Physiological and biochemical responses of greater yam (Dioscorea alata L.) to gibberellic acid (GA3) during storage

Dioscorea alata cv. Baligun-on follow a regular cycle of growth and dormancy. Yam tubers of various maturities sprouted at almost the same time (i.e., 318 to 331 days) if reckoned from the date of planting but sprouting significantly varied from 33 to 116 days if computed from the date of harvesting. Thus, tubers harvested later sprouted faster than those dug earlier at 30-day intervals increased in tuber weight but not in dry matter content. There was a decline in starch content corresponding with time of sprouting. Tubers harvested seven months after planting exhibited the highest initial respiration rate, weight loss and shriveling along with moderate sugar, starch and protein contents. The sprouting of bulbils treated with Ethrel (up to 10,000 ppm) and CaC2 (up to 100,000 ppm) was neither hastened nor delayed. In contrast, GA3 from 75 to 300 ppm progressively delayed sprouting and reduced weight loss and shriveling. The extension of dormancy was in the range of 16 to 33 percent (bulbils), 27 to 61 percent (less mature tubers), and 94 to 145 percent (mature tubers) and was greatest with 300 ppm GA3 treatment. GA3-treated tubers took longer to form the calyptrae, show purple color, initiate sprouting with lower sprouting coefficients, shorter and fewer sprouts, and a tendency for lower respiration rate particularly during advanced sprout growth. The role of GA3 in the reducing sugar and protein contents was not distinct. At the latter part of the storage period, sugar content was higher in GA3-treated tubers. Starch contents were also higher in tubers soaked in 150 and 300 ppm, GA3 indicating lower degradation due to delayed and slower sprouting. Growth inhibition by batatasin I was greater with increasing GA3 concentration but declined as storage progressed. Though low, GA3 levels was shown to be slightly higher in tubers treated with 150 and 300 ppm GA3 at 30 days after treatment. GA, ABA, and alpha-amylase levels did not appear to correspond with the termination of tuber dormancy. Peroxidase activity increased towards the end of storage. Polyphenol oxidase activity tended to be reduced with GA3 treatment indicating lowered metabolism