APOPLASTIC SUGARS, FRUCTANS, FRUCTAN EXOHYDROLASE, AND INVERTASE IN WINTER OAT - RESPONSES TO 2ND-PHASE COLD HARDENING

Changes in apoplastic carbohydrate concentrations and activities of ca rbohydrate-degrading enzymes were determined in crown tissues of oat ( Avena sativa L., cv Wintok) during cold hardening. During second-phase hardening (-3 degrees C for 3 d) levels of fructan, sucrose, glucose, and fructose in the apoplast increased significantly above that in no nhardened and first-phase-hardened plants. The extent of the increase in apoplastic fructan during second-phase hardening varied with the de gree of fructan polymerization (DP) (e.g. DP3 and DP4 increased to a g reater extent than DP7 and DP > 7). Activities of invertase and fructa n exohydrolase in the crown apoplast increased approximately 4-fold ov er nonhardened and first-phase-hardened plants. Apoplastic fluid extra cted from nonhardened, first-phase-hardened, and second-phase-hardened crown tissues had low levels, of symplastic contamination, as determi ned by malate dehydrogenase activity. The significance of these result s in relation to increases in freezing tolerance from second-phase har dening is discussed.