Effects of Si on Growth and Photosynthetic Characteristics of Lettuce Seedlings Under Drought Stress

Two lettuce varieties with different resistance, ‘Glass lettuce’ and ‘Rosa red’,were cultivated in nutrient solution and 15% polyethylene glycol was used to simulate drought.The alleviating effect of exogenous Na₂SiO₃ with 1.0 mmol·L^-1 as silicon donor on drought stress was studied, in order to explore the effects of silicon on the growth and photosynthetic characteristics of lettuce seedlings under drought stress.The results showed that compared with the control group, drought stress reduced the growth indexes of lettuce seedling such as plant height, root length, fresh weight and dry weight.The contents of chlorophyll a, chlorophyll b, carotenoid and total chlorophyll decreased.Net photosynthetic rate（Pn）,stomatal conductance（Gs） and transpiration rate（Tr） decreased significantly, and intercellular CO₂ concentration（Ci） increased significantly.PSⅡ photochemical efficiency（Fv/Fm）,the largest PSⅡ actual photochemical efficiency(Ф_PSⅡ),photosynthetic electron transport efficiency（ETR） and light chemical quenching coefficient（qP） significantly reduced, rather than the coefficient of photochemical quenching（NPQ） increased significantly.The growth potential and fresh dry weight of lettuce seedlings could be increased by adding proper concentration of Si.The photosynthetic pigment content and photosynthetic capacity of lettuce were also improved.At the same time, it could also improve the activity of PSⅡ reaction center and chlorophyll fluorescence parameters.It could be seen that exogenous addition of a certain concentration of Na₂SiO₃ could alleviate the damage of drought stress on lettuce seedlings, improve the ability of lettuce leaves to capture and transform light energy, enhance the photosynthetic efficiency and promote the growth of lettuce.