Improved drought tolerance through drought preconditioning associated with changes in antioxidant enzyme activities, gene expression and osmoregulatory solutes accumulation in White clover (Trifolium repens L.)

Improving drought tolerance through drought preconditioning is an important way to understand drought tolerance mechanism in plants. Some limited researches have focused on the gene expression patterns in conjunction with the underlying enzymes promoting drought tolerance and post-drought recovery in white clover. The objective of this study was to identify whether preconditioning-induced drought tolerance is in relation to changes of antioxidant enzyme activities, gene expression and osmoregulatory solutes in white clover. Plants of white clover cultivar, ‘Chuanyin Ladino’ (drought sensitive), were exposed to two cycles of drought preconditioning (non-preconditioned, well-watered plants, as a control) then exposed to drought stress for 11 days and rewatered for 6 days in a growth chamber with 14 h photoperiod at day/night temperature of 21/16 °C, 70 % relative humidity, and 300 μmol m s photosynthetic photon flux density. Results showed that drought preconditioning improved drought tolerance and post-drought recovery in white clover, as demonstrated by significantly lower (p≤0.05) lipid peroxidation, better cell membrane stability and higher relative water content in drought-preconditioned plants as compared to non-preconditioned plants under drought stress and rewatering. Drought-preconditioned plants exhibited higher superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and guaiacol peroxidase activities (POD), as well as higher transcript level of Cu/Zn SOD gene. Meanwhile, more accumulation of soluble sugars and betain and less accumulated proline were also observed in preconditioned plants. Enhanced antioxidant enzyme activities and significantly higher transcript level of Cu/Zn SOD gene may be the critical reasons in acquiring drought tolerance through drought preconditioning. This study also suggests that drought preconditioning improved white clover drought tolerance, which could be related to more accumulation of soluble sugars and betain, while the accumulated proline is associated with the degree of drought stress injury in white clover.