Non-structural carbohydrate dynamics in Robinia pseudoacacia saplings under three levels of continuous drought stress

KEY MESSAGE : Only severe drought stress induced lower non-structural carbohydrate concentrations in Robinia pseudoacacia saplings, and non-structural carbohydrate reduction occurs solely in branches and stems at the end of the growing season. A better understanding of non-structural carbohydrate (NSC) dynamics in trees under drought stress is critical to elucidate the mechanisms underlying forest decline and tree mortality that result from extended periods of drought. We performed a continuous controlled drought pot experiment from May 14 to September 7, 2013 using Robinia pseudoacacia saplings under 14, 10, 8, and 6 % soil water content that represented the absence of drought (AD), moderate drought (MD), severe drought (SD), and very severe drought (VSD) stress, respectively. Growth, leaf gas exchange, and NSC concentrations in different sapling tissues were measured once a month. The results showed that net photosynthetic rates and the relative increments of basal diameter and height decreased with the intensification of drought stress. Saplings showed lower starch and NSC concentrations and higher soluble sugar concentrations and ratios of sugars to starch only under SD and VSD. The concentrations of starch and NSC were significantly decreased under SD and VSD solely in branches and stems at the end of the growing season. Our results indicated that the timescale of physiological processes plays an important role in NSC dynamics, and that only severe drought stress significantly decreases NSC concentrations, and NSC reduction occurs solely in branches and stems at the end of the growing season.