WATER STRESS IN PLANT

WATER STRESS IN PLANT

Water stress is known to be one of the major constraints of agricultural productivity as most of the cultivation of our country is based on rain fed irrigation. Just as water scarcity leads to stress in plants, excess water is also a potential cause of stress. Excess water or water logging is mainly caused due to improper drainage and cultivation in low lying areas. Cultivation of maize is often seriously hampered by water logging conditions and the plant responds to this stress by an alternation of its metabolic processes.

Under water logging condition, the most common and immediate injury symptom was leaf yellowing that initiated from the base of the plants and proceeded towards the tip (Barnwal et al., 1999). With severity longevity of the stress condition, the yellow leaves get necrotic spots and finally the leaves along with leaf sheath get dried. Quantification of this response was undertaken in the present study, which clearly showed that the stress condition significantly reduced chlorophyll a, b and total chlorophyll content. It was noticed that chlorophyll b was comparatively more sensitive to water logging than chlorophyll a. Due to high sensitivity of chlorophyll b, the chlorophyll a/b ratio increased under the stress condition. Increase in chlorophyll a/b ratio was observed by Sinha et al.,1995 in 30 day old maize seedlings with 24-72 hr flooding.

Chlorophyll breakdown was positively correlated with super-oxide radical (O₂^-.) production in the leaves of water logged maize plants. There was a good generic variability with regard to sensitively of chlorophyll contents of water logging condition, which may be related to variability in super-oxide dismutase (SOD) activity in leaves and leaf sheath (Yan et al.,1995).

The impact of water logging stress was very remarkable on soluble sugar content in different plant parts. In the present study, an increase in the sugar content was recorded when the seedlings were subjected to water logging stress. Accumulation of soluble sugar was comparatively more in stem tissues than leaf. Schluter et al., 1996 suggest that high sugar accumulating under normal condition and ability to maintain the content under the stress, may be a desirable attribute for water logging tolerance in maize. Maintenance of high carbohydrate levels, in spite of stress condition, may be of vital importance for maintaining or recovery of growth when stress is released. Positive correlation between carbohydrate content and survival or tolerance during water logging condition has been suggested by many researchers (Palada and Vargara, 1972; Emes et al.,1987; Chaturvedi et al., 1995; Singh et al., 1998).

Analogous to total soluble sugar, the starch content in different plant parts was also affected severely due to water logging stress. This inhibition was possibly due to reduced current photosynthesis under the water logging (Lizaso and Ritchie, 1997) and increased utilization of existing starch content in maintenance respiration. Degradation of starch during water logging is a crucial biochemical event which affects survival and growth of plants under the stressful situation. Setter et al., 1989 observed 4-10 a fold reduction in starches in all plant parts of rice under flooding condition. The genotypes having high starch content under normal condition and maintained the concentration under stressful condition also, showed less injury and quick recovery growth after release of the stress (Singh et al., 1998). Data presented here demonstrated that during the initial stages of flooding stress (upto 72 hr) starch contents in leaves significantly decreased which may be due to utilization of stored carbohydrate to maintain respiration. But after prolonged stress condition (at 96 hr treatment) its contents become significantly increased due to mobilization of starch from root to leaf.

Stress caused an increase in the contents of proline (Cicek and Cakirlar, 2008). Many functions have been postulated for proline accumulation in high tissues. Proline could be involved in the osmotic adjustment of plants (Gzik, 1996) and could also be a protective agent of enzymes and membranes (Bandurska, 1993). Such a function coincided with the results associating proline content.

In order to determine the level of oxidative stress and membrane damage caused by waterlogging stress, in vivo H₂O₂ content was examined. Ahsan et al.,2007 showed that a higher amount of H₂O₂ had accumulated in waterlogged roots compared with the controls. Accumulation of higher amounts of H₂O₂ compared with those observed in their previous study on tomato. Hydrogen peroxide is the first stable compound among the ROS produced in the plant cell under normal conditions and as a result of various stresses. It has been reported that significant amounts of ROS such as O₂ and H₂O₂ are generated by waterlogging stress(Yan et al., 1996; Ahsan et al., 2007).

Exposure of plants to unfavourable environmental conditions such as water-stress (hypoxia/anoxia) can increase the production of reactive oxygen species (ROS) such as singlet oxygen (¹O₂), superoxide radical (O₂^-.), hydrogen peroxide (H₂O₂) and hydroxyl radical (OH^.) (Asada 1999; Dat et al., 2000). Plants possess both enzymatic and non-enzymatic mechanisms for scavenging of ROS. The enzymatic mechanisms are designated to minimize the concentration of O₂^-. and H₂O₂. In the present study, the activities of a number of antioxidative enzymes were assayed to determine their roles during different periods of water logging. Among the enzyme activities tested in this study only superoxide dismutase (SOD) registered a significant decrease, whereas other four antioxidative enzymes (i.e., catalase, peroxidase, ascorbate peroxidase and glutathione reductase) showed a remarkable increase in its activity when treated with water-stress.

Scavenging of super oxide radical (O₂^-.) by superoxide dismutase (SOD) results in the production of H₂O₂ which is removed by ascorbate peroxidase (APOX) or catalase. Thus the generation of active oxygen species, particularly H₂O₂ during water logging stress may be a part of signalling cascade leading protection from stress. Increased peroxidase activity has been seen due to flooding stress and was suggested to assist in the reduction of ROS (Lin et al., 2004). The high activity of peroxidase and expression of some newly synthesized proteins may be directly involved with plant’s defence and enhanced tolerance to oxidative stress induced by elevated stress periods.

The result of present study is inconsistent with that of Shi et al. (2008), who showed a drastic decrease in cytosolic ascorbate peroxidase activity under flooding stress in germinated soybean seedlings. An excessive accumulation of superoxide due to the reduced activity of SOD under flooding stress was shown (Yan et al.,1996). Results of the present study reveal the same. The intracellular level of H₂O₂ is regulated by a wide range of enzymes, the most important being catalase (Willekens et al.,1995). In rice seedlings SOD activities responded non-significantly to submergence, while catalase activity increased upon re-admission of oxygen (Ushimaru et al.,1999). The activities of glutathione reductase (GR) decreased slightly or remained unaltered under hypoxia, while anoxia caused a significant inhibition of enzyme activities (Biemelt et al.,1998). Inhibition of GR, ascorbate peroxidase (APX), CAT and SOD activities has been shown also by Yan et al., 1996 in corn leaves under prolonged flooding, while a short-term treatment led to an increase in the activities. Results of the present study revealed the same as in case of GR. Prolongation of the anoxic treatment led to a decline in the antioxidant level, both reduced and oxidized forms, in all plants tested.

Non-enzymatic components that were studied included ascorbate and carotenoids which are known also possess antioxidative activities. Ascorbate is one of the most studied and powerful antioxidants which can directly scavenge superoxide, hydroxyl radicals and singlet oxygen and reduce H₂O₂ to water via ascorbate peroxidase reaction (Noctor and Foyer, 1998). Results of that investigation showed that the contents of ascorbate and carotenoids was significantly increased during the different periods of stress.