Physiological response strategies of roots of different alkali-tolerant rice varieties to alkali stress

This study aimed to investigate the physiological differences and coping strategies of the roots of rice varieties with different tolerances to alkali stress. In this experiment, alkali-sensitive rice variety 'Zhonghua 11' and alkali-tolerant rice variety 'Ninggeng 52' were subjected to nine alkali stress using two types of alkaline salts (sodium bicarbonate and sodium carbonate) in three alkali concentration levels (10 mmol·L^-1, 20 mmol·L^-1, and 30 mmol·L^-1) and three pH levels (8.65, 9.55, and 10.50). The effects of alkali stress on the root growth and stress resistance of rice seedlings were examined. Through stepwise regression analysis, the optimal regression equation was established, and the key indices of the response of different rice varieties to alkali stress were screened. The results showed that: 1) the decline of root growth characteristics (total root length, total root surface area, average root diameter, and root volume) and root activity of 'Zhonghua 11' were greater than those of 'Ninggeng 52' (P < 0.05). The lysyl oxidase (LOX), malondialdehyde (MDA), superoxide anion (O₂^·-) producing rate, and hydrogen peroxide (H₂O₂) content in the roots of 'Zhonghua 11' were significantly higher than those of 'Ninggeng 52' (P < 0.05). The content of osmotic adjustment substancessoluble sugar (SS), soluble protein (SP), free proline (Pro), the activities of antioxidant enzymessuperoxide dismutase (SOD), peroxidase (POD), catalase (CAT), reduced ascorbic acid (ASA), and reduced glutathione (GSH) in the roots of 'Zhonghua 11' were lower than those of 'Ninggeng 52'. 2) In 'Zhonghua 11', the average root diameter and root activity of the 20C treatment (20 mmol·L^-1, pH 10.50) were lower than those of the 30A treatment (30 mmol·L^-1, pH 8.65). The MDA content, LOX activity, O₂^·- production rate, and H₂O₂ content of the 20C treatment were significantly higher than those of the 30A treatment, and the damage caused by high pH and a low alkali concentration was greater than that of low pH and a high alkali concentration. 3) Redundancy analysis showed that the increase in sodium ions (Na⁺) and pH was negatively correlated with the increased growth indexes and positively correlated with increases in the physiological indexes for the tested rice varieties. 4) Stepwise regression analysis showed that the ASA, SS, and H₂O₂ contents were sensitive indicators of alkali stress in 'Zhonghua 11'. The average root diameter, total root surface area, root activity, Pro content, SS content, SOD activity, POD activity, and GSH content were the sensitive indexes of alkali stress for 'Ninggeng 52'. In summary, alkali and pH stress affected the growth and physiological characteristics of rice roots of rice. The effects of alkali composition and pH must be considered when cultivating rice in alkaline soil, as different rice varieties initiate different defense strategies against different alkali stresses.