Photosynthetic response to deep soil water deficit in a semiarid apple tree plantation

Developing roots in deep soil is an important strategy for trees, enabling them to resist prolonged drought, particularly in drylands. However, the impact of deep root water uptake on photosynthesis is still not fully understood. An experiment partitioning deep soil and roots (below 200 cm) was undertaken at a dryland apple (Malus pumila Mill.) tree plantation on the Loess Plateau of China to examine the photosynthetic responses of apple trees to deep soil water deficit by characterizing seasonal changes in stomatal and nonstomatal limitation across years. It was evident that deep soil water deficit led to severe water stress. In the partitioned treatment, leaf hydraulic conductance (K_leaf, 28–58 %) and midday leaf water potential (ψ_md, 14–44 %) significantly decreased without adjustment, even in the wet season. Deep soil water deficit significantly impacted stomatal limitation by imposing average reductions of stomatal conductance (g_s), photosynthesis rate (A), and transpiration rate (T) by 38 %, 26 %, and 34 % while increasing leaf water use efficiency (WUE) by 36 %, during the dry seasons; however, these parameters recovered well in the wet season. In contrast, no significant differences in nonstomatal limitation occurred between treatments, despite the trees suffering from severe drought stress. These results indicate that deep soil water deficit affected photosynthesis by reducing stomatal conductance but not by damaging the photosynthetic apparatus even under severe drought.