Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2004
Publication Date: 1/1/2005
Citation: Mahan, J.R., Wanjura, D.F. 2005. Seasonal patterns of glutathione and ascorbate metabolism in field grown cotton under water stress. Crop Science. 45:193-201. Interpretive Summary: During periods of water stress plants produce harmful chemicals that may negatively affect their subsequent growth and development. Some of these chemicals, oxidants, can damage cells through a process called oxidation. Plants have a variety of mechanisms that help reduce damage caused by oxidation and thus improve the plant's ability to thrive in unfavorable environments. It has been previously proposed that 'antioxidant systems' of plants are not always effective and as a consequence plants are frequently damaged by oxidants. Efforts to analyze limitations in antioxidant systems have not clearly defined the limitations in antioxidant systems under agricultural production conditions. In order to determine the best approaches for enhancing antioxidant metabolism in plants grown under production conditions it is necessary to know when and to what extent antioxidant systems could be limiting. In this study we have monitored the status of several components of antioxidant metabolism from cotton in a effort to observe and define limitations. Cotton was grown under three levels of irrigation; dryland that received a pre-plant irrigation, deficit irrigated that received less than optimal water and, full irrigated that received optimal irrigations. Leaves were sampled from the treatments at various times during the growing seasons and the activitiy of antioxidant enzymes and the amount and form of antioxidants were determined. It was expected that changes in antioxidant metabolism would be observed in response to water stress. While antioxidant metabolism varied over the growing seasons in all treatments, there were no differences that could be attributed to water stress. These findings suggest that cotton plants either have a limited ability to alter antioxidant metabolism in response to oxidative stress, or alternatively that the plants were not experiencing oxidative stresses as would be predicted based on current understanding. These findings suggest that improvement in plant stress resistance by altering antioxidant metabolism may not be as simple as predicted from the scientific literature.
Technical Abstract: The exposure of plants to water deficits can result in the production of reactive oxygen species (ROS) that diminish plant performance. This study determines the variability in antioxidants in cotton under water stress in the field. Irrigation treatments resulted in relatively severe water stresses in both years of the study. In year 1 glutathione content varied 2-fold and the reduced form was predominant. The glutathione content in year 2 was relatively constant in all three water treatments. The ratio of reduced/oxidized glutathione did not vary with stress with the exception of an increase in the oxidized form at mid-season in the dryland treatment. The deficit irrigation treatment did not respond to water deficit. Glutathione reductase activity increased more than 2-fold from early to late season and there was no water effect. Ascorbate varied 4-fold seasonally with no effect of water. Ascorbate peroxidase rose more than 3-fold from early to late season. No differences were associated with the water. Malondialdehyde rose almost 2-fold in season with no effect of water deficit.The absence of elevated malondialdehyde due to water deficits coupled with the failure to detect stress related variation in antioxidants suggest that it was sufficient in all water levels. The variation in antioxidant metabolism over time but not in response to water deficits suggests that improvement of plant performance through the modification of antioxidant metabolism may be of limited value.