The effect of polyethylene glycol on seed germination of wheat (Triticum aestivum L.) genotypes/lines
S.M. Mujtaba, B. Khanzada, Muhammad Ali, Mazhar H. Naqvi, Sajida Mughal, S.M. Alam, M.U. Shirazi, M.A. Khan and S. Mumtaz.
Nuclear Institute of Agriculture (NIA), Tandojam-70060, Sindh, Pakistan
Water is generally considered as one of the limiting factors which affects the physiological and biochemical processes affecting crop productivity (Osborne et al. 2002). Water stress impairs seed germination, retards plant development and reduces crop yield (Bradford 1994). It also affects several morphological, biochemical alterations in plants ultimately leading to a massive loss in yield. If agriculture is to feed the world's burgeoning population, yields of water-limited crops must be improved substantially.
Seed germination is considered to be the most critical stage especially under stress conditions. The first requirement of seed germination is water for hydrolysis of reserves, as a medium of translocation and hydration of enzymes, for operational conformation of cell membranes, organelles and finally to provide the driving force for cell expansion reduced by germination. During germination, biochemical changes take place, which provides the basic framework for subsequent growth and development. The initial metabolic changes that occur immediately after the imbibition of water are the increase in the hydrolytic enzymes such as alpha amylase and protease. Alpha-amylase is an important starch degrading enzyme in the endosperm of cereal grains. The reaction products provide substrate and an energy source for the embryo during germination (Ahmed and Bolton 1988). Hydrolysis and solubilization of starch during seed germination is crucial because of the activity of starch hydrolyzing enzyme alpha-amylase. The synthesis of this enzyme during germination is regulated by gibberellic acid (Chrispeels and Varner 1976) and hence starch hydrolysis. Inhibition of seed germination of crop plants is also due to disturbance in the activities of peroxidase, alpha-amylase and acid-phosphatase (Murata et al. 1968). It has been suggested the phenolics as the cause of inhibition of metabolic process during germination (Rice 1984). Generally, the phenolic compounds caused interference with IAA metabolism, mitochondrial function, synthesis of protein and ion uptake and transport (Einhelling 1986).
Drought stress can affect seed germination through osmotic effects (Welbaum et al. 1990) or by ionic toxicity (Bliss et al. 1986; Huang and Reddman 1995). Physiological studies to distinguish between the two effects are limited (Bliss et al. 1986), but evidence suggests that low water potential of the germination medium is a major limiting factor (Bradford 1994). In the present study, the drought stress delayed germination in all genotypes/lines to varying degrees.