Genetical and physiological studies on sprouting in wheat

Abstract

Part I
Summary

The aims of this Cambridge Laboratory project were to assess risks associated with sprouting damage and to identify genetic markers for stable, high Hagberg Falling Number. Such genetic markers, together with increased insight into the mechanisms of sprout-damage, will enable plant breeders to select new resistant varieties with consequent benefits to the breadmaking quality and value of the U.K. wheat crop.

Special genetic stocks with known genetic markers have been examined in a series of field trials and artificially controlled tests, to evaluate their potential for controlling -amylase enzyme activity in the grain and sprouting in the ear. Part of the work has also been directed to produce new genetic stocks, for experiments described in this report and for future investigations.

Three distinct physiological disorders were observed in the 1987 harvest:

(a) Premature a-amylase production in sound, ungerminated grains. This behaviour was first observed in Maris Huntsman, but it has since become clear that many other varieties can be affected. The characteristic grain sample in this case has a low Falling Number without visible evidence of sprouting. The damage occurs early on in ripening, shortly after the onset of grain water loss.

(b) Premature germination during early ripening. Although this results in high -amylase activities in affected grains, and occurs in the same developmental stage as premature enzyme production, it differs in that the grain sample shows visible evidence of sprouting (embryo growth).

(c) Conventional pre-harvest sprouting can occur in ripe ears exposed to rainfall. Unless the mature grains are dormant, imbibition of water is sufficient to induce embryo germination and a-amylase production. In contrast to the two prematurity risks, this type of damage happens towards the end of (or after) ripening, typically when harvesting is delayed by wet weather.

The Rht3 dwarfing gene reduces a-amylase levels by some 90% in sprouted grains and in wheats susceptible to premature enzyme production. This gene inhibits enzyme production but does not affect grain dormancy or embryo germination rate. Thus, although Rht3 can help to maintain high Falling Number, additional sources of resistance are required to avoid sprouting. Other genetic effects on premature enzyme production have proved more difficult to analyse. Specific chromosomes responsible for high a-amylase in special genetic stocks have been identified, but it appears that different mechanism(s) may be involved in more recent U.K. varieties.

Resistance to visible sprouting is largely dependent upon genes controlling grain dormancy. The difference between red-grained and white-grained wheats is the single most important source of variation in artificial tests of sprout-resistance. Amongst the red-grained wheats, increasing the dosage of R genes for red pigment from 1 to 2 to 3 copies leads to smaller but significant increases in grain dormancy. The numbers of R genes carried by 83 modern varieties and breeding lines have been determined, allowing breeders to predict the frequencies of different types amongst new varieties bred out of this gene pool.

Other genetic markers for improved resistance include lack of awns (presence of awns is associated with more rapid germination in ears under simulated rainfall). Significant effects of other genes have been detected but these genes have yet to be identified.

Comparisons made between autumn and spring sowings of spring wheats indicate that the generally high sprout-resistance scores of spring versus winter types may be an artefact. When spring lines flower and ripen early they have lower resistance than predicted.

Construction of a Restriction Fragment Length Polymorphism genetic map of the entire wheat genome is underway at the Cambridge Laboratory and is being used to generate new markers for identifying and manipulating sprout-resistance genes.

The physiology and genetics of sprouting are complex. Much useful progress has been made towards understanding the control of this costly problem; much remains to be done, and we are continuing our research into this topic.

Part II
Summary

An investigation has been carried out into the composition and significance of wax crystals discovered in the embryo cavity of wheat grains. The crystals were found only in grains of red cultivars that had been stored for long periods.

The composition of the wax has now been tentatively defined. A wide range of organic compounds have been detected and details of these are contained in the report. Components with similar composition to the waxes of long-stored grains have been found in freshly harvested examples of both red and white wheats. There are significant differences however in the proportions of certain key components in the two types. Surface properties of the extracts from embryo cavities of red and white grains examined show significant differences, compatible with a water resistant function of the red grain extracts. There is a strong association between surface properties of the extracts and the tendency of the parent grains to sprout. The association now requires to be examined more systematically using breeder's substitution lines and the developmental biochemistry of the different waxes requires to be explored, particularly in regard to seasonal variation.