RESPONSE OF FRUCTAN TO WATER-DEFICIT IN GROWING LEAVES OF TALL FESCUE

Changes in dry matter and water-soluble carbohydrate components, espec ially fructan, were examined in the basal 25 mm of expanding leaf blad es of tall fescue (Festuca arundinacea Schreb.) to assess their roles in plant response to water deficit. Water was withheld from vegetative plants grown in soil in controlled-environment chambers. As stress pr ogressed, leaf elongation rate decreased sooner in the light period th an it did in the dark period. The decrease in growth rate in the dark period was associated with a decrease in local relative elongation rat es and a shortening of the elongation zone from about 25 mm (control) to 15 mm. Dry matter content of the leaf base increased 23% during str ess, due mainly to increased water-soluble carbohydrate near the ligul e and to increased water-soluble, carbohydrate-free dry matter at dist al positions. Sucrose content increased 258% in the leaf base, but esp ecially (over 4-fold) within 10 mm of the ligule. Hexose content incre ased 187% in the leaf base. Content of total fructan decreased to 69% of control, mostly in regions farther from the ligule. Fructan hydroly sis could account for the hexose accumulated. Stress caused the osmoti c potential to decrease throughout the leaf base, but more toward the ligule. With stress there was 70% less direct contribution of low-degr ee-of-polymerization fructan to osmotic potential in the leaf base, bu t that for sucrose and hexose increased 96 and 67%, respectively. Thus , fructan metabolism is involved but fructan itself contributes only i ndirectly to osmotic adjustment.