Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

The aridity influence on oxygen isotopes recorded in tree rings

The stable isotopes of oxygen in wood cellulose (δ18Ocell) have been widely used to reconstruct historical source water use in trees or changes in atmospheric humidity. However, in many cases, the δ18O of source water use is assumed to reflect that of precipitation, which is often not the case in semi-arid to arid ecosystems where trees use deeper and older water from previous precipitation events (or even groundwater). Furthermore, the degree to which δ18Ocell reflects source water and atmospheric aridity depends on pex, normally defined as the proportion of oxygen atoms that exchange between isotopically enriched carbohydrates from the leaf and unenriched xylem water during cellulose synthesis. Many studies treat pex as a constant. However, pex can only be estimated with direct measurements of δ18Ocell and the δ18O of tree source water and sucrose. Additionally, other physiological mechanisms (e.g., photosynthate translocation) can alter the isotopic signal before cellulose is produced. Thus, determining this 'apparent pex' (apex; which includes those other physiological mechanisms such as photosynthate translocation plus the exchange of oxygen atoms during cellulose synthesis), can be difficult. In this study, we collected δ18O of xylem water and δ18O of wood cellulose from seven stands of Ponderosa pine situated at the northern boundary of the North American Monsoon (NAM) climate system to assess how potential variability in apex influenced how source water and aridity were recorded in δ18Ocell. We compared measured and modeled values of δ18Ocell and found that more arid sites under-represented the vapor pressure deficit (VPD) signal in cellulose while wetter sites over-represented the VPD signal in cellulose. We also found that apex varied as a function of site aridity, where low precipitation and high VPD led to high apex, while high precipitation and low VPD led to low apex. Future studies can use our emerging understanding of the aridity-apex relationship in different portions of the annual ring to better disentangle the source water and VPD signals in cellulose, particularly for regions such as the NAM region.

Carbon Dynamics Under Drought and Recovery in Grapevine's Leaves

Drought stress reduces leaf net assimilation (AN) and phloem export, but the equilibrium between the two is unknown. Consequently, the leaf carbon balance and the primary use of the leaf nonstructural carbohydrates (NSC) under water deficit are unclear. Also, we do not know how quickly leaves can replenish their NSC storage and resume export after rehydration. Hence, we dried grapevines to either zero AN, leaf turgor loss, or complete wilting while following the leaf carbon dynamics. The vines ceased growth and minimized carbon export under drought, conserving the leaves NSC until AN zeroed. Subsequently, the leaves slowly depleted their NSC storage. However, the NSC depletion rate in the leaves was too slow to support the leaf's energetic requirements, potentially transforming the leaves into carbon sinks. Even under extreme drought (-2 MPa), the leaves had substantial NSC reserves (38% of the controls). After rehydration, all surviving leaves recovered their NSC storage within a week, and even leaves that were later shed had functional phloem export in the week after rehydration. The study reveals the leaf carbon relations under drought, highlighting the preference of the leaf to conserve its NSC storage rather than utilize it.

Generalized Stomatal Optimization of Evolutionary Fitness Proxies for Predicting Plant Gas Exchange Under Drought, Heatwaves, and Elevated CO2

Stomata control plant water loss and photosynthetic carbon gain. Developing more generalized and accurate stomatal models is essential for earth system models and predicting responses under novel environmental conditions associated with global change. Plant optimality theories offer one promising approach, but most such theories assume that stomatal conductance maximizes photosynthetic net carbon assimilation subject to some cost or constraint of water. We move beyond this approach by developing a new, generalized optimality theory of stomatal conductance, optimizing any non-foliar proxy that requires water and carbon reserves, like growth, survival, and reproduction. We overcome two prior limitations. First, we reconcile the computational efficiency of instantaneous optimization with a more biologically meaningful dynamic feedback optimization over plant lifespans. Second, we incorporate non-steady-state physics in the optimization to account for the temporal changes in the water, carbon, and energy storage within a plant and its environment that occur over the timescales that stomata act, contrary to previous theories. Our optimal stomatal conductance compares well to observations from seedlings, saplings, and mature trees from field and greenhouse experiments. Our model predicts predispositions to mortality during the 2018 European drought and captures realistic responses to environmental cues, including the partial alleviation of heat stress by evaporative cooling and the negative effect of accumulating foliar soluble carbohydrates, promoting closure under elevated CO2. We advance stomatal optimality theory by incorporating generalized evolutionary fitness proxies and enhance its utility without compromising its realism, offering promise for future models to more realistically and accurately predict global carbon and water fluxes.

Humboldt Review: Potassium may mitigate drought stress by increasing stem carbohydrates and their mobilization into grains

Potassium (K) deficiency occurs commonly in crop plants. Optimal K nutrition is particularly important when plants are exposed to stress conditions (especially drought and heat) because a cellular demand for K increases. Low K in plant tissues is known to aggravate the effects of drought stress by impairing the osmoregulation process and the photosynthetic carbon metabolism. However, despite numerous publications about the role of K in enhancing tolerance to drought stress in crop plants, our understanding of the major mechanisms underlying the stress-mitigating effects of K is still limited. This paper summarizes and appraises the current knowledge on the major protective effects of K under drought stress, and then proposes a new K-related drought stress-mitigating mechanism, whereby optimal K nutrition may promote partitioning of carbohydrates in stem tissues and subsequent mobilization of these carbohydrates into developing grain under drought stress. The importance of stem reserves of carbohydrates is based on limited photosynthetic capacity during the grain-filling period under drought conditions due to premature leaf senescence as well as due to impaired assimilate transport from leaves to the developing grains. Plants with a high capacity to store large amounts of soluble carbohydrates in stems before anthesis and mobilize them into grain post-anthesis have a high potential to yield well in dry and hot environments. In practice, particular attention needs to be paid to the K nutritional status of plants grown with limited water supply, especially during grain filling. Because K is the mineral nutrient deposited mainly in stem, a special consideration should be given to stems of crop plants in research dealing with the effects of K on yield formation and stress mitigation.

Bark structure is coordinated with xylem hydraulic properties in branches of five Cupressaceae species

The properties of bark and xylem contribute to tree growth and survival under drought and other types of stress conditions. However, little is known about the functional coordination of the xylem and bark despite the influence of selection on both structures in response to drought. To this end, we examined relationships between proportions of bark components (i.e. thicknesses of tissues outside the vascular cambium) and xylem transport properties in juvenile branches of five Cupressaceae species, focusing on transport efficiency and safety from hydraulic failure via drought-induced embolism. Both xylem efficiency and safety were correlated with multiple bark traits, suggesting that xylem transport and bark properties are coordinated. Specifically, xylem transport efficiency was greater in species with thicker secondary phloem, greater phloem-to-xylem thickness ratio and phloem-to-xylem cell number ratio. In contrast, species with thicker bark, living cortex and dead bark tissues were more resistant to embolism. Thicker phellem layers were associated with lower embolism resistance. Results of this study point to an important connection between xylem transport efficiency and phloem characteristics, which are shaped by the activity of vascular cambium. The link between bark and embolism resistance affirms the importance of both tissues to drought tolerance.

Phloem anatomy predicts berry sugar accumulation across 13 wine-grape cultivars

Introduction

Climate change is impacting the wine industry by accelerating ripening processes due to warming temperatures, especially in areas of significant grape production like California. Increasing temperatures accelerate the rate of sugar accumulation (measured in ⁰Brix) in grapes, however this presents a problem to wine makers as flavor profiles may need more time to develop properly. To alleviate the mismatch between sugar accumulation and flavor compounds, growers may sync vine cultivars with climates that are most amenable to their distinct growing conditions. However, the traits which control such cultivar specific climate adaptation, especially for ⁰Brix accumulation rate, are poorly understood. Recent studies have shown that higher rates of fruit development and sugar accumulation are predicted by larger phloem areas in different organs of the plant.

Methods

Here we test this phloem area hypothesis using a common garden experiment in the Central Valley of Northern California using 18 cultivars of the common grapevine (Vitis vinifera) and assess the grape berry sugar accumulation rates as a function of phloem area in leaf and grape organs.

Results

We find that phloem area in the leaf petiole organ as well as the berry pedicel is a significant predictor of ⁰Brix accumulation rate across 13 cultivars and that grapes from warm climates overall have larger phloem areas than those from hot climates. In contrast, other physiological traits such as photosynthetic assimilation and leaf water potential did not predict berry accumulation rates.

Discussion

As hot climate cultivars have lower phloem areas which would slow down brix accumulation, growers may have inadvertently been selecting this trait to align flavor development with sugar accumulation across the common cultivars tested. This work highlights a new trait that can be easily phenotyped (i.e., petiole phloem area) and be used for growers to match cultivar more accurately with the temperature specific climate conditions of a growing region to obtain satisfactory sugar accumulation and flavor profiles.