Termination of grain growth in cereals

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.