Temperature gradients assist carbohydrate allocation within trees

Or Sperling,Maciej A Zwieniecki,Guillaume Théroux-Rancourt,Lucas C R Silva,Aude Tixier

doi:10.1038/s41598-017-03608-w

Abstract

Trees experience two distinct environments: thermally-variable air and thermally-buffered soil. This generates intra-tree temperature gradients, which can affect carbon metabolism and water transport. In this study, we investigated whether carbohydrate allocation within trees is assisted by temperature gradients. We studied pistachio (Pistacia integerrima) to determine: (1) temperature-induced variation in xylem sugar concentration in excised branches; (2) changes in carbon allocation in young trees under simulated spring and fall conditions; and (3) seasonal variability of starch levels in mature orchard trees under field conditions. We found that warm branches had less sugar in perfused sap than cold branches due to increasing parenchyma storage. Simulated spring conditions promoted allocation of carbohydrates from cold roots to warm canopy and explained why starch levels surged in canopies of orchard trees during early spring. This driving force of sugar transport is interrupted in fall when canopies are colder than roots and carbohydrate redistribution is compartmentalized. On the basis of these findings, we propose a new mechanistic model of temperature-assisted carbohydrate allocation that links environmental cues and tree phenology. This data-enabled model provides insights into thermal “fine-tuning” of carbohydrate metabolism and a warning that the physiological performance of trees might be impaired by climatic changes.

Highlights

The effects of photoperiod on seasonal tree growth and phenology have been extensively studied and are known to be associated with changes in carbohydrate allocation in trees adapted to temperate climates[1, 2]
This hypothesis is an extension of our former conceptual model which proposes that internal temperature gradients in trees assist diurnal and seasonal carbon allocation and take part in regulating phenological cycles
The concentration of soluble carbohydrates (SC) in the solution perfused through P. integerrima excised branch segments was significantly affected by temperature (Fig. 1A)

Summary

Introduction

The effects of photoperiod on seasonal tree growth and phenology have been extensively studied and are known to be associated with changes in carbohydrate allocation in trees adapted to temperate climates[1, 2]. It could enable trees to utilize carbohydrates according to their seasonal requirements in a two-direction transport system that slowly moves concentrated sugars in the phloem downwards while delivering diluted sap through the xylem to distally located branches Research in this topic originated from farmers’ interest in increased productivity of tree crops and was recently evoked to explain tree mortality in forest ecosystems due to warming climates[20]. Based on recent theoretical developments[3], we hypothesize that NSC storage, release, and allocation are assisted by temperature gradients along transport pathways of temperate trees This hypothesis is an extension of our former conceptual model which proposes that internal temperature gradients in trees assist diurnal and seasonal carbon allocation and take part in regulating phenological cycles. We determined internal temperature gradients in mature trees under field conditions and empirically tested relationships between carbohydrate allocation and tree phenology to develop a new mechanistic model of temperature-assisted carbon allocation within trees

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