Modeling Competition for Water and Light Among Plants and Its Relationship with Grain Yield.

Heterogeneous communities of plants can be seen as individuals competing for resources including water. This competition can be interpreted as the extent to which each plant satisfies the water demand imposed by the atmospheric environment and its own shoot attributes. A simulation model capable of simulating plant competition was assembled and, as a first approximation, compared with information collected from field measurements of crop mixtures. The major assumptions in the model are that the root resistance to water flow per unit of root length and the shoot resistance per unit of crop cover (not leaf area) are stable. Using this model, it is shown that the grain yield of virtual cultivars of wheat with contrasting rooting patterns (shallow or evenly distributed roots) interact significantly with the environment and the plant density as a result of the timing and magnitude of water stress. For instance, in Mediterranean environments that depend on stored water during the grain filling phase, a shallow-rooted cultivar withstands better a water shortage in relatively shallow soils because it uses the deep-layer water at a slower rate than a cultivar with evenly distributed roots. These effects can be moderated by regulating the plant density. When these cultivars are mixed, the interaction among soil depth and water storage capacity, the precipitation pattern, and plant density determine the response of the community. Measurements of water potential in water stressed plants with different rooting patterns (maize and sunflower) in the same soil volume seem to indicate that plants appear to actively reduce the impact of dry layers on the water potential “seen” by the root system. Reconciling theory with measurements is a necessary step to design rooting systems suited to particular environments. These effects and their implication for grain determination are explored and discussed using a simulation model.