The effect of pea cultivar and water stress on root and shoot competition between vegetative plants of maize and pea.
Improvements in intercrop yields may be achieved through an understanding of yield advantages due to above-ground or below-ground interactions. Forage maize and two morphologically contrasting cultivars of pea (leafy cv. Bohatyr and semi-leafless cv. Grafila) were grown alone and in additive mixtures, under two contrasting levels of soil moisture (± water stress). The mechanism of competition between maize and pea was studied by separating the effects of root competition and shoot competition, using soil and aerial partitions. Plants were grown in rectangular tanks in a glasshouse. Leafy pea cv. Bohatyr was as competitive as maize, both belowground and aboveground, whereas semi-leafless pea cv. Grafila was less competitive than maize or pea cv. Bohatyr. The greater competitive ability of the leafy pea, both above and belowground, was probably due to its greater growth rate, associated with its greater leaf area. The competitive ability of maize, relative to peas, was considerably reduced by water stress. Both the root and shoot competitive abilities of pea were greater under water stress, compared with those of maize. Relative yield total (RYT) values were significantly greater when maize and pea were subjected to shoot competition only (RYT=1.76) than when subjected to root competition (RYT=1.17) or when subjected to both shoot and root competition (RYT=1.13). This reflects the fact that the effects of root competition were greater than those of shoot competition. Root competition decreased the shoot dry weights, plant height and leaf area of both maize and pea, whereas shoot competition had no significant effect on these attributes, indicating that soil resources, i.e. mineral nutrients and water, were more limiting than light.