRs tested. While the stimulation was only 37 to 50% in beech, it was 52 to 85% in spruce. The hypothesis that photosynthetic efficiency is stimulated by EC was confirmed in LRs when the leaves were pre-exposed to low light intensity and photosynthetic induction was required. By contrast, only a minor effect of EC treatment was found on the rate of induction loss and postillumination CO fixation in both species studied.

Sunflecks in trees and forests: from photosynthetic physiology to global change biology

Sunflecks are brief, intermittent periods of high photon flux density (PFD) that can significantly improve carbon gain in shaded forest understories and lower canopies of trees. In this review, we discuss the physiological basis of leaf-level responses to sunflecks and the mechanisms plants use to tolerate sudden changes in PFD and leaf temperature induced by sunflecks. We also examine the potential effects of climate change stresses (including elevated temperatures, rising CO(2) concentrations and drought) on the ability of tree species to use sunflecks, and advocate more research to improve our predictions of seedling and tree carbon gain in future climates. Lastly, while we have the ability to model realistic responses of photosynthesis to fluctuating PFD, dynamic responses of photosynthesis to sunflecks are not accounted for in current models of canopy carbon uptake, which can lead to substantial overestimates of forest carbon fixation. Since sunflecks are a critical component of seasonal carbon gain for shaded leaves, sunfleck regimes and physiological responses to sunflecks should be incorporated into models to more accurately capture forest carbon dynamics.

Functional regeneration and spectral reflectance of trees during succession in a highly diverse tropical dry forest ecosystem

PREMISE OF THE STUDY

The function of most ecosystems has been altered by human activities. To asses the recovery of plant communities, we must evaluate the recovery of plant functional traits. The seasonally dry tropical forest (SDTF), a highly threatened ecosystem, is assumed to recover relatively quickly from disturbance, but an integrated evaluation of recovery in floristic, structural, and functional terms has not been performed. In this study we aimed to (a) compare SDTF plant functional, floristic, and structural change along succession; (b) identify tree functional groups; and (c) explore the spectral properties of different successional stages.

METHODS

Across a SDTF successional gradient, we evaluated the change of species composition, vegetation structure, and leaf spectral reflectance and functional traits (related to water use, light acquisition, nutrient conservation, and CO(2) acquisition) of 25 abundant tree species.

KEY RESULTS

A complete recovery of SDTF takes longer than the time period inferred from floristic or structural data. Plant functional traits changed along succession from those that maximize photoprotection and heat dissipation in early succession, where temperature is an environmental constraint, to those that enhance light acquisition in late succession, where light may be limiting. A spectral indicator of plant photosynthetic performance (photochemical reflectance index) discriminated between early and late succession. This constitutes a foundation for further exploration of remote sensing technologies for studying tropical succession.

CONCLUSIONS

A functional approach should be incorporated as a regular descriptor of forest succession because it provides a richer understanding of vegetation dynamics than is offered by either the floristic or structural approach alone.

Steady and dynamic photosynthetic responses of seedlings from contrasting successional groups under low-light growth conditions

To test the hypothesis that leaf-level photosynthetic-related traits might confer late successionals a competitive advantage over early successionals in low-light growth conditions, steady photosynthetic assimilation- and dynamic photosynthetic induction-related traits were examined in low-light-grown seedlings with contrasting successional status. Compared with the early successionals, late successionals as a group significantly exhibited lower leaf gas exchange rates. While late successionals required a longer time to respond to simulated sunflecks, they had lower rates of induction losses after sunflecks. Such photosynthetic induction traits allowed late successionals to more effectively utilize subsequent sunflecks. It was observed that plants with lower gas exchange rates responded more slowly to simulated sunfelcks, but they had lower rates of induction losses after sunflecks. In addition, the rate of response to sunflecks was positively correlated with the rate of induction loss after sunflecks across the successional status of species. A principal components analysis (PCA) demonstrated that early and late successionals were separated along the first axis of the PCA, and that early successionals were grouped on the right and were associated with higher gas exchange rates, fast responses to sunflecks, and rapid rates of induction loss after sunflecks; late successionals held an opposite pattern. Overall, our results suggest that smaller respiratory carbon losses and lower metabolic costs give late successionals a competitive advantage in low-light growth conditions, that late successionals have an advantage over early successionals in utilizing sunflecks, and thus that the successional status of species are mainly associated with the leaf-level photosynthetic-related traits.

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.

Dynamics of stomatal water relations following leaf excision

We examined the stomatal response to leaf excision in an evergreen woody shrub, Photinia x fraseri, using a novel combination of gas exchange, traditional water relations and modelling. Plants were kept outdoors in mild winter conditions (average daily temperature range: -1 to 12 degrees C) before being transferred to a glasshouse (temperature range: 20-30 degrees C) and allowed to acclimate for different periods before experiments. 'Glasshouse plants' were acclimated for at least 9 d, and 'outdoor plants' were acclimated for fewer than 3 d before laboratory gas exchange experiments. The transient stomatal opening response to leaf excision was roughly twice as long in outdoor plants as in glasshouse plants. To elucidate the reason for this difference, we inferred variables of stomatal water relations (epidermal and guard cell turgor pressures and guard cell osmotic pressure: Pe, Pg and pi g, respectively) from stomatal conductance (gs) and bulk leaf water potential (psi l), using a hydromechanical model of gs. psi l was calculated from cumulative post-excision transpirational water loss using empirical relationships between psi l and relative water content obtained on similar leaves. Inferred Pg and Pe both declined immediately after leaf excision. Inferred pi g also declined after a lag period. The kinetics of pi g adjustment after the lag were similar in outdoors and glasshouse plants, but the lag period was much longer in outdoor plants. This suggests that the longer transient opening response in outdoor plants resulted from slower induction, not slower execution, of guard cell osmoregulation. We discuss the implications of our results for the mechanism of short-term stomatal responses to hydraulic perturbations, for dynamic modelling of gs and for leaf water status regulation.

In situ observation of stomatal movements and gas exchange of Aegopodium podagraria L. in the understorey

Observations of stomata in situ while simultaneously measuring CO(2) gas exchange and transpiration were made in field experiments with Aegopodium podagraria in a highly variable light climate in the understorey of trees. The low background photosynthetic photon flux density (PPFD) caused a slight opening of the stomata and no visible response to sporadic lightflecks. However, if lightflecks were frequent and brighter, slow opening movements were observed. Small apertures were sufficient to allow maximal photosynthetic rates. Therefore, the small apertures observed in low light usually only caused minor stomatal limitations of lightfleck photosynthesis. The response of stomata to step-wise changes in PPFD under different levels of leaf to air vapour pressure difference (Delta(W)) was observed under controlled conditions. High Delta(W) influenced the stomatal response only slightly by reducing stomatal aperture in low light and causing a slight reduction in the initial capacity to utilize high PPFD levels. Under continuous high PPFD, however, stomata opened to the same degree irrespective of Delta(W). Under high Delta(W), opening and closing responses to PPFD-changes were faster, which enabled a rapid removal of the small stomatal limitations of photosynthesis initially present in high Delta(W) after longer periods in low light. It is concluded that A. podagraria maintains a superoptimal aperture in low light which leads to a low instantaneous water use efficiency, but allows an efficient utilization of randomly occurring lightflecks.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : II. Stomatal versus biochemical limitations during photosynthetic induction

The relative importance of biochemical and stomatal limitations on assimilation (A) during photosynthetic induction were compared in sun and shade plants of Piper auritum, a pioneer tree, and shade plants of Piper aequale, a shade tolerant shrub native to a Mexican tropical rainforest. For non-induced leaves, increases in A during induction depended on the dynamics of stomatal conductance (g_s) and ribulose-1,5-bisphosphate carboxylase (RuBisCO) activation. At high leaf-air vapor pressure deficit (VPD), more of the limitation during induction was stomatal. Calculations of mesophyll conductance revealed longer time constants for shade than for sun plants. However, no differences in the time course of RuBisCO activity between sun- and shade-plants were found. We conclude on the basis of the similar RuBisCO responses that differences in induction can be accounted for by the differences in stomatal behavior. Differences in the time course of mesophyll conductance may be due to an artifact caused by stomatal patchiness. Experiments on induction loss of previously induced leaves revealed that under these circumstances biochemical limitations can be important. A more rapid induction loss was evident in sun as compared to shade plants. The rapid loss of induction in sum plants was not due to the decreases in g_s and RuBisCO activity, which both occurred slowly. Instead, a limitation, probably in RuBP regeneration capacity, appeared to develop during the low light periods. This limitation was much smaller or absent in shade plants.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : I. VPD effects on the transient stomatal response to lightflecks

The effects of leaf-air vapor pressure deficit (VPD) on the transient and steady-state stomatal responses to photon flux density (PFD) were evaluated in Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub, that are both native to tropical forests at Los Tuxtlas, Veracruz, México. Under constant high-PFD conditions, the stomata of shade-acclimated plants of both species were sensitive to VPD, exhibiting a nearly uniform decrease in g_s as VPD increased. Acclimation of P. auritum to high light increased the stomatal sensitivity to VPD that was sufflcient to cause a reduction in transpiration at high VPD's. At low PFD, where g_s was already reduced, there was little additional absolute change with VPD for any species or growth condition. The stomatal response to 8-min duration lightflecks was strongly modulated by VPD and varied between the species and growth light conditions. In P. aequale shade plants, increased VPD had no effect on the extent of stomatal opening but caused the rate of closure after the lightfleck to be faster. Thus, the overall response to a lightfleck changed from hysteretic (faster opening than closure) to symmetric (similar opening and closing rates). Either high or low VPD caused g_s not to return to the steady-state value present before the lightfleck. At high VPD the value after was considerably less than the value before whereas at low VPD the opposite occurred. Shade-acclimated plants of P. auritum showed only a small g_s response to lightflecks, which was not affected by VPD. Under sunfleck regimes in the understory, the stomatal response of P. aequale at low VPD may function to enhance carbon gain by increasing the induction state. At high VPD, the shift in the response enhances water use efficiency but at the cost of reduced assimilation.

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 μmol photons m^-2 s^-1) and light compensation (70 to 100 μmol m^-2 s^-1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO₂ fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.