Aims Two-year-old seedlings of Phoebe zhennan were used in this study to explore the responses of osmotic adjustment and active oxygen metabolism to drought stress and the mitigation effect of nitrogen application.
Methods The soil water content was firstly adjusted to four treatment levels, i.e. 80% of field water holding capacity (80% FC), 50% FC, 30% FC and 15% FC, respectively. The physiological variables of plants were measured after one week, and then three nitrogen application rates, control (N0), medium nitrogen (MN) and high nitrogen (HN) were performed at an interval of 7 days for four times (7 d, 14 d, 21 d and 28 d, respectively). The same physiological variables were determined again one month after the accomplishment of nitrogen application.
Important findings 1) The free proline (Pro) and soluble sugar (SS) contents in the leaves increased significantly with the aggravation of drought stress after 7 days of drought, but the content of soluble protein (SP) was firstly increased and then declined. The increase of Pro content was especially obvious under severe drought (15% FC). After nitrogen application, the content of Pro raise further, but the values varied in drought treatment. The SS contents under sufficient water supply (80% FC) and mild drought (50% FC) were decreased by MN, but it did not change significantly when supplied with HN despite the soil water content. After nitrogen application, the SP contents under 80% FC and 50% FC were lower than those of no exogenous N, while they were opposite response under 30% FC and 15% FC. 2) Before nitrogen application, with the aggravation of drought stress, the hydrogen peroxide (H2O2) content, superoxide dismutase (SOD) activity, catalase (CAT) activity increased significantly, and the peroxidase (POD) activity showed an up-down trend. After nitrogen application, the content of H2O2 was generally deceased at each water condition, with the maximum decrease at MN, while the HN treatment was not conducive to reduce the content of H2O2. The activities of three kinds of enzymes responded differently to the severity of drought and the level of nitrogen application. 3) Before nitrogen application, the content of malondialdehyde (MDA) in leaves increased significantly when the soil water content declined to and below 50% FC. The relative electrical conductivity (REC) was decreased at first, and followed by significant increase. Except severe drought (15% FC) stress, the MDA content showed a decreasing trend at MN, but a rebound at HN. As regards severe drought stress, however, the content of MDA increased at both MN and HN, indicating that nitrogen application is not a good choice to alleviate the damage caused by severe drought stress. 4)Two-factor ANOVA revealed an obvious interaction between nitrogen application and drought stress. In conclusion, a proper amount of nitrogen (1.35 g·a -1 for each sapling) could somewhat alleviate drought stress no severer than 15% FC on seedlings of Phoebe zhennan, but excessive nitrogen at rate of or more than 2.70 g·a -1 per sapling is not recommended.