Photosynthetically Related Physiological Responses in Maize to Multiple Plant Densities and Nitrogen Rates.

The capacity of the crop to assimilate CO₂ is one of the dominant factors of reaching high grain yield in maize. Light, water, and nutrient sources have significant influences on instantaneous leaf photosynthetic rate and on biomass formation. The objectives of the study are 1) to determine the individual and interacting effects of nitrogen application and plant density on maize leaf photosynthetic rate at critical periods, and 2) to detect the relationships of photosynthesis with SPAD, transpiration, crop growth, and grain yield. Two maize hybrids were grown on two research farms with three plant densities (low-54,000, medium-79,000 and high-104,000 pl/ha) and three N rates (low-0, medium-112 kg/ha, and high-224 kg/ha). Photosynthetic rate, transpiration rate, SPAD, and aboveground biomass were measured at five growth stages (V10, V15, R1, R3, and R5). Photosynthetic rate was highest at vegetative growth stage and then declined with time. Averaged across the two years and two locations, high plant density significantly reduced the photosynthetic rate about 5%, 12%, 14%, and 33% at V15, R1, R3, and R5, respectively, when compared to the low density treatment. Comparing with the sufficient N supply treatment, no side-dress N application could lower the photosynthetic rate about 2%, 6%, 23%, 28%, and 40% at V10, V15, R1, R3, and R5, respectively.  Transpiration rate was positively correlated with photosynthetic rate throughout the V10 to R5 period. The photosynthetic rate and crop growth rate correlation and the photosynthesis and SPAD correlation were stronger at reproductive stages than at vegetative stages. Effect of N rates on photosynthetic rates and yield was stronger at high plant density treatment than at low density. Photosynthetic rate of high density maize declined more at low N rate than at higher N rates. Effects of density on grain yield were negative at low N rate and slightly positive at higher N rates. Ensuring adequate plant N availability in high density maize is essential in maintaining high ear leaf photosynthetic rates during reproductive growth and obtaining high yield.