Drought and soil nitrogen (N) deficiency are the limiting factors for poplar plantation productivity improvement in semi-arid regions. N addition could alleviate the growth decline of trees caused by drought; however, the effectiveness under severe drought and the underlying ecophysiological understanding remains uncertain. To further clarify the mechanisms of N addition in regulating tree biomass accumulation under different drought levels, we investigated the effects of 6 g NH4NO3 per plant addition on the carbon and N assimilation and biomass accumulation of potted poplar seedlings under moderate or severe drought (40 % or 20 % of field capacity) conditions, with a particular emphasis on carbon and N interactions. We found that under moderate drought, N addition markedly promoted the activities of antioxidases, nitrate reductase (39 %), and N concentration (56 %) in leaves, significantly alleviated the damages of the membranes and photosystem II, and increased both leaf area (69 %) and chlorophyll content per unit leaf area, along with net photosynthesis rate (34 %), thereby significantly alleviating growth restrictions. However, under severe drought, although N addition increased the accumulation of both soluble sugars and N of the whole plant, it did not ameliorate the damage to membranes and photosystem II, nor did it improve chlorophyll content, leaf area, or biomass accumulation. Therefore, N addition could increase leaf area, enhance antioxidants, and positively influence leaf carbon assimilation (0.60, p < 0.001) in poplar seedlings under moderate drought. The restrictions on leaf area and carbon assimilation were exacerbated during severe drought, which mitigated the positive effects of N addition on carbon assimilation and biomass accumulation. The findings of this study suggest that the growth of hybrid poplar can be enhanced by applying N fertilizer under mild drought conditions. In contrast, N fertilization has no significant effect in severe drought conditions.

Damask rose is an important essential oil crop. In the present study, plants were subjected to three different water deficit levels (70, 40, and 10% available water content) for two periods (June-October). Plant phenology, growth, essential oil yield, gas exchange features, membrane stability and major antioxidant defense elements were monitored across two years. Soil water deficit was related to quicker completion of the growth cycle (up to 7.4 d), and smaller plants (up to 49.7%). Under these conditions, biomass accumulation was jointly constrained by decreased leaf area, chlorophyll content, CO2 intake, and photosynthetic efficiency (up to 82.8, 56.9, 27.3 and 68.2%, respectively). The decrease in CO2 intake was driven by a reduction in stomatal conductance (up to 41.2%), while the decrease in leaf area was mediated by reductions in both number of leaves, and individual leaf area (up to 54.3, and 64.0%, respectively). Although the reactive oxygen species scavenging system was activated (i.e., proline accumulation, and enhanced activity of three antioxidant enzymes) by water deficit, oxidative stress symptoms were still apparent. These effects were amplified, as soil water deficit became more intense. Notably, the adverse effects of water deficit were generally less pronounced when plants had been exposed to water severity during the preceding year. Therefore, exposure to water deficit elicited plant tolerance to future exposure. This phenotypic response was further dependent on the water deficit level. At more intense soil water deficit across the preceding year, plants were less vulnerable to water deficit during the subsequent one. Therefore, our results reveal a direct link between water deficit severity and plant tolerance to future water stress challenges, providing for the first time evidence for stress memory in damask rose.