Non-structural Carbohydrate Reserves Research Articles

Adjustment of storage capacity for non-structural carbohydrates in response to limited water availability in two temperate woody species.

Reserves of non-structural carbohydrates (NSC) stored in living cells are essential for drought tolerance of trees. However, little is known about the phenotypic plasticity of living storage compartments (SC) and their interactions with NSC reserves under changing water availability. Here, we examined adjustments of SC and NSC reserves in stems and roots of seedlings of two temperate tree species, Acer negundo L. and Betula pendula Roth., cultivated under different substrate water availability. We found that relative contents of soluble NSC, starch and total NSC increased with decreasing water availability in stems of both species, and similar tendencies were also observed in roots of A. negundo. In the roots of B. pendula, soluble NSC contents decreased along with the decreasing water availability, possibly due to phloem decoupling or NSC translocation to shoots. Despite the contrast in organ responses, NSC contents (namely starch) positively correlated with proportions of total organ SC. Individual types of SC showed markedly distinct plasticity upon decreasing water availability, suggesting that water availability changes the partitioning of organ storage capacity. We found an increasing contribution of parenchyma-rich bark to the total organ NSC storage capacity under decreasing water availability. However, xylem SC showed substantially greater plasticity than those in bark. Axial storage cells, namely living fibers in A. negundo, responded more sensitively to decreasing water availability than radial parenchyma. Our results demonstrate that drought-induced changes in carbon balance affect the organ storage capacity provided by living cells, whose proportions are sensitively coordinated along with changing NSC reserves.

Seasonal coordination of aboveground vegetative and reproductive growth and storage in apple trees subjected to defoliation, flower and fruit thinning

Key messageThe growth rates of current-year shoots, fruits and trunks in apple trees peak sequentially during the growing season. The period of most intense growth coincided with the lowest NSC reserves.Vegetative and reproductive growth and storage are major carbon sinks in fruit trees; however, little is known about their mutual seasonal coordination. In this study, we monitored growth dynamics of trunks, fruits and current-year shoots together with the concentration of non-structural carbohydrates (NSC) in trees subjected to defoliation, early season flower thinning, mid-season fruit thinning and their respective combinations across the season. We found that defoliation had a negative effect on both trunk radial growth and annual fruit yield. Flower and fruit thinning caused lower fruit number per tree, but the individual fruits were larger resulting in a similar annual fruit yield among the treatments. Shoot extension growth was not significantly affected by the defoliation and flower and fruit thinning treatments. The concentration of non-structural carbohydrates was also similar across treatments. Modelled daily growth rates of shoots, fruits and trunks peaked sequentially one after another throughout the growing season with a delay of 15 and 18 days, respectively. The period of most intense growth of tree’s organs corresponded well with the lowest NSC reserves and a temporary depletion of starch in 1-year-old branches. Taken together, our study illustrates a tight temporal coordination of major carbon sinks and improves our understanding of sink/source relations of commercially important apple trees.

Perturbation of tonoplast sucrose transport alters carbohydrate utilization for seasonal growth and defense metabolism in coppiced poplar.

Nonstructural carbohydrate reserves of stems and roots underpin overall tree fitness and productivity under short-rotation management practices such as coppicing for bioenergy. While sucrose and starch comprise the predominant stem carbohydrate reserves of Populus, utilization for fitness and agricultural productivity is understood primarily in terms of starch turnover. The tonoplast sucrose transport protein SUT4 modulates sucrose export from source leaves to distant sinks during photoautotrophic growth, but the possibility of its involvement in remobilizing carbohydrates from storage organs during heterotrophic growth has not been explored. Here, we used PtaSUT4-knockout mutants of Populus tremula × P. alba (INRA 717-1B4) in winter (cool) and summer (warm) glasshouse coppicing experiments to assess SUT4 involvement in reserve utilization. Conditions preceding and supporting summer sprouting were considered favorable for growth, while those preceding and supporting cool temperature sprouting were suboptimal akin to conditions associated with coppicing as generally practiced. Epicormic bud emergence was delayed in sut4 mutants following lower temperature 'winter' but not summer coppicing. Winter xylem hexose increases were observed in control but not in sut4 stumps after coppicing. The magnitude of starch and sucrose reserve depletion was similar in control and sut4 stumps during the winter and did not explain the sprouting and xylem hexose differences. However, winter maintenance costs appeared higher in sut4 based partly on Krebs cycle intermediate levels. In control plants, bark accrual of abundant defense metabolites, including salicinoids and condensed tannins, was higher in summer than in winter, but this increase of summer defense allocations was attenuated in sut4 mutants. Temperature-sensitive trade-offs between growth and other priorities may therefore depend on SUT4 in Populus.

Coupled hydraulics and carbon economy underlie age-related growth decline and revitalisation of sand-fixing shrubs after crown removal.

Crown removal revitalises sand-fixing shrubs that show declining vigour with age in drought-prone environments; however, the underlying mechanisms are poorly understood. Here, we addressed this knowledge gap by comparing the growth performance, xylem hydraulics and plant carbon economy across different plant ages (10, 21 and 33 years) and treatments (control and crown removal) using a representative sand-fixing shrub (Caragana microphylla Lam.) in northern China. We found that growth decline with plant age was accompanied by simultaneous decreases in soil moisture, plant hydraulic efficiency and photosynthetic capacity, suggesting that these interconnected changes in plant water relations and carbon economy were responsible for this decline. Following crown removal, quick resprouting, involving remobilisation of root nonstructural carbohydrate reserves, contributed to the reconstruction of an efficient hydraulic system and improved plant carbon status, but this became less effective in older shrubs. These age-dependent effects of carbon economy and hydraulics on plant growth vigour provide a mechanistic explanation for the age-related decline and revitalisation of sand-fixing shrubs. This understanding is crucial for the development of suitable management strategies for shrub plantations constructed with species having the resprouting ability and contributes to the sustainability of ecological restoration projects in water-limited sandy lands.

Season of drought affects growth, but not nonstructural carbohydrates dynamics, in Pinus taeda saplings.

In temperate evergreen conifers, growth occurs mostly in summer but photosynthesis precedes year-round; thus, nonstructural carbohydrates (NSC) increase in winter but decrease in summer. Given that mild drought reduces growth but not photosynthesis, a drought in summer should increase NSCs more than one in winter. However, the active regulation hypothesis suggests that to increase future drought resilience, plants might downregulate growth to increase NSCs after a winter drought even if NSCs do not increase during the drought. To test if so, potted Pinus taeda saplings (age $< 1$ yr) were subjected to six-month droughts in a greenhouse with one treatment receiving drought during winter (Sep-Mar), and another during summer (Mar-Sep). Both treatments were compared to a control. To measure dry biomass and NSCs, we harvested plants monthly following each drought, while to assess changes in growth rates, we measured height and diameter monthly. While we observed seasonal variation and an overall increase during the study, we found no drought-related changes in NSC dynamics; however, drought did reduce growth. Furthermore, drought in winter did reduce growth during the following summer, but the reduction was less than for a drought in summer. We conclude that the effect of drought on NSCs was too small to detect in our plants. While better control of soil water would have reduced a major source of uncertainty, plants with larger NSC reserves or more intense stress would also yield easier-to-detect effects. Although not definitive, our results suggest that water stress does not lead to dramatic changes in seasonal NSC dynamics in its aftermath, despite what one might expect under the active regulation hypothesis.

High resilience of campos rupestres plants to the interaction of drought and fire.

The concurrent impacts of multiple disturbances have the potential to modify ecosystem functioning by diminishing recovery capacity and resilience. Nevertheless, it remains uncertain how plant species from tropical communities respond to the cumulative effects of drought and fire. In this study, we evaluated the responses of six plant species from campos rupestres subjected to a mild drought followed by fire and tested if plants subjected to simulated drought show reductions in carbon uptake and depletion of non-structural carbohydrate (NSC) reserves, thus constraining their resprouting. We monitored monthly variations in leaf gas exchange and aboveground biomass over 18 months. Subsequently, an accidental fire occurred in the study area, leading us to collect samples of belowground structures for NSC analyses on the day of the burn. There were no differences in the frequency of resprouting between the above two conditions. Additionally, gas exchange in most species either remained stable or increased after the fire. Drought had no adverse effects on NSC reserves in the belowground structures and may have contributed to species resprouting after fire. The impact of drought pre-conditions on post-fire aboveground biomass was generally minor for most species, except Vellozia nivea, which displayed roughly a 5% reduction in biomass following the drought. Our findings highlight the remarkable resilience of campos rupestres species, even after enduring 18 months of reduced water availability and an unintended fire event. These species demonstrated the capacity to maintain their physiological functions and resprouting capacities after a fire event, underscoring their strong recovery potential.

Thinning effectively mitigates the decline of aging Mongolian pine plantations by alleviating drought stress and enhancing plant carbon balance

Intensifying drought around the globe poses strong challenges to the growth and survival of trees, emphasizing the urgency of understanding the physiological mechanisms of drought-induced tree decline and mortality and the necessity of using silvicultural approaches to mitigate drought effects on tree growth. We experimentally explored whether thinning enhances tree growth vigor and drought resilience in early aging Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations in a water-limited area of Northeast China using dendrochronological methods. Meanwhile, we compared plant hydraulic traits, water relations, and non-structural carbohydrate concentrations between trees growing in the thinned and un-thinned stands with the aim of unraveling the physiological mechanisms underlying the density effects on tree growth performance. The results showed that basal area increments of Mongolian pine trees in the high-density plantations started to decline continuously at tree cambium age of around 15 years over about a decade, while thinning effectively reversed their growth decline. Trees in thinned stands exhibited significantly higher predawn leaf water potentials, higher stem hydraulic conductivities, and larger carbon reserves than trees in un-thinned stands. Although trees growing in high-density stands maintained as low risks of xylem hydraulic dysfunction as trees growing in low-density stands, that was at the cost of substantially reducing tree crown size through branch mortality that unavoidably undermined their carbon balance at the whole-plant level. Consistently, trees growing in the thinned low-density stands showed significantly higher levels of non-structural carbohydrate reserve in stems and enhanced growth resilience to extreme drought than trees growing in the high-density reference stands. Our results suggest that thinning can effectively enhance xylem hydraulic transport efficiency and reduce the risk of carbon imbalance by increasing soil water availability and hence mitigates the early aging of Mongolian pine plantations in water-limited environments.

Moisture stress limits radial mixing of non-structural carbohydrates in sapwood of trembling aspen.

Dynamics in non-structural carbohydrate (NSC) pools may underlie observed drought legacies in tree growth. We assessed how aridity influences the dynamics of different-aged NSC pools in tree sapwood at two sites with differing climate conditions ('wet' vs. 'dry') that also experienced widespread regional drought five years earlier. We used an incubation method to measure radiocarbon (Δ14C) in CO2 respired from Populus tremuloides (aspen) tree rings to evaluate NSC storage and mixing patterns, coupled with measurements of NSC (soluble sugars, starch) concentrations and respired δ13C-CO2. At a wet site, CO2 respired from rings formed during 1962-1967 was only ~ 11years old, suggesting deep sapwood mixing of NSCs as starch. At a dry site, total NSC was about one-third of wet site totals, maximum ages in deep rings were lower, and ages more rapidly increased in shallow rings then plateaued. These results suggest historically shallower mixing and/or relatively higher consumption of NSCs under dry conditions. Both sites, however, had similar aged NSC (<1yr) in the most recent six rings, indicative of deep radial mixing following relatively wet conditions during the sampling year. We suggest significant differences in NSC mixing among sites are driven by moisture stress, where aridity reduces NSC reserves and restricts the depth of radial mixing. However, dynamic climate conditions in the southwestern US resulted in more complex radial patterns of sapwood NSC age than previously described. We suggest a novel conceptual framework to understand how moisture variability might influence the dynamics of NSC mixing in the sapwood.

Effects of NSC in different organs and at different growth stages on the yield of oil peony Fengdan with different ages.

Non-structural carbohydrates (NSC) as resource reserves of plants play important roles in energy supply for normal growth and reproduction under environmental stress. The yield of perennial crops is mainly determined by the carbohydrate production and allocation in the current growth season, as well as the re-allocation of NSC reserves. However, the contribution of NSC to crop yield has not been fully determined. Fengdan (Paeonia ostii) is a variety of oil Peony that is newly developed in China. The effects of tree age and NSC on yield were examined by investigated the variations of biomass, soluble sugars, starch, and NSC in the organ and whole tree levels in the dormant and ripening stages of Fengdan populations with 4-, 6-, and 8-year-old in 2020 and 5-, 7-, and 9- year old in 2021. Results showed that the biomass, yield (seed biomass), soluble sugars, starch, and NSC reserve of Fengdan at the whole tree level increased with the increase in age. Although consistent correlations were observed between soluble sugars, starch and NSC storage, and yield among the plants with different ages, Fengdan showed allometric growth relationships between the accumulation of soluble sugars, starch, and NSC and yield and biomass (standardized major axis analyses slope b ≠ 1). Tree age significantly affected biomass and its allocation and NSC levels, especially the yield of Fengdan plants. The results of the investigation of the variations in the relationships between the yield and seasonal fluctuations of NSC and biomass indicate that roots is the key storage structure, whereas stems both serve as sink and/or source functions for the adult (7-9a) plants. NSC level, particularly the concentration of soluble sugars, in stems mainly influences Fengdan yield. These findings together provide new insights into the mechanisms underlying the yield formation of Fengdan and have implications for manipulating sink-source relationship to achieve high yield.

The Role of Assimilate Dynamic in Rice Grain Filling Under Two Nitrogen Conditions

The assimilate obtained during the grain filling phase is sourced from photosynthesis and Non Structural Carbohydrate (NSC) reserves produced during the vegetative phase in the vegetative organs. Assimilate from different sources plays different roles in the grain filling process. This study aims to investigate varietal difference on assimilate distribution, nitrogen response and yield performance. The experiment was carried out in the rice fields of Petapahan Village, Kampar Regency, Riau. The experimental design used was a split-plot in randomized complete block design (RCBD) with 2 factors and 3 groups. The first factor, as the main plot, was the condition of N. Specifically without N (NoN) (0 kg ha−1 urea) and treatment with normal N (NN) (250 kg ha−1 urea). The second factor, as a sub-plot, was rice variety, spesifically Hipa 18, IPB 3S, Ciherang, and Way Apo Buru. Hipa 18 was a variety having the largest reserves of NSC. A positive correlation was found between plant characters observed in this research, they are stem and leaf dry weight, total carbohydrates in stems (proximate analysis), photosynthetic rate, and leaf nitrogen content (Kjeldahl method). As well as other agonomic characters such as leaf area index and yield components. The highest dry weight of stems and leaves of Hipa 18 was at the heading phase, 22.6 g. While the highest carbohydrate content of the stems of the variety was 7.93 g in the same phase. This large NSC reserve supported the largest sinks as well at Hipa 18. However, the source capacity of the varieties used was found to be lower than the sink capacity. It was also known that nitrogen affected the condition of all plant characters observed in this study. Therefore optimal nitrogen fertilization is needed to get a maximum yield. HIGHLIGHTS Production of dry matter through photosynthesis and remobilization of assimilate from vegetative organs are the main sources of grain filling There was a greater amount of assimilate remobilization in Hipa 18 compared to IPB 3S, Ciherang, and Way Apo Buru The yield was positively correlated with plant height at harvest, stem and leaf dry weight, leaf N content, leaf area index, total stem carbohydrates, and photosynthesis GRAPHICAL ABSTRACT

The effects of previous summer drought and fertilization on winter non-structural carbon reserves and spring leaf development of downy oak saplings.

It is still unknown whether the previous summer season drought and fertilization will affect the winter non-structural carbohydrate (NSC) reserves, spring leaf development, and mortality of trees in the next year. We, therefore, conducted an experiment with Quercus pubescens (downy oaks) saplings grown under four drought levels from field capacity (well-watered; ~25% volumetric water content) to wilting point (extreme drought; ~6%), in combination with two fertilizer treatments (0 vs. 50kg/ha/year blended) for one growing season to answer this question. We measured the pre- and post-winter NSC, and calculated the over-winter NSC consumption in storage tissues (i.e. shoots and roots) following drought and fertilization treatment, and recorded the spring leaf phenology, leaf biomass, and mortality next year. The results showed that, irrespective of drought intensity, carbon reserves were abundant in storage tissues, especially in roots. Extreme drought did not significantly alter NSC levels in tissues, but delayed the spring leaf expansion and reduced the leaf biomass. Previous season fertilization promoted shoot NSC use in extreme drought-stressed saplings over winter (showing reduced carbon reserves in shoots after winter), but it also showed positive effects on survival next year. We conclude that: (1) drought-stressed downy oak saplings seem to be able to maintain sufficient mobile carbohydrates for survival, (2) fertilization can alleviate the negative effects of extreme drought on survival and recovery growth of tree saplings.

Morpho-Physiological Responses of Two Multipurpose Species from the Tropical Dry Forest to Contrasting Light Levels: Implications for Their Nursery and Field Management.

Understanding the responses that some plants exhibit to acclimatize and thrive in different light environments can serve as a guideline to optimize their production or establishment. Morpho-physiological changes in Crescentia alata and Enterolobium cyclocarpum were examined in response to varying light levels: 25%, 35%, 55% and 70% of photosynthetic photon flux density (PPFD) of total solar radiation. One-month-old seedlings were subjected to the light treatments; subsequently, the effects on morphology, photosynthetic capacity, nutrient status, non-structural carbohydrate reserves (NSC) and growth were evaluated in three-month-old seedlings. Light levels affected several morpho-physiological parameters. C. alata responded better to higher light levels and E. cyclocarpum to lower levels. Particularly, C. alata with 70% PPFD increased its size in height and diameter, and accumulated more biomass in leaves, stems, and roots; it also exhibited higher net assimilation rates, improved nitrogen and phosphorus status and growth. In contrast, E. cyclocarpum with 25% PPFD increased aboveground biomass, nitrogen levels and NSC in leaves. Both species show morpho-physiological changes that determine their ability to acclimatize to different light conditions. This serves as a basis for designing better management strategies in the nursery or field by defining the light environments conducive to a proper functioning.

Seedling size and nutrient availability in the fall determine nitrogen resorption and storage compound allocation in Quercus variabilis

Soil fertility and resorption of leaf compounds in the fall can influence resource buildup in plants. However, whether intraspecific differences in seedling size can affect nutrient reserve buildup is unknown. This study examined the effects of seedling size and fall fertilization on the uptake and resorption of nitrogen (N), as well as the allocation of non-structural carbohydrates (NSC) and N in cultivated Quercus variabilis Blume. After the formation of terminal buds (T1), seedlings were stratified into small (shoot height < 30 cm) and large seedlings. During the hardening period, seedlings were treated with three different rates of 15 N-enriched fertilizer (0, 12, or 24 mg N seedling−1) and monitored until leaf fall (T2). Small seedlings had lower N resorption efficiency and resorbed proportionally less N than large seedlings. Fall fertilization notably improved N and NSC reserves, without reducing N resorption efficiency. Large seedlings allocated proportionally less N to leaves than small seedlings although both sizes seedlings absorbed similar amounts of N from fall fertilization. The priority perennial organ for NSC allocation was roots, while N allocation was dependent on the phenological growth stage of the seedling. Roots were prioritized during the rapid growth phase, while stems were prioritized during the hardening period. Under same fertilizer regime during the growth phase, large seedlings tends to have lower N concentration and have higher resorption efficiency compare to small seedlings, fall fertilization can increase N storage in large seedlings and NSC levels in both seedling sizes, without affecting growth.

Preferential allocation of carbohydrate reserves belowground supports disturbance-based management of American chestnut (Castanea dentata)

Non-structural carbohydrates (NSCs) are critical for the survival of trees, but the ability to accurately predict NSC levels for a given forest tree species is lacking. We evaluated NSC dynamics in American chestnut (Castanea dentata), a species of high restoration interest, to test the conventional model of seasonal and inter-organ dynamics and to facilitate informed conservation efforts. Chestnut trees were sampled over the course of one year at different phenological stages. Then, organ-specific NSC concentration data were paired with biomass estimates from a custom-built allometric model to generate NSC pool sizes. Organ-level NSC concentrations and pools generally peaked at leaf fall (October) and were lowest during shoot expansion (June), although interactions between organ type and collection period drove variation in pool sizes. Whole-tree NSC pools increased from 2.47 ± 0.42 kg at shoot expansion to 4.29 ± 0.65 kg at leaf fall. Coarse roots were the most important NSC reservoir, accounting for 46.5% − 56.6% of the total whole-tree NSC reserves, depending on the time of year. Coarse root NSC pools were larger and more dynamic than in previous studies with other temperate deciduous trees, and they were the primary supplier of NSCs to support spring leaf-out. Preferential belowground NSC allocation and the ability to mobilize root NSCs to fuel growth and metabolism suggest that chestnut could thrive under disturbance-based management.

Divergent allocations of nonstructural carbohydrates shape growth response to rainfall reduction in two subtropical plantations

Nonstructural carbohydrates (NSC) are indicators of tree carbon balance and play an important role in regulating plant growth and survival. However, our understanding of the mechanism underlying drought-induced response of NSC reserves remains limited. Here, we conducted a long-term throughfall exclusion (TFE) experiment to investigate the seasonal responses of NSC reserves to manipulative drought in two contrasting tree species (a broadleaved tree Castanopsis hystrix Miq. and a coniferous tree Pinus massoniana Lamb.) of the subtropical China. We found that in the dry season, the two tree species differed in their responses of NSC reserves to TFE at either the whole-tree level or by organs, with significantly depleted total NSC reserves in roots in both species. Under the TFE treatment, there were significant increases in the NSC pools of leaves and branches in C. hystrix, which were accompanied by significant decreases in fine root biomass and radial growth without significant changes in canopy photosynthesis; while P. massoniana exhibited significant increase in fine root biomass without significant changes in radial growth. Our results suggested that under prolonged water limitation, NSC usage for growth in C . hystrix is somewhat impaired, such that the TFE treatment resulted in NSC accumulation in aboveground organs (leaf and branch); whereas P . massoniana is capable of efficiently utilizing NSC reserves to maintain its growth under drought conditions. Our findings revealed divergent NSC allocations under experimental drought between the two contrasting tree species, which are important for better understanding the differential impacts of climate change on varying forest trees and plantation types in subtropical China.

Response of Merlot Grapevine to Drought Is Associated to Adjustments of Growth and Nonstructural Carbohydrates Allocation in above and Underground Organs

Studying changes in partitioning of dry matter and nonstructural carbohydrates (NSC) content in both aboveground and underground perennial tissues in drought-affected grapevines could provide insights into plant response and carbon allocation strategies during stress periods. The analysis of soluble NSC and starch content in leaf petioles, due to their role in hydraulic segmentation, should also be considered. In the present research, these aspects have been investigated in Merlot grapevines grown in pots and subjected to progressive and increasing soil dehydration, and in well-irrigated vines. Drought conditions caused drastic reduction of shoot elongation and total plant leaf area development in favor of a greater biomass allocation and partitioning towards roots, where most of the NSC reserves were also conserved. Dry matter content of the perennial organs increased in stressed vines due to growth reduction, allocation of carbon reserves and possible anatomical modifications. Vines subjected to drought showed a higher NSC content in petioles, supporting the hypothesis that they are involved as compatible solutes in osmotic adjustments.

Trunk radial growth, water and carbon relations of mature apple trees on two size-controlling rootstocks during severe summer drought.

The use of size-controlling rootstocks is central to modern high-density fruit production systems. While biological mechanisms responsible for vigor control are not fully understood, differences in water relations and carbohydrate storage ability have been suggested as two potential factors. To better understand the processes that control growth vigor, we analyzed the trunk radial variation at seasonal and diurnal timescales and measured the midday leaf water potential (ΨMD), leaf gas exchange and concentrations of non-structural carbohydrates (NSC) in apple trees of variety 'Jonagold' grafted on two rootstocks of contrasting growth vigor (dwarfing J-TE-G vs invigorating J-TE-H). The measurements were conducted during an exceptionally hot and dry summer. We found that smaller annual trunk radial increments in dwarfed trees were primarily due to an earlier cessation of trunk secondary growth. The interdiurnal trunk circumference changes (ΔC) were slightly lower in dwarfed trees, and these trees also had fewer days with positive ΔC values, particularly during the driest summer months. The trunks of dwarfed trees shrank gradually during the drought, showed less pronounced diurnal variation of trunk circumference and the maximum trunk daily shrinkage was only weakly responsive to the vapor pressure deficit. These results indicated that lower turgidity in the cambial region may have limited the trunk radial expansion in dwarfed trees during the hot and dry days. Dwarfed trees also maintained lower ΨMD and leaf gas exchange rates during the summer drought. These parameters decreased in parallel for both rootstock combinations, suggesting their similar drought sensitivity. Similar concentrations and seasonal dynamics of NSC in both rootstock combinations, together with their similar spring growth rates, suggest that NSC reserves were not directly limiting for growth. Our results support the prominent role of water relations in rootstock-induced size-controlling mechanisms and highlight the complexity of this topic.

Sodium Retrieval from Sap May Permit Maintenance of Carbohydrate Reserves in Mature Xylem Tissues of a Salt-tolerant Hybrid Pistachio Rootstock Exposed to 100 mm NaCl

Salinity’s many stresses may not kill a relatively salt-tolerant perennial in one season, but they can still deplete or modify nonstructural carbohydrate (NSC) pools. Any change to the quantity or quality of NSC reserves may have detrimental effects on phenology and reproduction, as well as yield, in tree crops. This study integrates salinity’s infringement on the energy margins of pistachio rootstock ‘UCB-1’ (an interspecific hybrid of Pistacia atlantica and P. integerrima) at senescence by measuring sugar and starch pools in wood, bark, and roots after treatment with ≈100 days of moderate to high salinity (50–100 mm NaCl and 10–20 mm CaCl2). Supported by a second experiment using sodium orthovanadate (NaOV) to block active xylem retrieval in the same hybrid pistachio rootstock, we conclude that retrieval of Na+ from xylem sap may allow for the preservation of NSC pools (particularly, starch) in mature xylem tissues by limiting the demand for carbon-based osmoticum (sugars). In contrast, younger growing tissues (bark and fine roots) were found to counteract salinity by degrading carbon-dense starch into osmotically active sugars at the expense of total NSC reserves, suggesting a physiological shift toward protection/isolation from environmentally pervasive but potentially toxic salts in these tissues.

Shade-induced reduction of stem nonstructural carbohydrates increases xylem vulnerability to embolism and impedes hydraulic recovery in Populus nigra.

Summary Nonstructural carbohydrates (NSCs) have been suggested to affect xylem transport under fluctuating water availability, but conclusive evidence is still lacking. We tested the effect of shade‐induced NSC depletion on xylem vulnerability to embolism and hydraulic recovery on Populus nigra saplings.Vulnerability was assessed in light‐exposed (L) and shaded (S) plants with the hydraulic method, and in vivo with the optical method and X‐ray micro‐computed tomography. Plants were stressed to 80% loss of hydraulic conductance (PLC) and re‐irrigated to check for possible recovery. We measured PLC, bark and wood NSC content, as well as xylem sap pH, surface tension (γ sap) and sugar concentration, before, during and after drought.Shading induced depletion of stem NSC (mainly starch) reserves. All methods converged in indicating higher xylem vulnerability in S than in L plants. This difference was not explained by xylem vessel and pit anatomy or by γ sap. Shading impeded sap acidification and sugar accumulation during drought in S plants and prevented hydraulic recovery, which was observed in L plants.Our results highlight the importance of stem NSCs to sustain xylem hydraulic functioning during drought and suggest that light and/or adequate stem NSC thresholds are required to trigger xylem sap chemical changes involved in embolism recovery.

Splitting the Difference: Heterogeneous Soil Moisture Availability Affects Aboveground and Belowground Reserve and Mass Allocation in Trembling Aspen.

When exploring the impact of resource availability on perennial plants, artificial treatments often apply conditions homogeneously across space and time, even though this rarely reflects conditions in natural systems. To investigate the effects of spatially heterogeneous soil moisture on morphological and physiological responses, trembling aspen (Populus tremuloides) saplings were used in a split-pot experiment. Following the division of the root systems, saplings were established for a full year and then subjected to either heterogeneous (portion of the root system exposed to non-lethal drought) or homogeneous (whole root system exposed to non-lethal drought or well-watered) treatments. Above- and belowground growth and non-structural carbohydrate (NSC) reserves (soluble sugars and starch) were measured to determine how allocation of reserves and mass between and within organs changed in response to variation in soil moisture availability. In contrast to saplings in the homogeneous drought treatment, which experienced reduced shoot growth, leaf abscission and fine root loss, saplings exposed to the heterogeneous conditions maintained similar aboveground growth and increased root system allocation compared to well-watered saplings. Interestingly under heterogeneous soil moisture conditions, the portion of the root system that was resource limited had no root dieback and increased carbon reserve concentrations, while the portion of the root system that was not resource limited added new roots (30% increase). Overall, saplings subjected to the heterogeneous soil moisture regime over-compensated belowground, both in mass and NSC reserves. These results indicate that the differential allocation of mass or reserves between above- and belowground organs, but also within the root system can occur. While the mechanisms and processes involved in these patterns are not clear, these responses could be interpreted as adaptations and acclimations to preserve the integrity of the entire sapling and suggests that different portions of plant organs might respond autonomously to local conditions. This study provides further appreciation of the complexity of the mechanisms by which plants manage heterogeneous conditions and offers evidence that spatial and temporal variability of resource availability, particularly belowground, needs to be accounted for when extrapolating and modeling stress responses at larger temporal and spatial scales.