Biotic Stress and Yield Loss

morphological changes that influence rates of herbivore attack, this area will not beaddressed within this review. Similarly, phytochemical inductions have already beenrecently reviewed. 29, 30 A shortened listing of potential changes from a wild todomesticated state has been compiled from these sources:a) Shifts in life history strategies 31–33b) Decreased sensitivity to photoperiod length 34c) Increased allocation to the harvested portion of the plant 28d) Changes in plant canopy including generally higher leaf area indices (LAI)and leaf area duration (LAD), alterations in leaf position and inclinationand corresponding rates of exposuree) Reductions in specific plant parts such as stem, root, or reserve allocation35f) General size increase 36–38g) Increased rates of polyploidy 24h) Increased uniformity in germination, synchronization of flower, andmaturation39, 40i) Shifts in duration of specific stages of plant development (either lengthenedor shortened)36, 41j) Loss of bitter and toxic substances10, 28, 42k) Adaptiveness to cultivation 43l) Seed retention enhanced 44m) Reduced seed coat thickness 45n) Increased sensitivity to system inputs (e.g., nutrient enhancements)Several factors have not been changed in any consistent pattern or to any great degree:a) Maximum carbon dioxide exchange rate (CER) 24b) Relative growth rates 24c) Sensitivity to low nutrient conditions 46To date, crop yield increases have not resulted from increased maximum carbonexchange rates (CER). However, more recent advances using molecular approachesmay produce significant changes as illustrated by the transfer of genes from C-4maize to C-3 rice plants that alter rates of expression of enzymes important for higherphotosynthetic rates. 47, 48 Higher CERs have been reported in wild progenitors orspecies of barley, sorghum, millets, soybean, cotton, cassava, wheat, rice, sugar cane,Brassicas, and sunflower. Generally, selection for increased CER in crop plants hasresulted in no increase or decreases in yield potential. Similarly, no systematicincreases in relative growth rates have been associated with crop domestication ifplant size is corrected, especially for initial seed size. 24Whereas crops have not become more sensitive to low levels of nutrient stress,they are more capable of exploiting and utilizing the higher inputs of modern, highintensityagriculture. Nitrogen use in the U.S. has increased by 355% from 1960 to1995, with almost all acres of corn, fall potatoes, and rice as well as 75% of cotton