Biotic Stress and Yield Loss

characteristics will have a critical impact on the quantity of light absorbed and subsequentplant growth within mixed plant canopies. In a sensitivity analysis of themodel INTERCOM, Lindquist and Mortensen 31 showed that parameters having thegreatest influence on maize yield in monoculture include rate of development duringreproductive and vegetative stages, the extinction coefficient, and relative height ofmaximum leaf area density (i.e., vertical leaf area distribution). Traits in maize resultingin the greatest reduction in yield loss in a mixed maize-Abutilon theophrasti (velvetleaf)canopy include maximum height, relative height of maximum leaf areadensity, rate of vegetative development, and thermal time from emergence to 50%maximum height and leaf area. Caton et al. 32 found similar results in their analysis ofa rice-Echinochloa oryzoides model.In an empirical field study with four diverse maize hybrids, Lindquist andMortensen 33 found that maize yield loss-velvetleaf density relationships variedamong hybrids. Lindquist et al. 34 determined that maize traits having strongest correlationto yield loss included maximum leaf area index and height, thermal timebetween emergence and 50% maximum LAI and height, and vertical leaf area distribution.Others have shown an increased LAI and subsequent reduction in light transmissionwithin maize canopies in response to increased crop population and narrowrow spacing, ultimately resulting in the reduction in weed productivity and theireffects on crop yield. 35, 36 These results corroborate those of the model sensitivityanalysis. Improving crop competitiveness with weeds is an important method ofreducing our current reliance on herbicides. Therefore, modification of these traits tooptimize crop yield and competitiveness needs to be an ongoing objective for plantbreeders and agronomists.13.3 COMPETITION FOR SOIL WATERThe CO 2required for photosynthesis is acquired by diffusion through stomata on thesurface of leaves followed by active uptake into the chloroplasts of the mesophyllcells. These stomata are essentially portholes that allow for the exchange of gasesbetween the atmosphere and the intercellular spaces within the leaf. Diffusion occursas the result of differences in concentration of gases in the air, the boundary layer ofthe leaf surface, and the intercellular spaces of the leaf. Although water is requiredfor photosynthesis, a far greater quantity of water is lost through stomata as a resultof diffusion. 37 In support of this process of transpiration, a continuous flow of waterfrom the soil through the outer cells of the root, into the xylem, and out through theleaf is required. If soil water content becomes limited, stomata at the leaf surface willclose, thereby reducing the quantity of CO 2that can be assimilated for plant growth.Kropff 38 assumed that the potential growth rate of a species (dW i/dt G p,ifromEquation 13.3 where only competition for light is assumed) is reduced in proportionto the ratio of actual (T a,i, cm( kg m 2 10)) to potential (T p,i, cm) transpiration.Hence:a,iG s,i T GT p,i[13.6]p,i