Wheat Quality

Despite the great emphasis placed on quality of wheat for nutritional and industrial uses (Heyne, 1987), there is little understanding of the interaction between environmental and genetic factors determining composition of wheat grain under current conditions. Supply of nutrients, particularly N, determines (in conjunction with other factors such as water supply and temperature) protein amounts relative to starch and other non-N components. Nutrient supply thus alters N-concentration, but only affects amino acid and protein composition to a limited degree. However, sulphur supply does affect the latter.

Elevated [CO2] tends to increase mass per grain and decrease percentage N because of the increased supply of carbohydrate from photosynthesis, either during grain-fill or from reserves (Mitchell et al., 1993; Tester et al., 1995; Batts et al., 1997; Rogers et al., 1998). Elevated [CO2] does not greatly alter the composition of the carbohydrates, e.g. types of sugars or starches, sugar/starch ratio (Rogers et al., 1998). It also does not affect protein composition (Shewry et al., 1994). Increased [CO2] interacted with nitrogen supply and temperature. Complex changes in lipid composition have been identified (Williams et al., 1995); and the effects of 700 |mmol CO2 mol-1 are smaller than those of a 4°C temperature rise but the mechanisms are not understood. The non-polar neutral lipids di- and mono-galactosyldiacylglycerol increased where temperatures were 4°C above ambient and N was deficient at elevated [CO2], but decreased in ambient temperature conditions. With elevated [CO2] and temperature, only starch lipids were affected. Fatty acid composition was affected by warmer conditions, which decreased oleate but increased linoleate of non-polar lipids.

Analysis of grain from spring wheat, cv. Hereward (Tester et al., 1995) showed that warmer temperatures substantially decreased mass per grain and starch content, due to fewer and smaller type A starch granules and fewer amyloplasts per endosperm. Starch gelatinization increased with temperature, but gelatinization enthalpy was unaffected. Also, total and lipid-free amylose increased, but amylolipid and lysophospholipids were not affected by warmth. The mechanisms of C and N accumulation related to grain filling are rather separate, and the biochemical pathways leading to the final stored products are complex and are likely to be affected by environmental conditions. In addition, variety/environment interactions are to be expected.

Given the dynamic nature of the processes and conditions, there is considerable scope for variation in final grain composition. Such alterations may affect the nutritional quality and industrial uses of wheat grain. It is surprising, therefore, that more attention has not been directed to assessing the impacts of GEC on wheat quality. Perhaps the perceived slowness of the environmental changes and the existing variability in wheat characteristics, coupled with the pragmatic approach of end-users, mitigate against scientific analysis of quality.

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