Alterations in sugar metabolism coincide with a transition of wheat seedlings to dehydration intolerance

Wheat seedlings are tolerant to dehydration up to the fourth day of germination. In the following days, appearance of the first leaf from the coleoptile coincides with development of seedling susceptibility to water deficiency. Glucose, at concentration as low as 5 mM, considerably increased survival of the 6-day-old seedlings, whereas mannose at the same concentration significantly decreased survival of either 4- or 6-day-old seedlings. The total relative capacity for sucrose metabolism, estimated as sucrose phosphate synthase (SPS)/sucrose synthase (SS) + acid invertase (AI) ratio was almost twofold higher in 6-day-old seedlings than in 4-day-old seedlings and almost threefold higher in dehydrated and rehydrated older seedlings than in the younger ones. Although hydrolysis of sucrose was also more intensive in older seedlings, the character of the hydrolytic process was different. In the 4-day-old seedlings the activity of SS prevailed, while in the 6-day-old seedlings AI was a dominant hydrolytic enzyme with SS playing a marginal role. These differences were not associated with the activities of hexokinase (HXK) and fructokinase (FK), since changes in the time course of these enzymatic activities and the levels of hexose phosphates were the same, irrespective of the seedling age. Seedling dehydration up to 50% water saturation deficit (WSD) resulted in a transiently increased activity of HXK and FK, but further development of water deficit inhibited the activity of both investigating enzymes almost completely and depleted the contents of glucose-6-phosphate and fructose-6-phosphate to practically negligible level. The observed dehydration-induced inhibition of HXK and FK activity was easily reversible. It is proposed that the lower intensity of invertase-hexokinase pathway of sucrose catabolism and the AI/SS balance associated with higher dehydration tolerance of wheat seedlings seems to be crucial for the regulation of dehydration tolerance level.