Plant and N Impacts on Maize Growth, What's Controlling Yield.

Plants compete for resources in several ways. Plants can modify their growth characteristics to aggressively compete, seen in enhanced shoot growth and up-regulation photosynthesis. A different mechanism is for plants to avoid competition with outcomes of changes in leaf orientation or the down regulation of photosynthesis. The research objective was to assess the mechanisms responsible for intraspecific competition in maize (Zea mays). Treatments were three shade levels (0, 40, and 60%), two water regimes (natural precipitation and + irrigation), two plant populations [single (74,500 plants/ha) and 2X] and two N rates (0 and 228 kg N/ha) placed in a randomized split-block design. Grain yields, N percentage, and carbon isotope discrimination, chlorophyll content, and soil water and inorganic N were measured. The per plant grain yields decreased with increasing shade and population level. If similar mechanisms reduced yields in shade and 2X population treatments, then similar plant responses to competition should be measured. However, for several key measurements this was not the case. For example: 1) grain N percent was minimally impacted by population level, whereas plants in shade had increased N; 2) well fertilized plants grown under irrigation had increased chlorophyll content that was not impacted by shade while in the population study, chlorophyll decreased in the 2X population; 3) in N fertilized plants grown under irrigation, shade increased isotope discrimination (0.27‰), whereas it was not impacted by population level; and 4) population level did not impact the amount of available water or inorganic N remaining in the surface 60 cm of soil at harvest. Differences between shade and population response suggest that the factors responsible for the per plant yield reduction differed. Reduced light availability was the mechanism in shade treatments, whereas in the 2X treatment shade avoidance mechanisms that reduced competition between adjacent plants was responsible for reduced yields.