Critical responses of photosynthetic efficiency of goldspur apple tree to soil water variation in semiarid loess hilly area

Abstract

Goldspur apple (Malus pumila cv. Goldspur) is one of the main fruit trees planted in semiarid loess hilly areas. The photosynthetic efficiency in leaves of eight-year-old trees were studied under different soil water conditions with a Li-6400 portable photosynthesis system and a Li-Cor1600 portable steady state porometer in order to explore the effects of soil water stress on photosynthesis and the suitable soil water content (SWC) for water-saving irrigation of apple orchards. The results showed that the leaf net photosynthetic rate (P N), transpiration rate (E), water-use efficiency (WUE), stomatal conductance (g s), intercellular CO2 concentration (C i), and stomatal limiting value (L s) displayed different threshold responses to soil water variation. When SWC was within a range of about 60%-86% of field capacity (FC), P N and E were maintained in a relative steady state. At an elevated level but below 60% of FC, both P N and E decreased evidently with decreasing soil moisture. The SWC needed to support WUE in a relatively steady state and at a high level was in the range of about 50%-71% of FC. When SWC was less than 48% of FC, g s and L s declined with decreasing soil moisture, while C i increased rapidly. Based on the analysis of the stomatal limitation of photosynthesis using two criteria (C i and L s) suggested by Farquhar and Sharkey, it was implied that the predominant cause of restricting P N had changed from stomatal limitation to nonstomatal one under severe water stress. In terms of water-saving irrigation for enhancing water-use efficiency, it was concluded that in semiarid loess hilly areas, the suitable range of SWC for water-saving irrigation in goldspur apple orchards is in the range of about 50%-71% of FC, and the most severe degree of soil water stress tolerated for photosynthesis is about 48% of FC.