Characterization of photosynthetic responses to shading and a light/shade switch during the rapid growth phase of tobacco (Nicotianatobacum L.) subjected to different fertilization regimes

Tobacco (Nicotianatobacum L.), a solanaceous annual grass, is a widely-planted cash crop. China has the largest acreage of tobacco in the world. Inefficient or excessive light negatively affect synthesis and transfer of photosynthate, while sufficient light is the foundation of tobacco production and quality. The rapid growth phase of tobacco is critical, since leaf differentiation, leaf number, leaf size, dry matter accumulation, yield and quality are mostly determined then. Inhibition of photosynthesis resulting from sunny or cloudy weather is common in the study region. When tobacco seedlings are subjected to changes in light intensity, the net photosynthetic rate (Pn) and chlorophyll fluorescence characteristics change accordingly. In addition to light intensity, fertilizers are another critical factor affecting the yield and quality of tobacco. Compared with chemical fertilizers, additions of organic amendments not only improve soil physical properties in favor of tobacco growth, but also provide essential nutrients for crop growth. However, it is not clear how different fertilizer types affect photosynthesis in tobacco under light/shade switch conditions. In order to determine the responses of photosynthetic characteristics of tobacco to a light/shade switch and fertilizer types, a pot culture study was conducted to investigate the changes of Pn and chlorophyll fluorescence characteristics of tobacco at the rapid growth stage. The experiment was arranged in a two factor factorial design with 15 replicates. The first factor was a light intensity switch at the rapid growth stage: (1) switch plants growing in natural light with those growing in 50% shade conditions, and (2) switch plants growing in the 50% shade conditions with those growing in natural light. The second factor was fertilizer type: (1) chemical fertilizer and (2) 50% chemical fertilizer and 50% cake fertilizer (50-50 fertilizer). The results showed that shading increased leaf chlorophyll (Chi) and carotenoid (Car) contents but decreased Pn, irrespective of fertilizer regimes. However, effects of shading on the contents of different types of leaf pigment among different fertilizer treatments were different. For the treatment of 100% mineral fertilizer, there was a large increase in leaf chlorophyll content, while for the 50 • 50 fertilizer treatment, leaf carotenoids increased significantly. Consequently, the ratio of Chi/Car of leaves increased in the treatment of 100% mineral fertilizer but decreased in the 50:50 fertilizer treatment. Whether tobacco seedlings were transferred from natural light to shading or vise versa, leaf Pn and the chlorophyll fluorescence parameters (ΦPS ii, Fv/Fmand qP) decreased instantly in both fertilizer treatments; however, reduction in these leaf photosynthetic parameters was milder in the 50-50 fertilizer treatment than in the treatment receiving only mineral fertilizer. Our data suggested that a light/shade switch at the rapid growth stage of tobacco significantly modified photosynthetic characteristics, and cake fertilizer might improve the light adaptability of tobacco.