Temperature Effect on Fructan Oligomer Contents and Fructan-Related Enzyme Activities in Stems of Wheat (Triticum aestivum L.) During Grain Filling

Two 2 m2 stands of wheat were grown under controlled conditions from 2 d after anthesis until harvest. Day and night temperature were 18 and 10 ⚬C respectively in crop 1, whereas they were shifted to 28 and 20 ⚬C 5 d after anthesis in crop 2. CO2 exchange rates of the canopies were continuously measured. Samples of 20 plants were collected from each crop two or three times a week. Grain filling was monitored as were various carbohydrates and enzyme activities in stems. Sugar reserves were highly correlated with the difference between cumulative photosynthesis and dry matter stored in grains. Fructan accumulated up to 150 mg culm-1 2 wk after anthesis in crop 1; but it ceased to accumulate in crop 2 as soon as high temperature was applied, because the increased metabolic rates resulted in higher carbon requirement for grain filling. The fructan was mostly phlein or highly 2-6 linked oligomers with a degree of polymerization up to 15. 6-Kestose was the most abundant, reaching 70 mg culm-1 in crop 1. 1-Kestose content never exceeded 2.5 mg culm-1 and it remained roughly constant from anthesis to harvest. No temperature-related difference in amount of 1-Kestose was noticed. Four kestotetraoses were quantified, 6,6- followed by 1&6-kestotetraose were the most abundant of them. The (2-1) linkages in bulk kestotetraose decreased from 40% of total at anthesis to 10% at maximum grain fresh weight. This chemical shift of the fructan reserves in the two crops was accelerated but not enhanced by elevated temperature. (2-1)-Fructan exohydrolase increased as soon as temperature was enhanced. However, in both crops its activity peaked at maximum grain dry weight only. Sucrose-sucrose fructosyl transferase was constant at 0.1 nkat culm-1 in crop 1, whereas its activity decreased in crop 2 when temperature increased. Little change in invertase activity can be reported in both crops later than 1 wk after anthesis.