Quality traits. Pre-harvest sprouting tolerance (PHS)
Contributed by Mark Sorrells (firstname.lastname@example.org)
Pre-harvest sprouting (PHS) is the condition where germination of grains occurs in the spike before harvest. Prolonged rainfall and high humidity contribute to premature germination of grains before harvest. The main effects of PHS are a lower yield due to harvest losses and, more importantly, a reduction in end-product quality. Flour obtained from sprouted grains loses its thickening power due to starch breakdown and the baked goods have a smaller volume and a compact, sticky crumb structure. The main cause for the loss of quality due to PHS is high α-amylase activity activity.
Several factors can contribute to increased tolerance to PHS: reduced levels of α-amylase activity in the grain, the presence of inhibitors of germination, reduced water absorption by the grains and altered responses to hormones, among others (4,6).
White-kernel wheats are usually more susceptible to PHS than red wheats. This association between PHS tolerance and red pigmentation is likely due to a pleiotropic effect of the genes controlling grain color (3, 10).
PHS tolerance is a quantitative trait influenced by many environmental factors and controlled by several dormancy related genes (5). A number of genes and QTLs involved in PHS tolerance have been found and mapped in wheat. One potential complication for analyzing the PHS phenotype is that it involves genes expressed in the maternal plant as well as tissues belonging to the next generation: the embryo and the endosperm. As a consequence, the breeding of tolerant variants is a difficult process.
For assessing PHS tolerance different methods are available. One is to measure the early α-amylase activity, another is measure starch viscosity using the "Hagberg falling number" apparatus. Finally, seed dormancy can be determined using germination tests or intact spikes can be placed under simulated rainfall conditions to estimate the rate and number of seeds showing early germination.
Clark's Cream is a hard white wheat with a high tolerance to PHS. It was developed by Earl G. Clark and placed in the germplasm collection by Kansas State University in 1972. This cultivar is the source of the QTL linked to the loci XBarc055 and Xbcd1434 which both contribute to resistance to PHS within the IFAFS project.
Cultivar Cayuga is a soft white winter wheat with a level of tolerance similar to many red wheats. This cultivar derives from Clark's Cream and carries the QTL linked to Xbarc055 and Xbcd1434. Cayuga was developed at Cornell University and seeds are available upon request from Dr. Mark Sorrells.
Many QTLs have been found in different mapping populations. However, not all them are suitable for breeding, because they do not show a significant effect across different environments or they have low significance values. Sources of PHS resistance are present in other species different from Triticum aestivum: Aegilops tauschii (2) or spelt, Triticum spelta (6). One disadvantage of using these species for breeding is the possibility that the transfer of the genes conditioning resistance might also bring along undesirable traits. For example, one of the QTLs with the most significant impact on PHS tolerance in spelt, cosegregates with a locus or loci responsible for the typical ear morphology of spelt, which is undesirable in cultivated wheat.
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8. Integrated physical maps of 2DL, 6BS and 7DL carrying loci for grain protein content and pre-harvest sprouting tolerance in bread wheat. Varshney, R.K.; Prasad, M.; Roy, J.K.; Roeder, M.S.; Balyan, H.S.; Gupta, P.K. In: Cereal Research Communications, 2001, 29(1-2):33-40. [abstract]
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10. Study of the relationship between pre-harvest sprouting and grain color by quantitative trait loci analysis in a white X red grain bread-wheat cross.. Groos, C.; Gay, G.; Perretant, M.-R.; Gervais, L.; Bernard, M.; Dedryver, F.; Charmet, G. In: Theoretical and Applied Genetics, 2002, 104(1):39-47. [abstract]