Activities of antioxidant and osmoprotective systems and photosynthetic gas exchange in maize seedlings under drought conditions

The influence of progressive soil drought (5 and 7 days) on photosynthetic CO₂/H₂O gas exchange, activities of ascorbate peroxidase (AscP) and glutathione reductase (GR), the contents of ascorbate (Asc), glutathione, soluble carbohydrates, proline, chlorophyll, and malondialdehyde (MDA) was investigated on leaves of maize seedlings (Zea mays L., cv. Troinaya sladost). Watering of plants destined for drought treatment was stopped on the 8th day after shoot emergence. On the first stage of drought (5 days) the activities of AscP, GR and the content of Asc, glutathione, MDA, and chlorophyll in leaves of drought-treated plants did not exhibit appreciable changes. The first plant response to water shortage consisted in the increased content of reducing sugars (glucose and fructose) and proline by 3.5 and 4.5 times, respectively. Despite insignificant decrease in transpiration, the photosynthetic CO₂exchange remained at the control level. After 7-day drought, the Asc content and AscP activity increased by a factor of 1.3 and 1.2, respectively, indicating the activation of the ascorbate/glutathione cycle. The content of glutathione and GR activity after 7-day drought remained unchanged. The content of reducing sugars doubled compared to control values, and the content of sucrose increased by a factor of 1.5. The proline content increased ninefold, whereas the content of chlorophyll and MDA remained unchanged. The photosynthetic rates decreased twofold, but the rate of dark respiration was unaffected. The results have shown that the drought-related recruitment of defense mechanisms follows a certain sequence. On the first stage of drought (5 days) the increase in carbohydrate and proline content ensured functioning of osmotic and antioxidant systems required for photosynthesis. On the second stage (7 days) the effectiveness of antioxidant systems increased notably, owing to the increase in AscP activity and the build-up of Asc, soluble carbohydrate, and proline content. Thus, the sequential mobilization of defense mechanisms in response to aggravating leaf water deficit sufficed to prevent oxidative stress.