Biotic Stress and Yield Loss

In soybeans, a leaf area index (LAI) of 3.5 at reproductive stage is consideredcritical for maximum yield. This critical LAI corresponds to about 90% canopy lightinterception. Reduction of the LAI below 3.5, or light interception below 90%,results in significant yield loss. 44, 75 When growing conditions are optimal for soybeans,including sufficient precipitation or irrigation water, soybeans can produce aleaf area index as high as 7. Consequently, these soybeans with ample water supplycan tolerate a significant defoliation injury without a significant yield loss. Forinstance, if the canopy has attained an LAI of 7, removal of half of the leaf area maynot result in a significant yield reduction.Although optimal moisture supply may enhance the level of tolerance to insectinjury, excess moisture can be harmful for plant growth. 76, 77 In some areas wherethere is excessive precipitation, drainage becomes a challenge to overcome waterlogging. Therefore, optimal water supply that favors rapid plant growth is desirable.The more vigorous the plants are, the more tolerant they can be to injuries by insectpests. Therefore, optimizing the supply of resources required for plant growth is onealternative to increasing the level of plant tolerance to insect injury and consequentlyreducing yield loss.In agricultural ecosystems, moisture stress can interact with arthropod injury toaffect plant gas exchange, dry-matter, and yield. 62, 63, 77, 78 Agronomic factors, suchas soil moisture available for plant growth, affect plant vigor altering the level of toleranceto arthropod injury. Therefore, consideration of agronomic inputs in a pestmanagement plan may be essential to increase plant tolerance to arthropod injury.8.9 CONCLUSIONSSeldom are environmental requisites in optimal supply for plants to achieve theirgenetic yield potential. In natural and agricultural systems, plants experience stressfrom both biotic and abiotic factors that can disrupt normal plant physiologicalprocesses and growth. As a result, stressed plants experience reduced fitness or yield.Moisture stress is one of the major abiotic factors that limit agricultural productivity.Plants experiencing moisture stress have reduced physiological processes suchas photosynthesis, transpiration, and reduced biomass and reduced yield. If there isextreme moisture stress, particularly at early growth stages, plants may not recoverfrom moisture stress and can die. However, if the level of moisture stress is notsevere, plants deploy different strategies to maintain fitness. Some of the adaptationsto moisture stress include increases in some hormone concentration to regulate stomatalconductance limiting transpirational water loss. In addition, moisture-stressedplants tend to increase root growth to absorb more water from the soil.Plants face multiple stressors that can limit their growth, development, and fitness.Because little research has focused on the interactions of abiotic stress andinsect injury, the broad importance of the interaction is not clear. Few studies indicatethat abiotic factors can alter plant response to insect injury. In agricultural ecosystems,yield loss occurs from both moisture stress and insect injury. However, themagnitude of yield loss from the interaction of the two stressors could be differentfrom the magnitude of yield loss if these stressors were acting independently. This