Non-structural Carbohydrates Depletion Research Articles

Nonstructural carbohydrates explain post-fire tree mortality and recovery patterns.

Trees use nonstructural carbohydrates (NSCs) to support many functions, including recovery from disturbances. However, NSC's importance for recovery following fire and whether NSC depletion contributes to post-fire delayed mortality are largely unknown. We investigated how fire affects NSCs based on fire-caused injury from a prescribed fire in a young Pinus ponderosa (Lawson & C. Lawson) stand. We assessed crown injury (needle scorch and bud kill) and measured NSCs of needles and inner bark (i.e., secondary phloem) of branches and main stems of trees subject to fire and at an adjacent unburned site. We measured NSCs pre-fire and at six timesteps post-fire (4days-16months). While all trees initially survived the fire, NSC concentrations declined quickly in burned trees relative to unburned controls over the same post-fire period. This decline was strongest for trees that eventually died, but those that survived recovered to unburned levels within 14months post-fire. Two months post-fire, the relationship between crown scorch and NSCs of the main stem inner bark was strongly negative (Adj-R2=0.83). Our results support the importance of NSCs for tree survival and recovery post-fire and suggest that post-fire NSC depletion is in part related to reduced photosynthetic leaf area that subsequently limits carbohydrate availability for maintaining tree function. Crown scorch is a commonly measured metric of tree-level fire severity and is often linked to post-fire tree outcome (i.e., recovery or mortality). Thus, our finding that NSC depletion may be the mechanistic link between the fire-caused injury and tree outcome will help improve models of post-fire tree mortality and forest recovery.

Unveiling resilience mechanisms of Quercus ilex seedlings to severe water stress: Changes in non-structural carbohydrates, xylem hydraulic functionality and wood anatomy

Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the β-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.

Limited mitigating effects of elevated CO2 in young aspen trees to face drought stress

Elevated atmospheric CO 2 concentration (eCO 2 ) is expected to mitigate the adverse effects of moderate drought on leaf and whole-tree functioning. However, tree responses to eCO 2 under severe drought and throughout the growing season remain largely unknown. One-year-old Populus tremula L. trees were grown in two controlled treatment chambers under ambient and elevated CO 2 conditions, while progressive drought was imposed early (spring/summer 2019) and late (summer/autumn 2018) during the growing season. Leaf level responses to eCO 2 (i.e., stomatal conductance, leaf carbon assimilation and leaf respiration) were monitored in concert with whole-tree level responses (i.e., canopy conductance, radial stem growth, stem CO 2 efflux, xylem water potential and non-structural carbohydrates (NSC)). At the leaf level, eCO 2 lowered the drought susceptibility of stomatal closure and delayed drought-induced reduction in leaf carbon assimilation during late season drought, but these responses were not observed during the early season drought. Drought effects on whole-tree functioning and NSC depletion remained unaltered by eCO 2 . Under moderate drought, stem volumetric growth ceased earlier than photosynthesis, while leaf and stem respiratory metabolism were maintained at 30 % of well-watered levels even under severe drought, independent of the CO 2 treatment and timing of drought. Therefore, the ability of eCO 2 to mitigate drought was mainly limited to leaf processes during the late season and under moderate drought (> − 2 MPa), while drought offset any beneficial effect of eCO 2 at the whole-tree level. These results urge us to revisit predictions of forests' potential to sequester carbon under climate change scenarios. • Mitigating effects of elevated CO 2 under drought are limited in young Populus trees. • Elevated CO 2 mitigates drought at the leaf level, while whole-tree processes remain unaffected. • Mitigating effects of elevated CO 2 were limited to a late-season drought event.

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.

Root carbon and nutrient homeostasis determines downy oak sapling survival and recovery from drought.

The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root C exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root C depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged C replenishment in roots might eventually lead to C starvation and further mortality.

Plant carbohydrate depletion impairs water relations and spreads via ectomycorrhizal networks.

Under prolonged drought and reduced photosynthesis, plants consume stored nonstructural carbohydrates (NSCs). Stored NSC depletion may impair the regulation of plant water balance, but the underlying mechanisms are poorly understood, and whether such mechanisms are independent of plant water deficit is not known. If so, carbon costs of fungal symbionts could indirectly influence plant drought tolerance through stored NSC depletion. We connectedwell-watered Pinus ponderosa seedling pairs via ectomycorrhizal (EM) networks where one seedling was shaded (D) and the other kept illuminated (LD) and compared responses to seedling pairs in full light (L). We measured plant NSCs, osmotic and water potential, and transfer of 13 CO2 through EM to explore mechanisms linking stored NSCs to plant water balance regulation and identify potential tradeoffs between plant water retention and EM fungi under carbon-limiting conditions. NSCs decreased from L to LD to D seedlings. Even without drought, NSC depletion impaired osmoregulation and turgor maintenance, both of which are critical for drought tolerance. Importantly, EM networks propagated NSC depletion and its negative effects on water retention from carbon stressed to nonstressed hosts. We demonstrate that NSC storage depletion influences turgor maintenance independently of plant water deficit and reveal carbon allocation tradeoffs between supporting fungal symbionts and retaining water.

Effects of drought regimes on growth and physiological traits of a typical shrub species in subtropical China

Drought represents the most important limiting factor controlling performance of subtropical plants, leading to reduced growth and die-offs. However, the mechanisms underlying growth and physiological responses of subtropical woody plants to different types of drought are poorly understood. Here, potted seedlings of a typical evergreen shrub species Ardisia lindleyana D. Dietr. in subtropical China, were exposed to three water regimes: well watered (WW), fast drought (FD) and slow drought (SD) until death. Leaf gas exchange and water potential were measured over the experimental period. Dry mass production and allocation, leaf nitrogen and carbon isotopic composition, and non-structural carbohydrates (NSC) were measured at mortality. Results showed that growth, leaf water potential and photosynthesis were significantly reduced in both drought regimes. FD induced significant increase of root soluble sugars, but promoted significant depletion of leaf and root starch, finally displaying depletion of root NSC. By contrast, SD led to significant depletion of stem and root starch, whereas the accumulation of soluble sugars in stem and root was significantly higher than that of WW, thereby resulting in no significant change of NSC. Neither FD nor SD has affected leaf carbon isotopic composition or nitrogen concentration at mortality. Our results demonstrate the divergent effects of two different drought treatments on carbon and water economy of A. lindleyana, which may have implications for understanding the mortality mechanism of understory plants in subtropical forests in China.

Patterns in nonstructural carbohydrate contents at the tree organ level in response to drought duration.

Nonstructural carbohydrates (NSCs) facilitate the adaptation of trees to drought stress. There have been a large number of studies exploring NSC changes in individual plant species and individual organ under drought and showed different trends; however, an understanding of the universal pattern of the plant NSCs responses to drought, particularly to drought duration, is still lacking. Here, we compiled data from 47 experimental studies on 52 tree species and conducted a meta-analysis to evaluate the responses of soluble sugars, starch, and TNSC (total nonstructural carbohydrates including both soluble sugars and starch) concentrations in different tree organs (leaf, stem, and root) to drought intensity and duration. We found that starch in all organs decreased and soluble sugars in leaf increased with prolonged experiment time, and the changes in soluble sugars in all organs were stronger under severe drought than under slight-to-moderate drought. Under slight-to-moderate drought, the NSC content of each organ varied with time, while with the extension of the drought duration, the NSCs gradually approached the control value (no drought stress); this trend remained inthe late drought, which means that trees activated physiological regulation processes to increase carbon storage and reduce the risks of carbon starvation. In contrast, long-term severe drought could lead to a net loss of carbohydrates, especially in the root, implying that prolonged severe drought could lead to NSC depletion in the whole plant. As prolonged drought duration has occurred in and is projected for many regions, this paper could shed light into studies on how trees respond and adapt extending drought duration through nonstructural carbon production, transportation, and reallocation.

The Possible Role of Non-Structural Carbohydrates in the Regulation of Tree Hydraulics.

The xylem is a complex system that includes a network of dead conduits ensuring long-distance water transport in plants. Under ongoing climate changes, xylem embolism is a major and recurrent cause of drought-induced tree mortality. Non-structural carbohydrates (NSC) play key roles in plant responses to drought and frost stress, and several studies putatively suggest their involvement in the regulation of xylem water transport. However, a clear picture on the roles of NSCs in plant hydraulics has not been drawn to date. We summarize the current knowledge on the involvement of NSCs during embolism formation and subsequent hydraulic recovery. Under drought, sugars are generally accumulated in xylem parenchyma and in xylem sap. At drought-relief, xylem functionality is putatively restored in an osmotically driven process involving wood parenchyma, xylem sap and phloem compartments. By analyzing the published data on stem hydraulics and NSC contents under drought/frost stress and subsequent stress relief, we found that embolism build-up positively correlated to stem NSC depletion, and that the magnitude of post-stress hydraulic recovery positively correlated to consumption of soluble sugars. These findings suggest a close relationship between hydraulics and carbohydrate dynamics. We call for more experiments on hydraulic and NSC dynamics in controlled and field conditions.

The effects of persistent drought and waterlogging on the dynamics of nonstructural carbohydrates of Robinia pseudoacacia L. seedlings in Northwest China

BackgroundThe nonstructural carbohydrates (NSCs) of plants are posited to be crucial traits for the resistance and resilience of plants to climate change-induced drought and flooding. However, the potential effects of persistent drought and waterlogging on the dynamics of the NSCs and the underlying mechanisms are still poorly understood.MethodsWe measured the NSCs concentrations and pool size, photosynthetic rate and biomass of Robinia pseudoacacia L. seedlings for both 2015 and 2016 under five soil water treatments: 3%, 8%, 17%, 24% and 34% soil water content, representing extreme drought (ED), moderate drought (MD), the control group (CG), field capacity (FC) and waterlogging (WL) stresses, respectively. We observed the relationship between the pool size of NSCs and the survival of seedlings under water stress (drought and waterlogging) for 30 days in greenhouse.ResultsCompared with CG, the net photosynthetic rate decreased 91%, 67%, 34% and 71%, and the biomass decreased by 37%, 15%, 16% and 33% under ED, MD, FC and WL, respectively. The total NSC (TNSC) concentration was significantly increased by 154% under ED after 10 days and sharply decreased by 50% under ED after 30 days. The concentrations of soluble sugars (SS) were significantly increased by 100% under MD after 10 days and sharply decreased by 60% under ED after 30 days. Compared with GC, the response of NSCs, photosynthetic rate and biomass under ED were more dramatic than that under WL. The pool sizes of fructose and sucrose were larger under ED and WL, but the maximum pool size of starch occurred under the CG. The depletion of NSCs was not observed under ED at the end of the experiments in both 2015 and 2016.ConclusionsOur results indicate that the dynamics of NSCs is an important physiological feature of plant adaptation and resistance to drought and waterlogging. In addition, high sugars concentrations are beneficial for the plants during the short-term extreme drought and the longer term mild drought or waterlogging.

Whole-tree nonstructural carbohydrate storage and seasonal dynamics in five temperate species.

Summary Despite the importance of nonstructural carbohydrates (NSC) for growth and survival in woody plants, we know little about whole‐tree NSC storage. The conventional theory suggests that NSC reserves will increase over the growing season and decrease over the dormant season. Here, we compare storage in five temperate tree species to determine the size and seasonal fluctuation of whole‐tree total NSC pools as well as the contribution of individual organs. NSC concentrations in the branches, stemwood, and roots of 24 trees were measured across 12 months. We then scaled up concentrations to the whole‐tree and ecosystem levels using allometric equations and forest stand inventory data.While whole‐tree total NSC pools followed the conventional theory, sugar pools peaked in the dormant season and starch pools in the growing season. Seasonal depletion of total NSCs was minimal at the whole‐tree level, but substantial at the organ level, particularly in branches. Surprisingly, roots were not the major storage organ as branches stored comparable amounts of starch throughout the year, and root reserves were not used to support springtime growth.Scaling up NSC concentrations to the ecosystem level, we find that commonly used, process‐based ecosystem and land surface models all overpredict NSC storage.

Trees use more non-structural carbohydrate reserves during epicormic than basal resprouting.

Non-structural carbohydrates (NSCs) are crucial to support tree resprouting after disturbances that damage the crown or stem. Epicormic resprouting (from stem) could demand more from NSC reserves than basal resprouting (following top-kill), since epicormically resprouting trees need to maintain a greater living biomass. Yet, little is known about NSC use during epicormic resprouting, particularly the relative importance of stem and below-ground NSC reserves. We compared the distribution and magnitude of NSC decreases during epicormic and basal resprouting by experimentally removing crowns or stems of 14-year-old Eucalyptus obliqua L'Hér. trees in native forest, then harvesting these trees over a 10-month period (start, sprouts emerged, sprouts expanded) to measure changes in NSC concentration and mass by organ (stem, lignotuber, roots). We hypothesized that (i) NSC depletion during resprouting is primarily due to decreases in starch rather than soluble sugars concentrations; (ii) during epicormic resprouting, stem NSC concentrations are decreased irrespective of any decreases in roots; and (iii) absolute decreases in NSC mass are greater for epicormic than basal resprouting during the leafless period due to the carbon demands associated with maintaining greater living biomass. Results confirmed our hypotheses; starch was the primary storage carbohydrate, stems were an important source of starch during epicormic resprouting and carbon demands of maintenance functions were greater for epicormic resprouting, leading to greater decreases in NSC reserves. Roots were a more important starch storage organ than the lignotuber for both epicormic and basal resprouting. The proportional decrease in starch was severe for both modes of resprouting due to a long leafless period, after which trees resprouting epicormically relied on starch reserves for longer than those resprouting basally. It remains to be seen how the timing of disturbance affects the timing and vigour of resprouting, and how long-term NSC recovery differs for epicormic and basal resprouting.

Carbohydrate Depletion during Lethal Infestation ofAulacaspis yasumatsuionCycas revoluta

Premise of research. The armored scale Aulacaspis yasumatsui Takagi has invaded numerous geographic regions to devastate cultivated populations of Cycas L. and the native population of Cycas micronesica K.D. Hill on the islands of Guam and Rota. Questions remain as to how this small hemipteran insect can so rapidly kill a large cycad tree. One possibility is depletion of nonstructural carbohydrates (NSCs) until the storage pool can no longer sustain plant viability.Methodology. We infested Cycas revoluta Thunberg plants with A. yasumatsui and harvested whole plants periodically until plant mortality at week 60. Harvested plants were separated into leaves, stems, and roots, and tissue was frozen and then lyophilized. The dry mass of each organ was measured, and then fructose, glucose, maltose, starch, and sucrose were quantified. Concentration and total pool size within each organ were calculated for each NSC.Pivotal results. Concentration and total pool of all five NSCs were greatest in stems. Concentrations exhibited linear or quadratic declines with time of infestation for all NSCs and organs, with the exception of glucose in stems and maltose in leaves. The total pool of all five NSCs declined linearly or quadratically in all three organs, with the exception of maltose in leaves. Initial plant dry mass of 152 g was 23% NSC, and ending plant dry mass of 59 g was 26% NSC. Carbohydrate stoichiometry shifted in favor of sugars over starch, and sugar stoichiometry shifted in favor of hexoses over disaccharides as time of infestation progressed.Conclusions. These results are consistent with carbohydrate depletion as the mechanism that explains rapid Cycas tree mortality during chronic A. yasumatsui infestation. This is the first demonstration of stress-induced expenditures of carbohydrates by cycads.

Homeostatic levels of nonstructural carbohydrates after 13yr of drought and irrigation in Pinus sylvestris.

Nonstructural carbohydrates (NSCs) are important for the growth and survival of trees. Drought may lead to a decrease in tree growth and to NSC depletion, whereas increased soil moisture in otherwise dry ecosystems may increase growth and NSC concentrations. A long-term (13yr) irrigation experiment was conducted in a Pinus sylvestris-dominated forest located at the dry margin of the species in southern Switzerland. We measured the relative leaf area, growth, NSCs, needle δ13 C, [N] and [P] in trees on control and irrigated plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with decreasing leaf area in both treatments, but NSC did not correlate with leaf-level gas exchange indices, such as foliar δ13 C, [N] or [P]. A legacy effect was shown, as trees with initially low leaf area had limited ability to respond to prolonged irrigation. The NSC constancy across treatments provides evidence that carbohydrate storage may stay constant when climate changes are sufficiently slow to allow acclimation. Moreover, we speculate that total leaf area, rather than leaf gas exchange per unit leaf area, drives the variation in whole-tree carbohydrate dynamics in this system.

Dynamics of non‐structural carbohydrates in terrestrial plants: a global synthesis

AbstractPlants store large amounts of non‐structural carbohydrates (NSC). While multiple functions ofNSChave long been recognized, the interpretation ofNSCseasonal dynamics is often based on the idea that storedNSCis a reservoir of carbon that fluctuates depending on the balance between supply via photosynthesis and demand for growth and respiration (the source–sink dynamics concept). Consequently, relatively highNSCconcentrations in some plants have been interpreted to reflect excess supply relative to demand. An alternative view, however, is thatNSCaccumulation reflects the relatively highNSClevels required for plant survival; an important issue that remains highly controversial. Here, we assembled a new global database to examine broad patterns of seasonalNSCvariation across organs (leaves, stems, and belowground), plant functional types (coniferous, drought‐deciduous angiosperms, winter deciduous angiosperms, evergreen angiosperms, and herbaceous) and biomes (boreal, temperate, Mediterranean, and tropical). We compiled data from 121 studies, including seasonal measurements for 177 species under natural conditions. Our results showed that, on average,NSCaccount for ~10% of dry plant biomass and are highest in leaves and lowest in stems, whereas belowground organs show intermediate concentrations. TotalNSC, starch, and soluble sugars (SS) varied seasonally, with a strong depletion of starch during the growing season and a general increase during winter months, particularly in boreal and temperate biomes. Across functional types,NSCconcentrations were highest and most variable in herbaceous species and in conifer needles. Conifers showed the lowest stem and belowgroundNSCconcentrations. MinimumNSCvalues were relatively high (46% of seasonal maximums on average for totalNSC) and, in contrast to average values, were similar among biomes and functional types. Overall, although starch depletion was relatively common, seasonal depletion of totalNSCorSSwas rare. These results are consistent with a dual view ofNSCfunction: whereas starch acts mostly as a reservoir for future use, soluble sugars perform immediate functions (e.g., osmoregulation) and are kept above some critical threshold. If confirmed, this dual function ofNSCwill have important implications for the way we understand and model plant carbon allocation and survival under stress.

Carbon starvation during drought-induced tree mortality – are we chasing a myth?

Drought-induced tree mortality has received much attention in the recent past. McDowell et al.’s (2008) hydraulic framework links tree hydraulics with carbon dynamics and proposes two non-exclusive mortality mechanisms: carbon starvation (CS) and hydraulic failure (HF). CS is often referred to as the (partial) depletion of non-structural carbohydrates (NSC) in response to stomatal closure, reduced C assimilation and sustained C storage dependency during longer droughts. HF describes a lethal level of xylem dysfunction from runaway embolism during severe droughts. While HF can be readily inferred from the percentage loss of conductivity in vascular tissues at the time of death, CS is much more difficult to assess.Starvation is usually defined as a lack of food leading to suffering or death. In plants photosynthetic sugars play many functional roles, not only as a source of catabolic energy. For example, sugars are important for osmotic regulation of cell pressure and recent studies suggest a potential link between xylem parenchyma sugars and embolism repair following drought. Hence, carbon limitation could have a direct impact on tree hydraulics and HF; however, empirical evidence for such a mechanism is still inconclusive.Although HF appears to be predominant during drought mortality, our limited understanding of the roles of NSC in hydraulic function precludes any premature refutation of CS as a mechanism in drought-induced tree mortality.

A carbohydrate quandary.

Carbon (C) is central to the flow and storage of mass and energy in plants and their ecosystems, but our understanding of C dynamics in plants is still developing. Trees and many plants have ∼50% C (45–60% of the dry mass, Thomas and Martin 2012), and a large portion of this C is in the form of carbohydrates, one of the four main macromolecules comprising plants. Carbohydrates are traditionally classified as either recalcitrant, ‘structural’ components of cell walls and other tissues, or as labile, ‘non-structural’ pools, although this dichotomy is often not so clear ( Hoch 2007). Non-structural carbohydrates (NSCs) can comprise a large portion of a tree’s C and the dry mass (up to ∼50 and ∼25%, respectively, e.g., Hoch et al. 2003), but they can also be scarce due to tight linkages of NSC to C uptake, NSC utilization and active regulation of pools. Several contemporary concepts in ecophysiology center on NSC as a potentially important balance point that links environmental stressors and physiological responses to whole-plant outcomes. Prominent examples include the growth-limitation hypothesis that low temperatures inhibit C allocation to structural growth more than they limit C uptake, leading to greater NSC ( Korner 1998); and the C-starvation hypothesis that drought kills plants through a loss of C uptake and NSC depletion more so than direct hydraulic failure ( McDowell and Sevanto 2010). As formulated, these and many other theories implicitly assume that NSCs are a discrete pool that does not intergrade with structural carbohydrates, and most importantly can be measured. In this issue, Quentin et al. (2015) present an ‘inconvenient truth’ for research addressing these theories that has important ramifications for tree physiology specifically, and plant and ecosystem sciences more generally. Quentin et al. (2015) sent the same set of five plant tissues to 29 NSC laboratories in 13 countries, and found large disagreement among the laboratories in their estimate of NSC content of the samples. Their findings suggest that we currently do not know which technique or laboratory could be most relied upon to develop plant tissue NSC standards needed for accuracy assessment.

Submergence tolerance in relation to application time of nitrogen and phosphorus in rice (Oryza sativa L.)

Transient complete submergence reduces survival and yield of more than 22millionha of rice in rainfed lowlands and flood prone areas in Asia. Progress has been made in developing more tolerant germplasm, but fewer efforts have been devoted to identifying suitable nutrient management options. This study compared IR-64 Sub1 and IR-64 for their tolerance to submergence at active tillering, panicle initiation and heading stages. The effects of N and P application and their application time on the plant's tolerance to submergence were also examined. We measured shoot elongation, plant survival, specific leaf area, chlorophyll and non-structural carbohydrates (NSC) concentration. Both the cultivars suffered more to submergence at heading stage than other stages with devastating effects on IR-64. Post-submergence N and basal P application increased the concentration of NSC, enhanced survival percentage and reduced shoot elongation. Pre-submergence N application enhanced under-water shoot elongation, which leads to reduced survival, depletion of chlorophyll and NSC. Soluble carbohydrates after submergence are more important for survival than at the initial level, IR-64 Sub 1 could survived better under submergence because it possessed 12.5% more NSC after submergence as compared to IR-64. Plant survival and productivity in areas where untimely flooding is anticipated could be enhanced by post-submergence N and basal P application.

Mobile carbohydrates in Himalayan treeline trees I. Evidence for carbon gain limitation but not for growth limitation

To test whether the altitudinal distribution of trees is determined by a carbon shortage or an insufficient sugar fraction (sugar:starch ratio) in treeline trees, we studied the status of nonstructural carbohydrates (NSC) and their components (total soluble sugars and starch) in Abies fabri (Mast.) Craib and Picea balfouriana var. hirtella Rehd. et Wils. trees along three elevational gradients, ranging from lower elevations to the alpine treeline, on the eastern edge of the Tibetan Plateau. For comparison, we investigated a low-altitude species (Tsuga yunnanensis (Franch.) Pritz.) which served as a warm-climate reference because it is distributed in closed montane forests below 3100 m a.s.l. in the study area. The carbon status of T. yunnanensis responded to altitude differently from that of the treeline species. At the species level, total NSC was not consistently more abundant in treeline trees than in trees of the same species growing at lower elevations. Thus there was no consistent evidence for carbon limitation of growth in treeline trees. For the three treeline species studied (P. balfouriana and A. fabri in the Kang-Ding Valley and A. fabri in the Mo-Xi Valley), winter NSC concentrations in treeline trees were significantly lower than in lower-elevation trees of the same species, suggesting that, in winter, carbon is limited in treeline trees. However, in no case was there total overwinter depletion of NSC or its components in treeline trees. Treeline and low-altitude species had similar sugar:starch ratios of about three at their upper-elevational limits in April. We conclude that survival and growth of trees at the elevational or latitudinal climate limit depend not only on NSC concentration in perennial tissues, but also on the maintenance of an overwintering sugar:starch ratio greater than three.

A test of the growth‐limitation theory for alpine tree line formation in evergreen and deciduous taxa of the eastern Himalayas

1 Whether the global high elevation tree line phenomenon is associated with a low-temperature-induced limitation of sink activities (i.e. direct impact on meristems and thus, growth) or by a limitation of the trees’ carbon source activities (net photosynthesis) still awaits detailed tests across a range of taxa and regions, especially for deciduous species in a short growing season. 2 Here, we test the sink limitation hypothesis in the highest tree lines of Eurasia at altitudes up to 4700 m. We assessed growth and tissue concentrations of non-structural carbohydrates (NSC) as a measure of the carbon source-sink balance in needles and branchwood of Abies, Juniperus (evergreen), Betula and Larix (deciduous). 3 The mean soil temperature in deep shade (a proxy for mean air temperature) across the growing season at tree line in this region is around 6·6 °C, which is consistent with the threshold temperature found at the natural climatic limit of the tree life-form worldwide. Mean tree height and stem diameter decreased significantly towards the upper tree line in all species studied. 4 Air temperature measurements at an inverted tree line site (valley bottom) indicate strong and rapid oscillations between nighttime freezing and mild daytime temperatures during late winter, which apparently eradicate Abies and select for Juniperus, offering potential explanations for the inferiority of Abies also at the upper Tibetan tree line. 5 At none of the four altitudinal transects studied did we observe a significant depletion of NSC (carbon limitation) at tree line. Instead, NSC increased in the majority of cases, suggesting direct (meristematic) low temperature constraints of growth. These results for these highest Eurasian tree lines suggest a common mechanism of alpine tree line formation for evergreen and deciduous species.