Abstract:

In order to determine the suitable time for 5-aminolevulinic acid（5-ALA）application to alleviate salinity injury in strawberry（Fragaria × ananassa Duch.‘Benihoppe’），exogenous 10 mg ? L-1 5-ALA solution was rhizospherically applied at 5 days before，simultaneously or 3 days after 100 mmol ? L-1 NaCl treatment，and the results showed that the alleviating effect was best in the pretreatment. This result implies that 5-ALA is more preventive than therapeutic for strawberry against salt stress. After then，another experiment was carried out，where 10 mg ? L-1 5-ALA solutions were rhizospherically pretreated to strawberry plants 5，10，15 or 20 days before NaCl treatment and the result showed that the pretreatment prior 20 d was the best effective treatment，which means that the earlier the strawberry was pretreated with 5-ALA，the less of salinity injury was found. It was observed that the possible alleviating mechanisms of 5-ALA against salt stress in strawberry may include the follows. (1) 5-ALA alleviated the decrease of leaf chlorophyll content and Chl.b/a ratio induced by salinity in strawberry. (2) 5-ALA induced the additional increase of antioxidant enzyme activities and decreased the production rate of superoxide anion in the leaves and roots of strawberry under salt stress. (3) 5-ALA was beneficial to intersystem electron transport from PSⅡ to PSⅠ，increasing the energy conversion efficiency during light energy absorption，harvest and transport，reducing heat dissipation，and improving photosynthetic capacity under salt stress. (4) 5-ALA alleviated the inhibition of plant growth induced by salt stress，including shoots and roots，especially the latter.

Key words: strawberry, 5-ALA；salt stress, chlorophyll fluorescence, reactive oxygen metabolism, time effect