Effects of silicon on photosynthetic and fluorescence characteristics oftomato leaves under drought stress

【Objectives】 The study of the effect of different silicon supplement levels on the physiological parameters and the growth of tomato(Lycopersicon esculentum)seedlings exposure to drought stress will provide a theoretical basis for understanding the mechanism of drought stress alleviation with Si.【Methods】Tomato(Lycopersicon esculentum) cultivar ‘jinpeng.1#’ was used as tested material, and a hydroponic culture based on Hoagland solution with different silicon levels was conducted. Drought stress was simulated by addition of 1% polyethylene glycol(PEG-6000)for this experiment. Na2SiO3·9H2O was used as the silicon source, 0(CK), 0.6(T1), 1.2(T2), 1.8(T3)mmol/L of Na2SiO3·9H2O were contained in the Hoagland solution plus 1% PEG-6000, and none of PEG and Na2SiO3·9H2O was CK0. The pigment content of tomato seedlings, water status, gas exchange parameters and chlorophyll fluorescence parameters were investigated. 【Results】Compared with CK0, the relative water content(RWC),the photosynthetic pigments, net photosynthetic rate(Pn), stomatal conductance(Gs), maximum photochemical efficiency(Fv/Fm), actual photochemical efficiency(ΦPSⅡ), and photochemical quenching(qP)of tomato seedlings treated with different silicon levels all showed steadily declining with extension of drought time, except non-photochemical quenching(NPQ). The stomatal limitation value(Ls)first increased and afterwards decreased. However, the intercellular carbon dioxide concentration(Ci)first decreased and afterwards increased. The differences in both the increasing amplitude and decline range of the above parameters were significant among the treatments exposured to different silicon concentrations. In contrast to tomato plant without silicon(CK), in treatments of Si 0.6 and 1.2 mmol/L on the day 12, the increments in RWC were 18.03% and 30.25%, in the chlorophyll content were 64.56%, 88.24%, in Pn were 48.78%, 131.71%, in ΦPS were 31.68%, 62.70%, in qP were 18.92%, 40.54%, and NPQ were decreased by 9.54%, 13.35%, respectively. While in treatment of Si 1.8 mmol/L, the corresponding parameters were significantly lower than CK, except NPQ. The decrements in RWC, chlorophyll content, Pn, ΦPSⅡ and qP were respectively 17.53%, 21.79%, 21.95%, 10.16% and 5.41%. 【Conclusions】 Under 1% of PEG-6000 drought stress, the addition of silicon 1.2 mmol/L in the Hoagland solution could significantly improve the water status of tomato leaves, reduce the degradation of photosynthetic pigment, improve photochemical efficiency in the leaves pigment, and reduce photoinhibition. Consequently, the tomato seedlings are able to maintain high photosynthetic rate and regular growth of ‘jinpeng 1#’ tomato.