Effects of fertilization on photosynthetic characteristics and growth of Coffea arabica L. at juvenile stage under drought stress

To understand the enhancing effects of nitrogen (N) and phosphorus (P) on drought resistance of Coffea arabica L. at juvenile stage, a pot experiment was conducted at the Division of Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Kunming, Yunnan Province, China. The experiment consisted of eighteen treatments (in 3 × 3 × 2 design) with three N rates N₀ (0 g·plant^-1), N₁ (2.5 g·plant^-1) and N₂ (7.5 g·plant^-1), three P rates P₀ (0 g·plant^-1), P₁ (2.5 g·plant^-1) and P₂ (7.5 g·plant^-1) and two water treatments (normal water supply and drought stress). To learn the effects of different fertilizer and water treatments on various coffee traits, we mainly investigated coffee leaf relative water content (LRWC), photosynthetic characteristics, chlorophyll content, morphology and biomass allocation. The results showed that LRWC, maximum net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (T_r), chlorophyll content, specific leaf area (SLA) and biomass allocation under drought stress were lower than those under normal water supply. Both N and P slowed down LRWC depression induced by drought stress. The LRWC of C. arabica under fertilizer treatments (both single applications of N₁, P₁ and P₂ and combined applications of N₁ with P₁ or P₂) exceeded 72%, and was significantly higher than that of non-fertilizer treatment (N₀P₀). Both N and P enhanced photosynthetic characteristics of C. arabica. The maximum P_n and water use efficiency (WUE) of fertilizer treatments (N₀P₁, N₀P₂ and N₁P₂) were significantly higher than those of other treatments. P_n of N₀P₁, N₀P₂ and N₁P₂ fertilizer treatments were 2.09 times, 2.09 times and 2.40 times that of N₀P₀ (non-fertilizer treatment), while the corresponding WUEs were 1.37 times, 1.46 times and 1.58 times that of N₀P₀. Compared with N₀P₀, both gs and T_r of fertilizer treatments increased obviously, but with no significant difference among treatments. N and P increased chlorophyll content of coffee leaves and eased chlorophyll degradation rate caused by drought stress. Moreover, combined application of N and P performed better than single application of N or P. The morphology and biomass allocation of C. arabica were obviously influenced by both N and P. And drought stress promoted the distribution of photosynthetic products to underground system, increasing root mass fraction (RMF) and root-to-shoot ratio (R/S), all fertilizer treatments had higher RMF and R/S under drought stress than under normal water supply. N₀P₁ and N₀P₂ had the largest RMF and R/S, followed by N₁P₂ treatment, and root dry weight of N₁P₂ under drought stress was bigger than under normal water supply. The findings demonstrated that both N and P fertilization were critical for improving drought-resistance of C. arabica at juvenile stage, and N₁P₂ was the optimum treatment for C. arabica.