| Population levels had a pronounced effect on spike length. Treatments
of low densities had longer spikes than their counterparts at higher densities
(Table 2). Competition for nutrients, water and
light in the very early stages of ear development results in decreased spike
lengths in dense population. These results are in good hormony with MADDENS
(1974) and WILLEY & HOLLIDAY (1971). These authors reported that spikes
growing under shade remain shorter than those growing under open light.
Spikelets per spike were affected significantly (P>=.01) by plant spacing. However, it showed no response to row spacing (Table 1). Spikes at low population levels had more spikelets than their dense population equivalents (Table 2). The number of spikelets is controlled by competition among plants for light, assimilates and nitrogen (PUCKRIDGE 1968) and the proportion of spikelets surviving from ear emergence to ripeness displayed a trend from 52.4% for the low density to 37.5%for the high density population (LEAKY 1971). Increasing competition between the plants with increasing plant densities affected the number of grains per ear in the same manner as the spike length and spikelets per spike (Table 2). The number of grains per ear is controlled by the number of spikelets present and the number of grains formed in each spikelet. In the present experiment some basal and a large number of terminal spikelets failed to develop at higher densities. The fertility of initiated spikelets depends largely on growing conditions. The physiological factors determining the number of ripe grains produced per ear are not clearly understood, but it is obvious that more spikelets are always available at ear emergence than are subsequently filled. The cause of the failure of spikelets to develop is not evident but may be related to competition for assimilates between spikelets, possibly controlled by gradients of growth regulatory substances. KIRBY & FARIS (1970) have reported that at high densities an increase in the level of GA-like substances coupled with a lower nutrient supply, would together produce a steeper-than-normal nutrient gradients in the apex early in the life of the plant, thus causing the reduced development of terminal spikelets. However, there is some evidence that with a higher light intensity more spikelets produce grains (FRIEND 1965, WILLEY & HOLLIDAY 1971) and that within each ear the number of florets and grains produced are closely connected with the rate of spikelet differentiation (KIRBY, 1974). 1000-kernel weight decreased progressively with increasing plant density (Table 2). In dense populations competition for light and assimilates from flowering to ripening results in reduced seed weight. The cause of reduced grain weight can be ascribed to the competition for photosynthetic material which occur between individual grains on the ear (NAZIR et al. 1975) and growing the spikes under shade from flowering to ripening resulted in reduced grain weight (WILLEY & HOLLIDAY 1971, KASIMOV 1976). 1000-kernel weight is an important factor for seed production. Bolder seeds can produce healthier plants. For this reason wider spacing is recommended for seed production. |
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