Abstract
This work examines the processes taking place during grain
growth and maturation in wheat which may be concerned in the
termination of dry matter deposition.
Three different cultivars were used in the study and were grown
under field, glasshouse and growth -room conditions. Some caryopses and
detached ears were also grown in liquid culture.
Starch is a major component of dry weight, accounting for about
70% 0a/w) of the endosperm. Termination of dry matter and starch
deposition were shown to occur at the same point. The results suggest
that reduction in assimilate supply is unlikely to lead to the
cessation of dry matter deposition since sucrose levels in the
endosperm were maintained beyond the stage at which starch content
had reached its maximum level. Measurement of photosynthetic and
respiratory activity showed that pericarp photosynthesis starts to
decline before respiratory activity, and both start to fall before
termination of dry matter accumulation. Photosynthetic activity
paralleled changes in the chlorophyll content of the pericarp.
Comparison of starch deposition in the cultivars Fenman and
Broom showed a higher final level of starch and dry weight in Fenman.
This was due to higher rates of starch deposition coupled with higher
rates of starch synthase activity in cv. Fenman. Affinity of starch
synthase for ADPG substrate as indicated by Km was the same in both
cultivars. High temperature stress reduced starch content and starch
synthase activity in both cultivars, but the effects on both were
greater in Fenman than in Broom.
The granule -bound form of starch synthase was found to be the
most active in wheat endosperms. This activity appears to be rate limiting during the grain -filling period but is maintained after
termination of starch deposition. However, evidence is presented that
its activity may be inhibited in vivo towards the end of grain -
filling. Although activity of ADPG pyrophosphorylase is much higher
than that of starch synthase during grain -filling, it was shown to be
much more sensitive to reduced water levels, induced by polyethylene
glycol, when activity was greatly reduced. In this situation, starch
deposition ceased. Some results indicate that onset of water loss
slightly precedes termination of grain growth and starch deposition.
Thus, it appears that while starch synthase activity is rate - limiting
during grain -filling, reduction in ADPG pyrophosphorylase activity
caused by falling water levels may be very important in determining
when termination of starch deposition occurs in developing wheat
endosperms.
It is suggested that termination of grain growth is the result of
a programmed sequence of events which may have its origins before or very early in grain development.
