Nitrogen availability affects stem development and response to differential root-zone drought stress in Catalpa bungei

A greenhouse experiment was conducted to explore how nitrogen availability affects drought responses and stem development of Catalpa bungei seedlings under partial root-zone drought. A factorial design consisting of two N rates [adequate-N (AN) and low-N (LN)] and four water treatments [well-watered (WW), horizontally partial root-zone drought (H-PRD), vertically partial root-zone drought (V-PRD) and full root-zone drought (FRD)] was used. On the whole, the effects of nitrogen applications on seedling growth, physiological parameters and gene expression of key genes may vary among different water treatments. For example, A, E, g_s and CO_2int were significantly increased under WW and V-PRD conditions while A and WUE_i were decreased under FRD by N addition. As compared with the sharp stoma closure under FRD, stoma was moderately regulated under H-PRD and V-PRD conditions by triggering a network of signaling including phytohormones, NO and Ca²⁺. Despite the more sensitive drought responses under H-PRD, the intrinsic water use efficiency (WUE_i) was higher under V-PRD, mainly due to the homeostasis of abscisic acid (ABA) signals. The higher WUE_i and the efficient carbohydrate production contributed to the dominant xylem development under V-PRD. In contrast, the constitutive activation of ABA signaling over long-term H-PRD reduced WUE_i and thus suppressed stem growth under H-PRD. The dominant WUE_i under V-PRD condition contributed to the positive effect of N addition on xylem development. Moreover, N application increased IAA levels and the transcription of critical PIPs genes, which contributed to the dominant WUE_i and xylem development under V-PRD conditions.