Coordination Between Phloem Loading and Structure Maintains Carbon Transport Under Drought

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.