EFFECTS OF VARYING NITROGEN RATES ON PREHARVEST SPROUTING AND ALPHA-AMYLASE ACTIVITY IN CEREALS

In many parts of the world pre-harvest sprouting, induced by unfavourable ecological conditions, can affect the grain grower's success considerably. Positive correlations are reported between resistance to pre-harvest sprouting and primary dormancy. Genotypes with a short dormancy period have a high pre-harvest sprouting risk. In the case of a premature germination of caryopses in the head of grain, a hydrolysis of intact starch granules caused by the endoenzyme alpha-amylase takes place in the endosperm. In some publications negative correlations between falling number and protein content are reported, however, it is unknown if a varying N-application influences pre-harvest sprouting rates, dormancy periods and amylase activity. For this reason, both greenhouse and field trials were conducted with different N-fertilization rates and (additionally in the greenhouse) a rain simulation treatment. With regard to alpha-amylase the results show that high amounts of the enzyme a few days post anthesis are opposed to small enzyme activities in mature kernels. Stratificating temperatures and germination inducing precipitation at the same time are inducing pre-harvest sprouting and a high alpha-amylase activity, especially in rye and triticale. Varying N-rates had a significant influence on the alpha-amylase activity in the milk ripeness stage and also - in the case of pre-harvest sprouting - at harvest time. It seems as if N-deficiency reduces the possibility of pre-harvest sprouting, on the other hand high N-rates increase the enzymes' activity and promote germination processes in the kernel. Significant effects of N-fertilization on dormancy under field conditions could not be found. In the discussion of reasons for an increase of alpha-amylase activity in sprouted grain caryopses, changes in the relation of the phytohormones gibberellic acid (promoter of enzyme activities) and abscisine acid are presumed to be the main factor.