Alkaline soils have a great influence on apple production in Northern China. Therefore, comprehensive evaluations of tolerance to such stress are important when selecting the most suitable apple rootstocks. We used hydroponics culturing to test 17 genotypes of apple rootstocks after treatment with 1:1 Na₂CO₃ and NaHCO₃. When compared with the normally grown controls, stressed plants produced fewer new leaves, and had shorter roots and shoots and lower fresh and dry weights after 15 d of exposure to alkaline conditions. Their root/shoot ratios were also reduced, indicating that the roots had been severely damaged. For all stressed rootstocks, electrolyte leakage (EL) and the concentration of malondialdehyde (MDA) increased while levels of chlorophyll decreased. Changes in root activity (up or down), as well as the activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) were rootstock-dependent, possibly reflecting their differences in alkali tolerance. Using alkali injury index (AI), adversity resistance coefficients (ARC), cluster analysis, and evaluation of their physiological responses, we classified these 17 genotypes into three groups: (1) high tolerance: Hubeihaitang, Wushanbianyehaitang, Laoshanhaitang Ls2, Xiaojinbianyehaitang, and Fupingqiuzi; (2) moderate tolerance: Pingyitiancha, Laoshanhaitang Ls3, Hubeihaitang A1, Deqinhaitang, Balenghaitang, Maoshandingzi, Shandingzi, and Xinjiangyepingguo; or (3) low tolerance: Pingdinghaitang, Hongsanyehaitang, Xiaojinhaitang, and Sanyehaitang. These results will significantly contribute to the selection of the most suitable materials for rootstocks with desired levels of tolerance to alkali stress.

The effects of light intensity on photosynthesis and photoprotective mechanisms under progressive drought were studied on apple trees (Malus domestica Borkh.) Fuji. The potted trees were exposed to drought stress for 12 days and different light conditions (100, 60 and 25% sunlight). During the progressive drought, the relative water content (RWC) in leaf declined and was faster in full light than in 60 and 25% sunlight. However, the decrease in the net photosynthetic rate (Pn), stomatal conductance (Gs) and Rubisco activity were slower under 100% sunlight condition than other light conditions. After the 6 days of drought, the maximum PSII quantum yield (Fv/Fm), the capacity of electrons move beyond QA − (1–VJ) and electron move from intersystem to PSI acceptor side (1–VI)/(1–VJ) decreased, with greater decline extent in brighter light. While RWCs were >75%, the variations in different light intensities of Gs and Rubisco activity at identical RWC, suggested the direct effects of light. While the little difference in the state of photosynthetic electron transport chain among tested light intensities indicates the results of faster water loss rate of light. Our results also demonstrated that the enhancement the de-epoxidations of xanthophyll cycle, activities of ascorbate peroxidase (APX) and catalase (CAT) were directly regulated by light intensity. While the higher photorespiration rate (Pr) under stronger light condition was mainly caused by faster water loss rate of light.

To understand how drought stress affects CO2 assimilation and energy partitioning in apple (Malus domestica Borkh.), we investigated photosynthesis and photo-protective mechanisms when irrigation was withheld from potted Fuji trees. As the drought progressing, soil relative water content (SRWC) decreased from 87 to 24% in 15 d; this combined the decreasing in leaf relative water content (LRWC), net photosynthesis rate (Pn) and stomatal conductance (Gs). However, the concentrations of chlorophylls (Chl) remained unchanged while Pn values were declining. Photochemistry reactions were slightly down-regulated only under severe drought. Rubisco activity was significantly decreased as drought conditions became more severe. The actual efficiency of photosystem II (ΦPSII) was diminished as drought became more intense. Consequently, xanthophyll-regulated dissipation of thermal energy was greatly enhanced. Simultaneously, the ratio of ΦPSII to the quantum yield of carbon metabolism, which is measured under non-photorespiratory conditions, increased in parallel with drought severity. Our results indicate that, under progressive drought stress, the reduction in photosynthesis in apple leaves can be attributed primarily to stomatal limitations and the inhibited capacity for CO2 fixation. Xanthophyll cycle-dependent thermal dissipation and the Mehler reaction are the most important pathways for dispersing excess energy from apple leaves during periods of drought stress.