Any environmental variable, which can induce a potentially injurious strain in plants, is known as stress. It may be Biotic stresses which include animals, weeds, pathogens (fungi, bacteria, virus,) or a-biotic stresses which may be due to water, salinity, temperature, radiation, heavy metals accumulation etc. which adversely the plant growth and productivity. Although one third of the total land area is considered as potentially suitable for arable agriculture, only 10% of the world 13 billion hectares is cultivated. The average yield from the major crops may reduce by more than 50% owing to stresses. By 2030, global cereal demand for food and animal feed alone is expected to be 2.8 billion tonnes per year, or 50% higher than in 2000. Various form of the a-biotic stress limit production on most of the world’s 1.4 billion ha of cultivated land.
Water comprises 80-90% of the biomass of the non-woody plants, it is the central molecule in all physiological processes of plants by being the major medium for transporting metabolites and nutrients. Drought and flooding can cause stress for plants. In the drought condition less availability of the water to plants for its normal functioning, and in the flooding condition roots cannot uptake water due to suffocation. The modern scientific study of drought tolerance started in 1702 with Anthony Von Leeuwenhoeks discovery of the survival of rotifers without water for month.
Water stress effect the plant on different stages. Photosynthesis is particularly sensitive with respect to water deficiency. Plants resistance to water deficiency results in metabolic changes along with functional and structural rearrangements of photosynthesizing apparatus. Photosynthesis of higher plants decreases with the reduction in the relative water content (RWC) and leaf water potential. And increase in the photosynthesis with the increase in RWC upto certain limit. Because rate of transpiration also increase with the increase RWC. Lower photosynthesis rate in a usual effect of water stress in plants and has been attributed primarily to stomatal limitation and secondary to metabolic impairment.
The photosynthesis rate of leaves in both C3 and C4 plants decrease under the drought conditions. Photosynthesis in C4 plants is more sensitive to water stress such as corn ( zea mays L.) and C3 plants is less susceptible to water deficiency such as wheat. C4 plants are in hot, arid region areas prone to frequent drought. The process of photosynthesis is take place in the chloroplast, which contain green pigment known as chlorophyll. The rate of chlorophyll increases the rate of photosynthesis. Decrease in chlorophyll content of leaves under water stress is well known. Water stress inhibit chlorophyll synthesis at four different stages.
- The formation of 5-aminole-vulinuc acid (ALA)
- ALA condensation into porphobilinogen and primary tetrapyrrol, which is transformed into protochlorophyllide.
- Light-dependent conversion of protochlorophyllide into chlorophyllide.
- Synthesis of chlorophyll a and b along with their inclusion into developing pigment-protein.
Water deficiency reduce the supply of carbon dioxide from the environment due to closure of stomata. And closure of stomata directly affects the rate of photosynthesis. Due to the closure of stomata reactive oxygen species are produced. In addition to RuBisCO, water stress can reduce activity of other photosynthetic enzymes to different extends such as NADP dependent glyceraldehyde phosphate dehydrogenase, phosphoenolpyruvate carboxylase and sucrose phosphate synthase.
Water stress has negative effects on dark reaction of photosynthesis. It stress also disturb the cyclic and non-cyclic type of electron transport chain. The disruption is clearer in the transfer of electron from P700. ATP formed as well as NADP+ reduction. Photosystem-I (PS I) of some plants are more severely damaged compared to photosystem-II (PS II).
Muhammad Shazab Awan, Muhammad Wajid Khan, Mubasshir Sohail, Rashad Mukhtar Balal, Muhammad Adnan Shahid, Muhammad Zubair and Usama Asad
Faculty of Agriculture, University College of Agriculture, University of Sargodha