Whole-plant versus leaf-level regulation of photosynthetic responses after partial defoliation in Eucalyptus globulus saplings.
ABSTRACT: Increases in photosynthetic capacity (A1500) after defoliation have been attributed to changes in leaf-level biochemistry, water, and/or nutrient status. The hypothesis that transient photosynthetic responses to partial defoliation are regulated by whole-plant (e.g. source-sink relationships or changes in hydraulic conductance) rather than leaf-level mechanisms is tested here. Temporal variation in leaf-level gas exchange, chemistry, whole-plant soil-to-leaf hydraulic conductance (KP), and aboveground biomass partitioning were determined to evaluate mechanisms responsible for increases in A1500 of Eucalyptus globulus L. potted saplings. A1500 increased in response to debudding (B), partial defoliation (D), and combined B&D treatments by up to 36% at 5 weeks after treatment. Changes in leaf-level factors partly explained increases in A1500 of B and B&D treatments but not for D treatment. By week 5, saplings in B, B&D, and D treatments had similar leaf-specific KP to control trees by maintaining lower midday water potentials and higher transpiration rate per leaf area. Whole-plant source:sink ratios correlated strongly with A1500. Further, unlike KP, temporal changes in source:sink ratios tracked well with those observed for A1500. The results indicate that increases in A1500 after partial defoliation treatments were largely driven by an increased demand for assimilate by developing sinks rather than improvements in whole-plant water relations and changes in leaf-level factors. Three carbohydrates, galactional, stachyose, and, to a lesser extent, raffinose, correlated strongly with photosynthetic capacity, indicating that these sugars may function as signalling molecules in the regulation of longer term defoliation-induced gas exchange responses.
Project description:BACKGROUND AND AIMS:Sink-source imbalance could cause an accumulation of total non-structural carbohydrates (TNC; soluble sugar and starch) in source leaves. We aimed to clarify interspecific differences in how sink-source imbalance and TNC causes the downregulation of photosynthesis among three legume plants. The TNC in source leaves was altered by short-term manipulative treatments, and its effects on photosynthetic characteristics were evaluated. METHODS:Soybean, French bean and azuki bean were grown under high nitrogen availability. After primary leaves were fully expanded, they were subjected to additional treatments: defoliation except for two primary leaves; transfer to low nitrogen conditions; transfer to low nitrogen conditions and defoliation; or irradiation by light-emitting diodes. Physiological and anatomical traits such as TNC content, maximum photosynthetic rate, cell wall content and ?13C values of primary leaves and whole-plant growth were examined. KEY RESULTS:Among the three legume plants, the downregulation of maximum photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content was co-ordinated with an increase in TNC only in French bean. Rubisco did not decrease with an increase in TNC in soybean and azuki bean. The defoliation treatment caused an increase in cell wall content especially in soybean, and maximum photosynthesis decreased despite resulting in a higher Rubisco content. This indicates that a decrease in mesophyll conductance could cause photosynthetic downregulation, which was confirmed by an increase in ?13C. CONCLUSION:The present results suggest that a downregulation of photosynthesis in response to increased levels of TNC in source leaves can result not only from decreases in Rubisco content, but also from anatomical factors, such as an increase in cell wall thickness leading to reduced chloroplast CO2 concentrations.
Project description:Photosynthetic activity is affected by exogenous and endogenous inputs, including source-sink balance. Reducing the source to sink ratio by partial defoliation or heavy shading resulted in significant elevation of the photosynthetic rate in the remaining leaf of tomato plants within 3 d. The remaining leaf turned deep green, and its area increased by almost 3-fold within 7 d. Analyses of photosynthetic activity established up-regulation due to increased carbon fixation activity in the remaining leaf, rather than due to altered water balance. Moreover, senescence of the remaining leaf was significantly inhibited. As expected, carbohydrate concentration was lower in the remaining leaf than in the control leaves; however, expression of genes involved in sucrose export was significantly lower. These results suggest that the accumulated fixed carbohydrates were primarily devoted to increasing the size of the remaining leaf. Detailed analyses of the cytokinin content indicated that partial defoliation alters cytokinin biosynthesis in the roots, resulting in a higher concentration of trans-zeatin riboside, the major xylem-translocated molecule, and a higher concentration of total cytokinin in the remaining leaf. Together, our findings suggest that trans-zeatin riboside acts as a signal molecule that traffics from the root to the remaining leaf to alter gene expression and elevate photosynthetic activity.
Project description:The hydraulic traits of plants, or the efficiency of water transport throughout the plant hydraulic system, could help to anticipate the impact of climate change and improve crop productivity. However, the mechanisms explaining the role of hydraulic traits on plant photosynthesis and thus, plant growth and yield, are just beginning to emerge. We conducted an experiment to identify differences in growth patterns at leaf, root and whole plant level among four wild olive genotypes and to determine whether hydraulic traits may help to explain such differences through their effect on photosynthesis. We estimated the relative growth rate (RGR), and its components, leaf gas exchange and hydraulic traits both at the leaf and whole-plant level in the olive genotypes over a full year. Photosynthetic capacity parameters were also measured. We observed different responses to water stress in the RGRs of the genotypes studied being best explained by changes in the net CO2 assimilation rate (NAR). Further, net photosynthesis, closely related to NAR, was mainly determined by hydraulic traits, both at leaf and whole-plant levels. This was mediated through the effects of hydraulic traits on stomatal conductance. We observed a decrease in leaf area: sapwood area and leaf area: root area ratios in water-stressed plants, which was more evident in the olive genotype Olea europaea subsp. guanchica (GUA8), whose RGR was less affected by water deficit than the other olive genotypes. In addition, at the leaf level, GUA8 water-stressed plants presented a better photosynthetic capacity due to a higher mesophyll conductance to CO2 and a higher foliar N. We conclude that hydraulic allometry adjustments of whole plant and leaf physiological response were well coordinated, buffering the water stress experienced by GUA8 plants. In turn, this explained their higher relative growth rates compared to the rest of the genotypes under water-stress conditions.
Project description:Early leaf removal significantly alters the source-sink balance within grapevine shoots, leading to a reduction in fruit set. However, no research has previously examined the conditions controlling this process in terms of carbon allocation among major sink organs following defoliation. In this study, the impact of defoliation at bloom on the distribution dynamics of leaf assimilates among clusters and growing shoot apices was investigated on Vitis vinifera, cv. Pinot noir, grown in Michigan, a cool climate viticultural region. Three levels of defoliation: no leaves removed (LR-0); six leaves removed from six basal nodes (LR-6); and ten leaves removed from ten basal nodes (LR-10), were imposed at full bloom. A 13C pulsing was performed 1 week after the treatment application to the defoliated shoots. Single leaf gas exchange (Pn), diurnal changes of the leaf net CO2 assimilation rate, carbon distribution, fruit-set, yield, and fruit composition were measured. Higher Pn was recorded in diurnal measurements of gas exchange in leaf removal (LR) treatments compared to LR-0. The shoot apex of LR-10 experienced the highest 13C allocation (%) after 3 and 7 days following the carbon pulsing. LR-10 had lower percentage of 13C allocated to clusters, which decreased fruit set by 60%, compared to the control, and enhanced the concentration of phenolic compounds in fruit. Alteration of carbon portioning among shoot sink organs indicated that an increasing severity of leaf removal significantly reduced fruit set, and was linearly correlated to shoot apex sink strength, which occurred at the expense of the cluster.
Project description:Abstract The tonoplast sucrose transporter PtaSUT4 is well expressed in leaves of Populus tremula × Populus alba (INRA 717?IB4), and its inhibition by RNA?interference (RNAi) alters leaf sucrose homeostasis. Whether sucrose partitioning between the vacuole and the cytosol is modulated by PtaSUT4 for specific physiological outcomes in Populus remains unexplored. In this study, partial defoliation was used to elicit compensatory increases in photosynthesis and transpiration by the remaining leaves in greenhouse?grown poplar. Water uptake, leaf gas exchange properties, growth and nonstructural carbohydrate abundance in source and sink organs were then compared between wild?type and SUT4?RNAi lines. Partial defoliation increased maximum photosynthesis rates similarly in all lines. There was no indication that source leaf sugar levels changed differently between wild?type and RNAi plants following partial defoliation. Sink levels of hexose (glucose and fructose) and starch decreased similarly in all lines. Interestingly, plant water uptake after partial defoliation was not as well sustained in RNAi as in wild?type plants. While the compensatory increase in photosynthesis was similar between genotypes, leaf transpiration increased less robustly in RNAi than wild?type plants. SUT4?RNAi and wild?type source leaves differed constitutively in their bulk modulus of elasticity, a measure of leaf turgor, and storage water capacitance. The data demonstrate that reduced sucrose partitioning due to PtaSUT4?RNAi altered turgor control and compensatory transpiration capacity more strikingly than photosynthesis and sugar export. The results are consistent with the interpretation that SUT4 may control vacuolar turgor independently of sink carbon provisioning.
Project description:Variation in xylem structure and function has been extensively studied across different species with a wide taxonomic, geographical, and ecological coverage. In contrast, our understanding of how xylem of a single species can adjust to different growing condition remains limited. Here phenotypic and developmental plasticity in xylem traits of hybrid poplar (Populus trichocarpa×deltoides) was studied. Clonally propagated saplings were grown under experimental drought, nitrogen fertilization, and shade for >30 d. Xylem hydraulic and anatomical traits were subsequently examined in stem segments taken from two different vertical positions along the plant's main axis. The experimental treatments affected growth and development and induced changes in xylem phenotype. Across all treatments, the amount of leaf area supported by stem segments (A(L)) scaled linearly with stem native hydraulic conductivity (K (native)), suggesting that the area of assimilating leaves is constrained by the xylem transport capacity. In turn, K (native) was mainly driven by the size of xylem cross-sectional area (A(X)). Moreover, the structural and functional properties of xylem varied significantly. Vulnerability to cavitation, measured as the xylem pressure inducing 50% loss of conductivity (P50), ranged from -1.71 MPa to -0.15 MPa in saplings subjected to drought and nitrogen fertilization, respectively. Across all treatments and stem segment positions, P50 was tightly correlated with wood density. In contrast, no relationship between P50 and xylem-specific conductivity (K (S)) was observed. The results of this study enhance our knowledge of plant hydraulic acclimation and provide insights into common trade-offs that exist in xylem structure and function.
Project description:Drought-induced tree mortality has been observed worldwide. Nevertheless, the physiological mechanisms underlying this phenomenon are still being debated. Potted Robinia pseudoacacia and Platycladus orientalis saplings were subjected to drought and their hydraulic failure and carbon starvation responses were studied. They underwent simulated fast drought (FD) and slow drought (SD) until death. The dynamics of their growth, photosynthesis, water relations and carbohydrate concentration were measured. The results showed that during drought, growth and photosynthesis of all saplings were significantly reduced in both species. The predawn water potential in both species was ~ -8 MPa at mortality. The percentage loss of conductivity (PLC) was at a maximum at mortality under both FD and SD. For R. pseudoacacia and P. orientalis, they were >95 and ~45 %, respectively. At complete defoliation, the PLC of R. pseudoacacia was ~90 % but the trees continued to survive for around 46 days. The non-structural carbohydrate (NSC) concentrations in the stems and roots of both FD and SD R. pseudoacacia declined to a very low level near death. In contrast, the NSC concentrations in the needles, stems and roots of P. orientalis at mortality under FD did not significantly differ from those of the control, whereas the NSC concentrations in SD P. orientalis stems and roots at death were significantly lower than those of the control. These results suggest that the duration of the drought affected NSC at mortality in P. orientalis. In addition, the differences in NSC between FD and SD P. orientalis did not alter mortality thresholds associated with hydraulic failure. The drought-induced death of R. pseudoacacia occurred at 95 % PLC for both FD and SD, indicating that hydraulic failure played an important role in mortality. Nevertheless, the consistent decline in NSC in R. pseudoacacia saplings following drought-induced defoliation may have also contributed to its mortality.
Project description:As trees grow taller, hydraulic resistance can be expected to increase, causing photosynthetic productivity to decline. Yet leaves maintain productivity over vast height increases; this maintenance of productivity suggests that leaf-specific conductance remains constant as trees grow taller. Here we test the assumption of constant leaf-specific conductance with height growth and document the stem xylem anatomical adjustments involved. We measured the scaling of total leaf area, mean vessel diameter at terminal twigs and at the stem base, and total vessel number in 139 individuals of Moringa oleifera of different heights, and estimated a whole-plant conductance index from these measurements. Whole-plant conductance and total leaf area scaled at the same rate with height. Congruently, whole-plant conductance and total leaf area scaled isometrically. Constant conductance is made possible by intricate adjustments in anatomy, with conduit diameters in terminal twigs becoming wider, lowering per-vessel resistance, with a concomitant decrease in vessel number per unit leaf area with height growth. Selection maintaining constant conductance per unit leaf area with height growth (or at least minimizing drops in conductance) is likely a potent selective pressure shaping plant hydraulics, and crucially involved in the maintenance of photosynthetic productivity per leaf area across the terrestrial landscape.
Project description:Drought-induced tree death has become a serious problem in global forest ecosystems. Two nonexclusive hypotheses, hydraulic failure and carbon starvation, have been proposed to explain tree die-offs. To clarify the mechanisms, we investigated the physiological processes of drought-induced tree death in saplings with contrasting Huber values (sapwood area/total leaf area). First, hydraulic failure and reduced respiration were found in the initial process of tree decline, and in the last stage carbon starvation led to tree death. The carbohydrate reserves at the stem bases, low in healthy trees, accumulated at the beginning of the declining process due to phloem transport failure, and then decreased just before dying. The concentrations of non-structural carbohydrates at the stem bases are a good indicator of tree damage. The physiological processes and carbon sink-source dynamics that occur during lethal drought provide important insights into the adaptive measures underlying forest die-offs under global warming conditions.
Project description:The effects of herbivory can shape plant communities and evolution. However, the many forms of herbivory costs and the wide variation in herbivory pressure, including across latitudinal gradients, can make predicting the effects of herbivory on different plant species difficult. Functional trait approaches may aid in contextualizing and standardizing the assessment of herbivory impacts. Here we assessed the response of 26 old-field plant species to simulated defoliation in a greenhouse setting by measuring whole plant and leaf level traits in control and treated individuals. Simulated defoliation had no significant effects on any plant traits measured. However, the baseline leaf level traits of healthy plants consistently predicted the log response ratio for these species whole plant response to defoliation. The latitudinal mid-point of species' distributions was also significantly correlated with aboveground biomass and total leaf area responses, with plants with a more northern distribution being more negatively impacted by treatment. These results indicate that even in the absence of significant overall impacts, functional traits may aid in predicting variability in plant responses to defoliation and in identifying the underlying limitations driving those responses.