The influence of water stress on starch production
seems to be dependent on the growth stage of
the plant. Mature plants are able to adapt by
initiating regulatory mechanisms enabling them to
withstand prolonged stress conditions. By reducing
leaf canopy and establishing a root system to
utilize deep soil water, older plants are able to
maintain photosynthetic activity. The plants, after
surviving the stress conditions, will recover
quickly by forming new leaves, the energy for this
process is obtained by utilizing reserved starch.
The net effect is a reduction in starch content. In
contrast, water stress at the start of plant development
does not seem to elicit a regulatory mechanism,
starch is, therefore, not effectively
synthesized and the plants will not develop. On
removing the stress plants start growing and accumulating
starch. Starch content remains lower
than normal despite the application of sufficient
water (maximal starch content was 12.6–21.1%
and 25.9–30.3% for roots of with and without
initial water stress at 10 and 8 months, respectively).
Varieties responded differently to the adverse
conditions, of the six varieties R90 and KU
50 recovered the quickest after stress conditions
were removed. These two varieties also had the
highest starch content under both trial conditions
and are recently developed varieties that are distributed
to Thai farmers.