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[Effects of shading on photosynthetic characteristics and yield at different growth stages of potato.]

We examined the effects of shading photosynthetic characteristics, yield, and tolerance to low light in two potato varieties (Jizhangshu 12 and Jizhangshu 8) at four growth stages (seedling, seedling/budding, budding/early flowering and flowering/harvest). There were three shading treatments (0(CK), 20% and 50%). The results showed that at both 20% and 50% shading rates, the SPAD value (a measure of leaf cellular chlorophyll content) of the two varieties decreased significantly at the seedling stage compared with CK treatment. No significant change in the SPAD value occurred at the seedling/budding stage or the budding/early-flowering stage. However, the SPAD value increased marginally after shading at the initial flowering stage. Under the 50% shading regime, the SPAD values of both varieties followed the same trend as the 20% regime. The range of changes at different growth stages remained similar. The only exception was that shading at the beginning of flowering increased SPAD value. Shading had little effect on leaf stomatal conductance (gs) at each developmental stage. There was no significant difference between all treatments and the control, except that the gsvalue of Jizhangshu 8 decreased significantly (43.9%) compared with the control under 50% shading at the beginning of flowering. After the shading treatment, the intercellular CO2 concentration (Ci) of leaves showed an upward trend. 50% shading at the seedling and seedling/budding stages could significantly increase Ci, but not at other stages. The net photosynthetic rate (Pn) of the four periods were all decreased after 15 d of shading. The Pn reduction of the two varieties, 50% shading, was greater than 20% shading. The decline range of Pn of Jizhangshu 12 was less than that of Jizhangshu 8 at all stages, except for that at the seedling stage. The yield of shading treatments decreased in all four stages, with the decrease rate of 50% shading treatment being greater than that of the 20% shading treatment. 'Jizhangshu 12'was not tolerant to low light at the seedling stage but performed better than 'Jizhangshu 8' at other stages. The comprehensive analysis of two test varieties implied that varieties with strong tolerance to low light experienced a smaller boost in intercellular CO2 concentration and a smaller drop in net photosynthetic rate, stomatal conductance, and yield post-shading.