Stored non-structural carbohydrates (NSC) in plants have been implicated in responses to drought, including drought-induced mortality. While there is plenty of evidence that NSCs interact with plant hydraulics during drought-induced mortality, direct evidence via NSC manipulations that low NSC impair water relations is very limited. Several questions about the role of NSC in drought responses deserve more research. First, does low NSC availability impair the water relations of plants? If so, what are the specific processes affected and their underlying mechanisms? Are there NSC thresholds below which plants cannot move and retain water and, therefore, survive? The answers to these questions have important implications for our understanding and modeling of plant responses to drought.
Results/Conclusions
We first review what we know about the role of NSC in plant water relations. We then present experimental evidence with ponderosa pine seedlings (subjected to incremental periods of dark to deplete NSC) which show that reductions of NSC impair their water relations. Compared to controls with higher NSC (not subjected to dark periods), seedlings with reduced NSC had lower water potentials immediately after removal from the dark, even though they were watered throughout the experiment. These results indicate that low NSC impair the ability of plants to move and retain water. More research is needed to elucidate the processes involved and their underlying mechanisms. Overall, plants rarely deplete NSC acutely, suggesting that minimum NSC thresholds are required for survival. Regardless of carbon availability, lack of water supply due to complete hydraulic failure will unavoidably cause desiccation and death. However, our results suggest that plants cannot die of carbon starvation with intact water relations. Thus, hydraulic failure (broadly defined) is the only independent mechanism leading to mortality under drought. Reduced NSC availability accelerates this process, but the underlying mechanisms and how they vary across species remain unknown.