THE EFFECT OF NITROGEN AND GROWTH-REGULATORS ON STEM AND ROOT CHARACTERISTICS ASSOCIATED WITH LODGING IN 2 CULTIVARS OF WINTER-WHEAT

The effects of nitrogen and plant growth regulators (stem shorteners) on root and shoot characteristics associated with lodging resistance were investigated in two winter wheat (Triticum aestivum L.) cultivars of contrasting lodging resistance: the susceptible Galahad and the resistant Hereward, The morphology and mechanical strength of the stems and anchorage systems grown at two levels of nitrogen and with or without growth regulators were measured and related to the incidence of lodging recorded in a field trial. In both cultivars high levels of nitrogen increased the height of the stem, thereby increasing the 'self-weight' moment transmitted into the ground and weakened both the stems and the anchorage coronal roots. As a result, the anchorage strength was also reduced, plants failing in the root system in simulated lodging tests. Growth regulators, in contrast, had little effect on the bending strength of the shoots and root systems, but reduced plant height so that the overturning moments generated by the weight of the shoot were less. There were also differences between cultivars: Galahad plants had weaker anchorage due to the smaller number and lower strength of the coronal roots. The morphological and mechanical measures were used to calculate a safety factor against both stem and root lodging. Five factors were found to influence the safety factors, these were: cultivar type, the type of lodging, the rate of nitrogen and growth regulator application, and time, being lowest in Galahad plants at high levels of nitrogen and without growth regulators and at grain filling when the ears were heaviest. This was consistent with the observed pattern of lodging: root lodging occurred at grain filling and only in Galahad which had been treated with high nitrogen rates, most strongly in plants without growth regulators.