Sucrose metabolism and cellulose biosynthesis in sunflower hypocotyls.

The relationships between cellulose accumulation, changes in specific activities of enzymes of sucrose catabolism, levels of UDP-glucose and rate of dark respiration were investigated in the subapical 1 cm-hypocotyl region of 10- to 14-day-old-sunflower seedlings (Helianthus annuus L). The plants were grown under a light/dark regime in vermiculite that was soaked either with distilled water or half-strength Hoagland nutrient solution. At this stage of seedling development, the hypocotyl had ceased to elongate but increased in width. Stem thickening and the rate of cellulose accumulation were promoted by nutrient solution. The levels of the soluble (vacuolar) and wall-associated acid invertases (EC 3.2.1.26) were not correlated with these processes. However, the activities of the soluble (cytoplasmic) and membrane-bound sucrose synthases (EC 2.4.1.13) were larger in hypocotyls that were grown in the presence of nutrient solution. The concentration of UDP-glucose was reduced, and the rate of dark respiration was enhanced in the hypocotyls that were grown in Hoagland solution. The results support the hypothesis that both forms of the enzyme sucrose synthase play a critical role in cellulose biosynthesis of hypocotyl cells that had ceased to elongate and continue to grow by wall thickening.