Photosynthetic performance of walnut leaves during the occurrence of leaf scorch.

To clarify the effect of leaf scorch on walnut leaf photosynthesis, photosynthetic parameters were measured in Juglans regia 'Wen185' and 'Xinxin2' symptom trees (WS, XS) and symptomless trees (WH, XH). At the early stage of infection and under the low-grade leaf scorch, WS showed significantly lower net photosynthetic rate (P _N), stomatal limitation (L_s), and higher intercellular CO₂ concentration (C _i) than those of WH. However, at the mid to late stage of infection and under the high-grade leaf scorch, WS showed significantly lower P _N, C _i, the maximal quantum yield of PSII photochemistry and higher L_s, minimal fluorescence yield of the dark-adapted state, nonphotochemical quenching than those of WH, which would occur once Juglans regia 'Xinxin2' was infected. The above results indicated the effect of leaf scorch on walnut leaf photosynthesis was related to the walnut varieties and the duration and severity of the disease. Under the influence of leaf scorch, the decline in photosynthesis of Juglans regia 'Wen185' leaves changed from stomatal to nonstomatal restriction, while Juglans regia 'Xinxin2' leaves showed always nonstomatal restriction.