Physiological Response of Rhododendron under Alkali Stress and Screening of Alkali Tolerance Indexes

This study explored the response mechanisms of rhododendrons to alkali stress and identified indicators for evaluating alkali tolerance, thus providing technical support for the rapid identification and assessment of alkali tolerance in rhododendrons and for the breeding of new alkali-tolerant cultivars. The 10-month-old cuttings of two rhododendron cultivars with contrasting alkaline tolerances were subjected to alkaline stress treatment using hydroponic cultivation. Changes in their growth performance along with physiological and biochemical parameters were systematically examined. Rhododendron growth was inhibited and biomass was significantly lower under alkali stress. The alkali-sensitive cultivar exhibited symptoms of alkali damage, such as leaf chlorosis and desiccation. Under high alkali stress, quinic acid in rhododendron roots decreased, which promoted the accumulation of citric and malic acids. Alkali stress resulted in a significant increase in root Na + content, while leaf Na + content in the R. Zihe cultivar remained stable and was not significantly different from that in the control. In contrast, leaf Na + content in the R. Kirin cultivar was significantly higher than that in the control following 5 days of high alkali stress. Alkali stress also led to a notable accumulation of malondialdehyde in rhododendron roots. Superoxide dismutase (SOD) activity in R. ‘Kirin’ decreased with increasing stress, whereas it increased significantly in R. ‘Zihe’, indicating intercultivar differences in SOD activity. The degrees of leaf chlorosis, degree of root browning, root biomass, significantly different leaf Na + content compared with that in the control, and the direction of change in SOD activity can be used as indicators when screening alkali-tolerant cultivars of rhododendron. In addition, significant differences in growth and physiological indicators were observed in the two cultivars under high-alkali conditions, thus making this condition suitable for screening alkaline tolerance.