Large seasonal fluctuations in whole-tree carbohydrate reserves: is storage more dynamic in boreal ecosystems?

Feeding Preferences Shift from Protein to Carbohydrates Across Life Stages in a Phloeophagus Bark Beetle Species

Understanding stage-specific nutritional requirements is essential for determining insect feeding strategies and developing targeted pest management approaches. We examined the feeding preferences, developmental duration, survival rates, and digestive efficiency of the mountain pine beetle across different life stages under different nutritional conditions. We tested three artificial diets with varying protein-to-carbohydrate ratios, including high-protein-low-carbohydrate (HP-LC), medium-protein-medium-carbohydrate (MP-MC), and low-protein-high-carbohydrate (LP-HC). The results showed stage-specific differences in feeding preference among beetle larvae. Early-instar larvae preferred HP-LC and MP-MC diets, whereas late-instar larvae preferred LP-HC diets. Adults of both sexes strongly favoured LP-HC diets. Larvae on MP-MC diets exhibited the fastest development and highest digestive efficiency, indicating optimal protein-carbohydrate balance for growth. Survival was highest on the HP-LC and MP-MC diets but was lower on the LP-HC diets, especially in early instars. Poor digestive efficiency in LP-HC diets suggests that excessive carbohydrates hinder nutrient assimilation. These findings show that mountain pine beetle developmental stages have distinct nutritional needs, with early instar larvae requiring higher protein for survival and development. The observed dietary shifts may be linked to seasonal changes in the nutrient composition of host trees and fungal symbionts of the mountain pine beetles. These stage-specific nutritional preferences further suggest opportunities to disrupt beetle growth through targeted, nutrition-based pest management strategies.

Seasonal dynamics of non-structural carbohydrates in new twigs and old branches of Vitex negundo Var. heterophylla under three densities of Robinia pseudoacacia forests

Non-structural carbohydrates (NSCs) are vital for plant growth, with their levels influenced by light intensity and seasonal changes. However, research on how varying light conditions due to forest density and seasons affect carbon allocation in new twigs and old branches is scarce. Vitex negundo var. heterophylla is a leading shrub species in the warm temperate zone's shrub layer. In this study, we conducted a detailed sampling of V. negundo var. heterophylla branches, differentiating new twigs and old branches across phenological stages under three densities of Robinia pseudoacacia forests. Our sampling schedule was as follows: March (dormant period), May (sprouting period), July (leaf spreading period), September (flowering and fruiting period), and December (deciduous period). The results showed that the seasonal patterns of carbon allocation in the new twigs and old branches were largely in harmony. The starch concentration in the old branches under the high density was significantly lower than in the other two densities during the growing season, but the NSC concentration in December remained at a high level and did not significantly decrease. These indicated even though the light environment was unfavorable to understory V. negundo var. heterophylla during the growing season, cold tolerance in December was not inhibited. And the concentrations of soluble sugars and starch in the new twigs were typically higher than those found in the old branches. This dynamic suggests a strategic prioritization of resources to fuel the growth and development of the plant during the current year. Findings from this study not only contribute to our understanding of carbon allocation strategies in V. negundo var. heterophylla but also provide critical insights for managing and predicting the resilience of warm temperate shrub ecosystems to environmental change.

Use of stored carbon for new organ development in apple saplings in early spring for two consecutive years after 13C labelling

The use of stored carbon is essential for new organ development in deciduous trees during early spring. However, the contribution of carbon to the development of new organs in early spring of subsequent years is not well understood. Using a 13C labelling approach, we investigated the reallocation of assimilated carbon into new aboveground organs on apple (Malus domestica) saplings in the following two years. Eight three-year-old potted saplings were exposed to 13CO2 in an exposure chamber on each of eight different dates during the growth season. Some of the trees were harvested in the late autumn of the same year. The remaining trees were transferred to a field and cultivated during the two following growing seasons. We directly showed that the assimilated 13C was used to develop terminal and flower buds for two consecutive years after labelling. The proportions of the concentration of 13C remobilized to the terminal and flower buds in the second year were 5 and 24% of those in the first year after labelling, respectively. The concentration of assimilated 13C was higher in the terminal buds than in the flower buds in the first year after the labelling, while opposite results were found in the second year. This study demonstrates that the stored carbon used for the development of new organs was a mixture of recent- and old-stored carbon and indicates that recently-stored carbon was preferentially used to develop new organs. We also indicated that the stored carbon was remobilized to flower buds during development.

Tracing carbon and nitrogen reserve remobilization during spring leaf flush and growth following defoliation

Woody plants rely on the remobilization of carbon (C) and nitrogen (N) reserves to support growth and survival when resource demand exceeds supply at seasonally predictable times like spring leaf flush and following unpredictable disturbances like defoliation. However, we have a poor understanding of how reserves are regulated and whether distance between source and sink tissues affects remobilization. This leads to uncertainty about which reserves-and how much-are available to support plant functions like leaf growth. To better understand the source of remobilized reserves and constraints on their allocation, we created aspen saplings with organ-specific labeled reserves by using stable isotopes (13C,15N) and grafting unlabeled or labeled stems to labeled or unlabeled root stocks. We first determined which organs had imported root or stem-derived C and N reserves after spring leaf flush. We then further tested spatial and temporal variation in reserve remobilization and import by comparing (i) upper and lower canopy leaves, (ii) early and late leaves, and (iii) early flush and re-flush leaves after defoliation. During spring flush, remobilized root C and N reserves were preferentially allocated to sinks closer to the reserve source (i.e., lower vs upper canopy leaves). However, the reduced import of 13C in late versus early leaves indicates reliance on C reserves declined over time. Following defoliation, re-flush leaves imported the same proportion of root N as spring flush leaves, but they imported a lower proportion of root C. This lower import of reserve C suggests that, after defoliation, leaf re-flush rely more heavily on current photosynthate, which may explain the reduced leaf mass recovery of re-flush canopies (31% of initial leaf mass). The reduced reliance on reserves occurred even though roots retained significant starch concentrations (~5% dry wt), suggesting that aspen prioritizes the maintenance of root reserves at the expense of fast canopy recovery.

Distribution characteristics of photoassimilates in walnut leaves to different organs

The understanding of photoassimilate distribution serves as the fundamental basis for scientific regulation of fruit quality. Currently, there is a scarcity of research on whole-plant scale photoassimilate distribution in walnut. In order to clarify the characteristics of leaf photoassimilates translocation to various organs in 5-year-old 'Wen185' (J. regia 'Wen185') walnut during the growing season, this study used the 13C isotope pulse labeling technique to label the whole plant of walnut trees in the growing season, temporal variations of 13C abundance (δ13C), 13C partition rate (R13C), leaf source strength and fruit sink strength were analyzed in various organs at different days after tree flowering. The findings indicated that during the periods of 30-70 days and 90-110 days after flowering, there was a higher distribution of 13C in fruits and vegetative branches. However, at 110-130 days after flowering, the predominant allocation of 13C shifted towards main trunk and roots. In-depth study of source leaves and sink fruits showed that chlorophyll content in leaves increased significantly 30-50 days after anthesis, indicating that they gradually became mature functional leaves. The increase of net photosynthetic rate led to increase of source strength, and the retention of photoassimilates in leaves was higher at this time. From 30 to 70 days after flowering, the fresh weight and volume of fruit increased rapidly, which increased the capacity of the sink and enhanced the competition ability against photoassimilates. The recovery of photosynthetic capacity of leaves from 90 to 110 days promoted the output of photoassimilates. At this time, walnut entered the oil conversion period, and the demand for photoassimilates increased. All these factors jointly promoted the unloading of photoassimilates in fruit. In summary, maintaining adequate material conditions and optimizing tree structure at 30-70d and 90-110d after anthesis are important for more efficient distribution of photoassimilates to fruit.

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 dynamics and punctuated carbon sink reduction suggest photosynthetic capacity of boreal silver birch is reduced by the accumulation of hexose

The 'assimilates inhibition hypothesis' posits that accumulation of nonstructural carbohydrates (NSCs) in leaves reduces leaf net photosynthetic rate, thus internally regulating photosynthesis. Experimental work provides equivocal support mostly under controlled conditions without identifying a particular NSC as involved in the regulation. We combined 3-yr in situ leaf gas exchange observations (natural dynamics) in the upper crown of mature Betula pendula simultaneously with measurements of concentrations of sucrose, hexoses (glucose and fructose), and starch, and similar measurements during several one-day shoot girdling (perturbation dynamics). Leaf water potential and water and nitrogen content were measured to account for their possible contribution to photosynthesis regulation. Leaf photosynthetic capacity (A/Ci) was temporally negatively correlated with NSC accumulation under both natural and perturbation states. For developed leaves, leaf hexose concentration explained A/Ci variation better than environmental variables (temperature history and daylength); the opposite was observed for developing leaves. The weaker correlations between NSCs and A/Ci in developing leaves may reflect their strong internal sink strength for carbohydrates. By contrast, the strong decline in photosynthetic capacity with NSCs accumulation in mature leaves, observed most clearly with hexose, and even more tightly with its constituents, provides support for the role of assimilates in regulating photosynthesis under natural conditions.

Seasonal patterns of nonstructural carbohydrate storage and mobilization in two tree species with distinct life-history traits

Nonstructural carbohydrates (NSC) are essential for tree growth and adaptation, yet our understanding of the seasonal storage and mobilization dynamics of whole-tree NSC is still limited, especially when tree functional types are involved. Here, Quercus acutissima Carruth. and Pinus massoniana Lamb, with distinct life-history traits (i.e. a deciduous broadleaf species vs an evergreen coniferous species), were studied to assess the size and seasonal fluctuations of organ and whole-tree NSC pools with a focus on comparing differences in carbon resource mobilization patterns between the two species. We sampled the organs (leaf, branch, stem and root) of the target trees repeatedly over four seasons of the year. Then, NSC concentrations in each organ were paired with biomass estimates from the allometric model to generate whole-tree NSC pools. The seasonal dynamics of the whole-tree NSC of Q. acutissima and P. massoniana reached the peak in autumn and summer, respectively. The starch pools of the two species were supplemented in the growing season while the soluble sugar pools were the largest in the dormant season. Seasonal dynamics of organ-level NSC concentrations and pools were affected by organ type and tree species, with above-ground organs generally increasing during the growing season and P. massoniana roots decreasing during the growing season. In addition, the whole-tree NSC pools of P. massoniana were larger but Q. acutissima showed larger seasonal fluctuations, indicating that larger storage was not associated with more pronounced seasonal fluctuations. We also found that the branch and root were the most dynamic organs of Q. acutissima and P. massoniana, respectively, and were the major suppliers of NSC to support tree growth activities. These results provide fundamental insights into the dynamics and mobilization patterns of NSC at the whole-tree level, and have important implications for investigating environmental adaptions of different tree functional types.

Dynamically optimizing stomatal conductance for maximum turgor-driven growth over diel and seasonal cycles

Stomata have recently been theorized to have evolved strategies that maximize turgor-driven growth over plants' lifetimes, finding support through steady-state solutions in which gas exchange, carbohydrate storage and growth have all reached equilibrium. However, plants do not operate near steady state as plant responses and environmental forcings vary diurnally and seasonally. It remains unclear how gas exchange, carbohydrate storage and growth should be dynamically coordinated for stomata to maximize growth. We simulated the gas exchange, carbohydrate storage and growth that dynamically maximize growth diurnally and annually. Additionally, we test whether the growth-optimization hypothesis explains nocturnal stomatal opening, particularly through diel changes in temperature, carbohydrate storage and demand. Year-long dynamic simulations captured realistic diurnal and seasonal patterns in gas exchange as well as realistic seasonal patterns in carbohydrate storage and growth, improving upon unrealistic carbohydrate responses in steady-state simulations. Diurnal patterns of carbohydrate storage and growth in day-long simulations were hindered by faulty modelling assumptions of cyclic carbohydrate storage over an individual day and synchronization of the expansive and hardening phases of growth, respectively. The growth-optimization hypothesis cannot currently explain nocturnal stomatal opening unless employing corrective 'fitness factors' or reframing the theory in a probabilistic manner, in which stomata adopt an inaccurate statistical 'memory' of night-time temperature. The growth-optimization hypothesis suggests that diurnal and seasonal patterns of stomatal conductance are driven by a dynamic carbon-use strategy that seeks to maintain homeostasis of carbohydrate reserves.

Plant photosynthetic overcompensation under nocturnal warming: lack of evidence in subtropical evergreen trees

BACKGROUND AND AIMS

Increased plant photosynthesis under nocturnal warming is a negative feedback mechanism to overcompensate for night-time carbon loss to mitigate climate warming. This photosynthetic overcompensation effect has been observed in dry deciduous ecosystems but whether it exists in subtropical wet forest trees is unclear.

METHODS

Two subtropical evergreen tree species (Schima superba and Castanopsis sclerophylla) were grown in a greenhouse and exposed to ambient and elevated night-time temperature. The occurrence of the photosynthetic overcompensation effect was determined by measuring daytime and night-time leaf gas exchange and non-structural carbohydrate (NSC) concentration.

KEY RESULTS

A reduction in leaf photosynthesis for both species and an absence of persistent photosynthetic overcompensation were observed. The photosynthetic overcompensation effect was transient in S. superba due to respiratory acclimation and stomatal limitation. For S. superba, nocturnal warming resulted in insufficient changes in night-time respiration and NSC concentration to stimulate overcompensation and inhibited leaf stomatal conductance by increasing the leaf-to-air vapour pressure deficit.

CONCLUSIONS

The results indicate that leaf stomatal conductance is important for the photosynthetic overcompensation effect in different tree species. The photosynthetic overcompensation effect under nocturnal warming may be a transient occurrence rather than a persistent mechanism in subtropical forest ecosystems.