Abstract:

Reaumuria soongorica is an important and extremely drought-resistant tree species found in arid and semi-arid desert ecosystems in the Northwest of China. R. soongorica plays an important role in maintaining the stability of plant communities in semi-desert environments. In recent years， because of frequent occurrence of extreme drought conditions， R. soongorica desert plant communities have sustained large areas of death and are exhibiting degradation. To counter this trend， adoption of artificial measures to improve the drought resistance of plants in arid desert areas may be helpful. In this study， 5 proline （Pro） concentrations （50， 100， 150， 200， 250 mg·L^-1） were applied as a spray treatment to R. soongorica seedlings to investigate the extent of alleviation of damage caused by drought stress. Antioxidant enzyme activities， metabolic regulatory indicators， photosynthetic pigment levels and photosynthetic performance in leaves of R. soongorica seedlings were measured on the day before Pro treatments （day 0）， and on the 1st， 3rd， 6th and 9th days after Pro treatments. A growth index of R. soongorica seedlings was compiled on the 9th day after Pro treatments， in order to elucidate the mechanism of action of Pro protection of plants under drought stress. It was found that the various exogenous Pro treatments had significant effects on the antioxidant enzyme system， metabolic regulation system， photosynthetic pigment levels， photosynthetic performance and growth status of R. soongorica seedlings. Among the measured traits， significant differences between year， among times of spraying， among Pro concentrations， and the interaction between treatment concentration and treatment time were detected. However， chlorophyll a/b （Chl a/b） showed no significant difference among treatment times， and soluble sugar （SS） showed no significant difference among treatment concentrations. The 100 mg·L^-1 Pro treatment had the best overall effect. Soluble protein （SP）， Pro， catalase activity （CAT） and peroxidase activity （POD） on the 9th day after the 100 mg·L^-1 Pro treatment were increased by 4.48%， 131.07%， 30.66% and 12.30%， respectively. SS levels on the 9th day after the 100 mg·L^-1 Pro treatment were decreased by 6.42%， and superoxide dismutase activity （SOD） was decreased by 1.95%. Photosynthetic pigment concentrations ［Chl （a+b）， Chl a， Chl b］ on the 9th day after the 100 mg·L^-1 Pro treatment were decreased by 5.81%， 1.47% and 22.22%， respectively， compared with the results for day 0， and these reductions were the smallest among the five Pro treatments tested. The photosynthetic traits： net photosynthetic rate， stomatal conductance， transpiration rate， intercellular CO₂ concentration and water use efficiency on the 9th day after Pro treatment were decreased by 16.07%， 10.00%， 6.44%， 7.68% and 9.92%， respectively， compared with results for day 0. On the 9th day after treatment， the mean plant height for 100 mg·L^-1 Pro treatment was 2.48 cm， the mean number of side buds was 3.67， the mean growth of side buds was 1.87 cm， and the mean stem diameter at ground level was 0.27 mm； these values were， respectively， 0.68 cm， 0.34， 0.32 cm， and 0.07 mm higher than those of the 250 mg·L^-1 treatment， respectively. Therefore， it was concluded that the 100 mg·L^-1 Pro treatment significantly improved the resistance of R. soongorica plants to drought stress， compared with other Pro rates tested.

Key words: exogenous proline, drought stress, Reaumuria soongorica, physiological property, drought relief mechanism