We examined changes in nonstructural
carbohydrate biosynthesis and activities of related
enzymes in leaves of micropropagated apple plants (Malus domestica Borkh. cv. 'NaganoFuji') in response to
water stress, with particular emphasis on the
enzymes associated with
sorbitol,
sucrose and
starch metabolism.
Water stress resulted in the accumulation of photosynthates in leaves, mainly
sorbitol,
sucrose,
glucose and
fructose, accompanied by a reduction in
starch concentration. Correlation and path analysis indicated that
water stress affected the partitioning of newly fixed
carbon among terminal products. In response to
water stress,
ADP-glucose-pyrophosphorylase (ADPGPPase) activity decreased, becoming a critical and limiting step in shifting partitioning of photosynthetically fixed
carbon.
Amylase and ADPGPPase affected
sucrose and
sorbitol metabolism, mainly by regulating substrate supply; however, competition for limited substrate had a greater effect on the biosynthesis of
sorbitol than of
sucrose.
Starch metabolism was also strictly regulated by ADPGPPase and
amylase, whereas other related
enzymes were downstream of the pathway for synthesis and degradation of
carbohydrates and thus had relatively little effect on
starch metabolism.
Sorbitol dehydrogenase and
sucrose phosphate synthase were critical regulators of
sorbitol and
sucrose metabolism, respectively.