Carbon and water dynamics of two Populus species under drought stress: The role of hydraulic transport as modulator of carbon reserves

Background/Question/Methods

Carbon reserves play a fundamental role in physiological processes in plants. The interaction between carbon reserves and water transport domains, especially under hydraulic stress, has gained attention as a possible key factor modulating plant mortality.  Nonetheless, the interaction between these two domains is far from understood and likely more complex than currently thought.  The goal of this study was to determine: (1) How physiological and morphological responses differ in two closely related Populus species (Populus balsamifera (Pb) and P. tremuloides (Pt)) with different drought tolerance and (2) how the allocation of carbon reserves at the tissue and whole-plant scale differs between these two species in response to drought. In a controlled experiment potted seedlings were exposed to a desiccation process to simulated severe drought conditions and grown under outside conditions. Over an eight week period, we destructively sampled seedlings every two weeks. At each sampling measurements of gas exchange, water relations, and above and below mass were taken.  Soluble sugar and starch (NSC) reserves of different tissues were measured and expressed on a per unit leafarea. 

Results/Conclusions

Stomatal behavior during the desiccation process was distinctly different between the two species.  The more drought tolerant Pt closed it stomates much earlier and faster than less drought tolerant Pb. However in both species aboveground growth and root volume were lower in droughted seedlings while root specific density was consistently higher in droughted seedlings. After 8 weeks, NSC content scaled by leaf area (Cont/LA) doubled in roots and tripled in stems of droughted Pt while it only moderately increased in stems of droughted Pb. NSC concentration based on leaf area (Conc/LA) in both species was consistently higher in all tissues of droughted seedlings. Conc/LA in droughted Pb was composed of  >90% of sugars in all tissues, while in droughted Pt Conc/LA was >90% sugars in leaves and stems only, but contained 40% of starch in roots. Total NSC content per leaf area at the whole plant scale (TNSC-LA_plant ) more than double in droughted Pt while it decreased by 43% in droughted Pb. Based on the results of previous and this experiment, we propose that TNSC-LA_plant could be useful as a predictor of mortality risk for seedlings. Hence, we predict that, as long as there is functional xylem capable of providing water to tissues allowing reserve remobilization, Pt would have a greater chance of survival of a severe drought event than Pb.