Effect of lead on carbohydrate mobilization in oat seeds during germination

In response to various concentrations of lead, the total amylolytic activity in the endosperm or embryonic axis of oat was suppressed 33 percent and 11 percent respectively while increased as germination advances towards 12 days at all treatment levels. Suppression of enzyme activity suggested that lead tended to retard the initial rise of enzyme activity. The results on starch utilization show reduced hydrolysis of major carbohydrate reserves in endosperm. Same trend was observed in the developing embryonic axis. It is suggested that seed metabolism is adversely affected by lead. @ JASEM Heavy metal pollution has serious consequences for plant because of their toxicity, long persistence, bio accumulation and biomagnficaton in food chain (Whitton, 1970; Meagher, 2000; Raskin and Ensley, 2000). Cd, Ni and Pb toxicity to plant growth has been well documented in maize (Osuji et al., 1998), pea (Bansal, 2001), rye grass (Allinson and Dziala, 1981), Eichhornia (Odjegba and Fasidi, 2006) and in tomato (Khan and Khan, 1983; Jaja, E.T. and Odvemena CSI, 2004). Heavy metals including Pb (lead) have a profound deleterious effect on metabolism including reserve mobilization of germinating seeds. One of the crucial events during seed germination entails mobilization of seed reserves because it supplies substrate for functioning of different metabolic processes which are vital for growth of embryonic axis (Bewley, 1997). Many workers (Bansal, 2001; Deep et al., 2002; Mishra and Choudhuri, 1997) reported the toxic effects of lead on various aspects of seed germination. Therefore, it was of curiosity to examine the influence of lead on mobilization of starch, which happens to be a major reserve substrate during germination. MATERIALS AND METHODS The experiment were conducted on oat (Avena sativa L.) CV HJ-8 seeds procured from Forage Section, Department of Plant Breeding, CCS Haryana Agricultural University, Hisar, India. Seeds of uniform size and colour were selected and surface sterilized with 0.2 percent (w/v) mercuric chloride solution for one minute, washed thoroughly several times with glass distilled water and kept for germination in petriplates (15cm diameter) lines with two layers of whatmann No. 1 filter paper, C in a B.O.D. incubator maintained at 25oC±1oC. Seedling were irrigated with distilled water (control) or aqueous solutions of various concentrations (25, 50, 100, 200, 300 or 400 ppm) of lead. Seedlings were withdrawn at three days interval thoroughly washed with distilled water, endosperm and embryonic axis were separated, dried in an oven at 70oC until a constant weight was observed, ground to a fine powder and stored in a desicator over anhydrous calcium chloride. Soluble sugars were extracted (Marbach and Mayer, 1976) and estimated by Anthrone reagent method (Hedge and Hofreiter, 1962). Reducing sugars were estimated by Nelson’s method (Nelson, 1994). Total, α and β amylase activity were assayed (Swain-Dekker, 1966) and liberated maltose was estimated (Yomo-Varner, 1973) in the dialyzed extract. RESULT AND DISCUSSION A 21 percent retardation in starch utilization was recorded in endosperm of seeds germinated in 25 ppm lead solution while 33, 48, 67, 75 and 94 per cent retardation was observed in 50, 100, 200, 300 and 400 ppm lead treatment after 3 days of germination as compare to that of control (Table 1). This retardation was found to be concentration dependent. Approximately 3.64 mg of starch was utilized in endosperm sown in distilled water. Similar decrease in utilization of starch was also observed on other days after germination. Amount of starch increased with passage of time in embryonic axis upto 12 days after germination in all the treatments. The amount of starch in embryonic axis of control seeds between 3 and 12 days was 3.28 mg. After 12 days of germination, starch accumulation was considerably inhibited (39 percent) in embryonic axis when germinated in solution containing 400 ppm lead. After 3 days of germination, amount of starch in the embryonic axis of seeds germinated in 25 ppm lead was decreased by 29 percent as compared to the control. Similarly 30, 33, 41, 49 and 59 percent depression in starch accumulation in embryonic axis was registered in response to 50, 100, 200, 300 and 400 ppm of lead respectively. Curtailed degradation of starch in response to heavy metal stress was proposed by Mishra and Choudhuri (1997). They recorded a greater dry mass of endosperm tissues of rice seeds germinated in solution of either lead or mercury and suggested that starch in the endosperm is not much solubilized by amylase and could not be consumed by the embryo. Effect of lead on carbohydrate mobilization in oat seeds during germination * Corresponding author: Gupta, K 30 Table 1: Effect of lead on starch content in endosperm and embryonic axis of germinating oat seeds Conc. of lead (ppm) mg starch /endosperm* Days after germination mg starch/embryonic axis** Days after germination 3 6 9 12 3 6 9 12 0 8.240±0.17 6.110±0.08 3.756±0.05 1.586±0.02 1.154±0.007 2.226±0.003 3.114±0.018 4.434±0.007 25 9.236±0.14