Analysis of photosynthetic and fluorescence characteristics of low-light tolerant genotype potato under shade condition

With constant development of potato cultivation in winter agriculture fields in south China and in relay-intercropping systems in recent years, low light stress has been becoming the emerging limitation to potato development in these regions and it has increasingly gained considerable research attention. In order to explore the effects of low light stress on photosynthetic and chlorophyll fluorescent characteristics along with the distribution of absorbed light energy of low-light tolerant genotype potato, a field experiment was conducted with low-light sensitive potato cultivar 'Lishu 6' and low-light tolerant potato cultivar 'Zhongshu 20'. The experiment consisted of a shade treatment using black net that can filter 70% of sunlight (T) and then a light treatment with full natural sunlight (CK) after seed emergence. The variations in photosynthesis, light and CO₂ response curves, light induced curve and chlorophyll fluorescence parameters of the different potato genotypes following 30 days of shade treatment were measured. The results following the shade treatment were as follows: 1) the net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), maximum net photosynthetic rate (P_max), light saturation point (LSP) and light compensation point (LCP) under T were significantly lower than those under CK. While P_n, P_max and LSP of 'Zhongshu 20' potato cultivar were relatively higher than those of 'Lishu 6' potato cultivar, LCP and dark respiration rate (R_d) of 'Zhongshu 20' potato cultivar were lower than those of 'Lishu 6' potato cultivar. 2) The parameters of CO₂ response curve were not significantly different between the shade and control treatments, but 'Zhongshu 20' potato cultivar had higher maximum carboxylation rate (V_cmax), higher maximum electron transportation rate (J_max) and lower CO₂ compensation point (CCP) than 'Lishu 6' potato cultivar. 3) Potato cultivar 'Zhongshu 20' reacted quicker to high introduction rate and performed better in terms of photosynthetic capacity than potato cultivar 'Lishu 6'. 4) Compared with the control (CK), the increases of initial fluorescence (F_o), maximum fluorescence (F_m) and maximum photochemical quantum efficiency (F_v/F_m) were significantly higher in T treatments. Also reductions in actual photochemical quantum efficiency of PSⅡ(ΔF/F_m'), apparent electron transfer rate (ETR) and photochemical quenching coefficient (qP) were significantly higher in T treatments. Furthermore, there was an increasing trend in non photochemical quenching coefficient (NPQ). 5) Compared with the control, the ratios of non-photochemical dissipation (Ф_NPQ) and fluorescence dissipation (Ф_{f, d}) pathways increased remarkably under shade treatment. The ratio of photochemical quenching pathway (Ф_PSⅡ) decreased significantly, making Ф_NPQ as the main pathway of dissipation. After shade treatment, the low-light tolerant potato genotype 'Zhongshu 20' overall performance for NPQ and Ф_NPQ was higher than the low-light sensitive potato genotype 'Lishu 6', suggesting that 'Zhongshu 20' cultivar had more stronger photosynthetic apparatus protective capability. The above comprehensive analysis indicated that low-light tolerance genotype potatoes had relatively higher P_n, lower LCP and CCP, faster light induced reaction rate and higher non-photochemical dissipation capacity under shade conditions compared with that of low-light sensitive potato cultivar.