| The increase in dry matter during the post-heading stages, 65 days to
maturity, mounted to 9.0% of the total dry matter at site 1 compared to
25.0% formed at site 2. Root samples taken from the upper 60 cm of the soil
were also lighter in weight for site 1 compared to site 2 (Table
3). The average grain yield harvested at site 1 was reduced to 25.0% of that harvested at site 2. This was accompanied by a reduction in the three yield component characters namely; number of fertile spikes/m2, number of kernels/spike and 100-grain weight, of 29.0, 33.0 and 12.0%, respectively, (Table 3). Path-Coefficient analysis of the two sites is presented in Table 4. At site 1, kernels/spike had the largest direct effect on grain yield in addition to an indirect effect through spikes/m2. At site 2, 1000-grain weight had the largest direct effect on grain yield. Spikes/m2 was second to 1000-grain weight despite its very small correlation with grain yield. The degree of association between the two sites showed significant correlations for plant height, days to heading, days to maturity and 1000-grain weight. The rest of the characters showed insignificant correlations indicating diflerent levels of response to drought stress. In conjunction with drought tolerance, only five cultivars, three bread and two durum wheat, ranked within the best ten yielders at both sites (Table 5). They included the top yielder at each site. The widely grown cultivar, Mexipak and the triticale line Maya-I ranked within the top ten at site 1 but dropped below average at site 2. Cultivar Arz and Jori 69 ranked below average at both sites. Discussion The plants at site 1 were under moisture stress during the pre-heading stages. They suffered from severe drought when the irrigation was halted. The maximum number of tillers was established prior to heading with twice as much formed at site 2. The number of dead tillers at maturity was 9.0% at site 1 compared to 30.0% at site 2. The non-fertile tillers seemed to be unwanted luxury at site 1 since they represented wasteful soil moisture and assimilates needed for ear development (1, 4). Competition among tillers for assimilates could have limited the spike size (kernels/spike). This supports the concept of a limited number of tillers for dry conditions, which onset and mature about the same time (4, 8). Drought stress during inflorescence development and anthesis reduced the number of kernels/spike to two-thirds of the potential as produced at site 2. Therefore, kernels/spike had the largest direct effect on grain yield. The 1000-grain weight at site 1 was reduced by only 12.0% despite the fact that few green areas on the plants were functioning during the grain filling period. This indicated that assimilates from the stems were translocated to the grains to compensate for the deficit in dry matter production (3). |
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