Carbohydrate metabolism in Nerine sarniensis bulbs developing in liquid culture

Abstract

Anatomical and physiological changes accompanying enhanced Nerine sarniensis cv. Salmon Supreme bulb growth in vitro were examined. Small bulbs, 2--3 mm in diameter, grown in vitro on a semi-solid medium were subcultured to liquid medium with elevated levels of sucrose (Suc) and inorganic phosphate. Bulbs' fresh and dry weights, carbohydrate contents and the activities of enzymes related to carbohydrate metabolism were determined at different stages of bulb development. Starch was the dominant storage carbohydrate in these bulbs, and the leaf bases parenchyma cells were the principal storage tissue. During the first month of bulb growth, only small changes in starch content were detected. However, an increase in starch level was observed at later stages of development. The activity of ADP-glucose pyrophosphorylase (EC 2.7.7.27), a key enzyme of starch synthesis, increased just before the increase in starch accumulation. Sucrose was the dominant soluble sugar in the bulbs, only traces of glucose and fructose were detected. The activity of alkaline invertase (INV, EC 3.2.1.26) was higher than that of acid INV during the growth period. Sucrose synthase (EC 2.4.1.13) exhibited the highest Suc-degrading activity during bulb growth. Suc was hydrolyzed in the medium by the cell wall bound acid INV during the growing period. The results are discussed in relation to enhanced nerine bulb growth and development in vitro.