Concurrent time course of xylem hydraulic dysfunction and non-structural carbohydrates under contrasting water deficits and nitrogen supplies in poplar

Abstract

We compared the temporal dynamics of growth, leaf gas exchange, xylem hydraulic dysfunction and non-structural carbohydrates (NSC) of two poplar hybrid genotypes subjected to a differential nitrogen (N) supply under contrasting water deficits. Moderate water deficit generated intermediate embolism rates (45–70%) but had marginal effects on NSC. Severe water deficit generated progressive hydraulic failure and NSC reduction, leading to tree death within 90 days. At death, NSC in perennial tissues were not entirely depleted because of remaining soluble sugars. Higher N availability primarily affected growth and NSC dynamics (mainly starch), not embolism dynamics. The faster growing genotype benefiting most from N addition ceased growth and photosynthesis almost simultaneously, with no effect of N addition on process cessation, leading to progressive starch depletion during severe drought. In contrast, the slower growing genotype ceased growth before photosynthesis, leading to a transient increase in starch concentration during early drought stages, but this tended to be suppressed by higher N availability. Altogether, our findings indicate that carbon starvation alone is unlikely in hydraulically vulnerable species such as poplar even under prolonged moderate water deficit, but starch relative depletion covaries in time with progressive hydraulic failure under lethal water deficit. Nutritional status has the potential to shape drought responses, primarily through adjustments in carbon source-sink relations rather than in intrinsic xylem hydraulics, in a genotype-dependent manner. Exploring how hydraulic and carbon safety margins can be coordinated within species would be valuable in understanding how variation in drought response strategies can be exploited in breeding/selection.