Effects of exogenous abscisic acid pre-treatment of cucumber (Cucumis sativus) seeds on seedling growth and water-stress tolerance

Abstract

The effect of treating cucumber seeds (Cucumis sativus ‘Zhongnong 12’) with exogenous abscisic acid (ABA) on seedling growth and various plant characteristics, including chlorophyll a fluorescence, net photosynthesis, antioxidant enzyme activity and water-stress tolerance under dehydration was investigated. The results showed that the photosynthesis rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and the apparent quantum yield of photosynthesis (AQY) all declined gradually, but the decline in ABA seed pre-treated seedlings (ASPS) was less than in non-ABA seed pre-treated seedlings (NASPS), especially for Pn during dehydration. Water use efficiency (WUE) was observed in ASPS, which was three times greater than that in NASPS, implying that the corresponding increase in WUE, calculated on the basis of water uptake, was the result of increased Pn and a reduction in transpiration in ASPS. During dehydration, the levels of electron transport rate (ETR), photochemical efficiency of PSII (Fv/Fm), photochemical quenching (qP), quantum efficiency of PSII photochemistry (ΦPSII) and the ratio of photochemicals (Pc) all decreased; in particular, the decline in NASPS was more than that in ASPS. The levels of non-photochemical quench (NPQ), ratio of antenna heat dissipation (Hd) and ratio of heat dissipation in PSII (Ex) all increased more in NASPS than in ASPS, suggesting that ABA could protect cucumber seedlings from the damage of photoinhibition by improved heat dissipation. Dehydration caused an increase in antioxidant enzyme activities in the first two days, especially in ASPS, and then declined slightly. In comparison with NASPS, ASPS showed a marked decrease in malondialdehyde (MDA) content during dehydration. On the other hand, rehydration resulted in nearly 100% recovery of Pn, Gs and Tr in ASPS, as well as chlorophyll fluorescence parameters, but only a small recovery in NASPS. These results suggest that imbibition of cucumber seeds with ABA protected cucumber seedlings from water-loss damage and increased their resistance or tolerance of dehydration, especially improving their WUE by improved Pn rather than by reduced water uptake under dehydration. The beneficial effect of exogenous ABA applications on reducing susceptibility to gradual dehydration of cucumber seedlings was confirmed in our study by applying ABA to imbibition cucumber seeds.