Plants of malting barley variety Schooner were exposed to 5 days of high
temperatures (up to 35°C) during mid grain filling under controlled
environment conditions. Grains from heat treated plants accumulated
c. 30% less starch than grains from control
plants (21/16°C; 14 h day). Reduced starch deposition was not limited
by assimilate levels in heat treated grains, but high temperature reduced the
conversion of sucrose to starch. The reduction in starch synthesis appeared to
result from the effects of diminished catalytic activity of a number of
enzymes in the committed pathway of starch synthesis, and/or delayed
recovery of enzyme activity in the cooler recovery conditions. Reductions of
11–75% in the activity of the enzymes under investigation
followed high temperature exposure. In addition, ADPglucose pyrophosphorylase,
branching enzyme and granule bound starch synthase showed increased activity
during exposure to moderate temperatures (28–32°C), but reduced
activity at high temperatures, while soluble starch synthase showed an
immediate loss of activity, even at moderate temperatures. Sucrose synthase
and UDPglucose pyrophosphorylase showed the greatest reduction in catalytic
activity after plants were returned to cooler conditions. Individual enzymes
showed variation in the level of recovery under the cooler temperature
conditions which followed the heating period.
Starch accumulation is reduced when endosperms develop at elevated temperatures. Reduced starch deposition does not appear to be due to limiting assimilate levels during the grain filling period; on the contrary, endosperm sucrose may even be increased at the elevated temperature. Results indicate that elevated temperatures significantly reduce the activity of the sucrose cleavage enzyme UDPsucrose synthase (EC 2.4.1.13), found in the endosperm during grain development, and that these effects may be initiated by a relatively short period of thermal stress applied close to anthesis. It would appear that, when developing barley ears are exposed to elevated temperatures, there is an irreversible reduction in the capacity of the endosperm to convert sucrose to starch, caused by a decrease in the activity of at least one of the enzymes involved in this conversion pathway.
Short periods of high temperatures (up to 35°C) during mid grain filling
appear to reduce yield and quality in barley. Plants of 3 malting barley
varieties, Schooner, Arapiles, and Sloop (a new South Australian malting
variety), were grown under constant environment conditions from germination to
maturity and exposed to 5 days of high temperatures (up to 35°C) during
mid grain filling. Schooner and Sloop showed similar patterns of accumulation
of dry matter under control conditions (21°C/16°C, day/night
temperature) and in response to high temperatures. In all varieties, the
reduction in starch accumulation represented the most significant detrimental
effect of high temperature and made the greatest contribution to the reduction
in final grain weight. The reduction in absolute grain nitrogen (N) in
heat-treated Arapiles grains represents a potentially important response under
high temperature conditions.
In this study, water loss did not have a decisive role in the termination of
grain filling. Continued accumulation of endosperm dry matter at low moisture
levels suggested that water distribution and/or components of water
potential may be more important than overall water content in the cessation of
grain filling.
Final grain composition depended not only on the amount of endosperm storage
component present in the grain but also on the contribution of the
non-endosperm components (including the embryo and husk) to final grain dry
weight. In some cases, changes in the contribution made by the non-endosperm
components of the grain to final grain weight masked important high
temperature effects on key endosperm storage components.
Hot water extract (HWE) values were similar within treatments and ranged from
73% to 78%. High temperature exposure reduced HWE for all
varieties. Malt b-glucan was lower in heat-treated grains than in control
grains. Despite relatively high malt protein levels in all varieties, higher
free amino N levels in heat-treated grains indicated a higher protein
modification than in control grains.
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