Photosynthesis-related physiological responses of field-grown maize to plant density and nitrogen stress during vegetative and reproductive growth stages.

摘要

Photosynthesis is the only process of biological importance that can harvest the energy from the sun and produce carbohydrates for biomass formation. Even though photosynthesis processes are basic and critical to maize ( Zea mays L.) yield formation, direct measurements of photosynthesis in field-grown maize have been very rare. A three-year (2009 to 2011), two-site (West Lafayette and Wanatah, IN) field experiment with three plant densities (54,000, 79,000, and 104,000 plants ha-1) and three nitrogen rates (0, 112 or 150, and 224 or 300 kg N ha-1) were conducted with two commercial maize hybrids. Leaf photosynthesis, as well as whole-plant biomass per unit area, was measured at V10, V15, R1, R3 and R5 growth stages to directly examine photosynthesis changes over time and the complimentary relationships between source (at the fundamental photosynthesis level) and sink in explaining how plant density and N rate interacted to affect final yield outcomes. The objectives of the research were i) to determine the leaf photosynthesis response to plant density, and nitrogen stress at different growth stages, including both vegetative and reproductive growth, ii) to understand the relationship of leaf photosynthesis with leaf transpiration and with plant nitrogen status, iii) to determine the dependence of grain yield and yield components on photosynthesis levels at various growth stages, and iv) to identify the key factors for successfully measuring photosynthesis in field conditions. Mean air temperatures during the photosynthetic measurements narrowly ranged between 30°C and 38°C for all sampling occasions, and varied less than 4°C between reproductive stage measurements within a single site-year. Photosynthesis measurements were responsive to maize management system interactions at discrete vegetative and reproductive growth stages, and were meaningfully related to actual maize plant growth rates. High plant density significantly reduced leaf photosynthesis (A), transpiration ( E), stomatal conductance (gs), SPAD value, as well as leaf N concentration, especially during reproductive stages. Nitrogen deficiency lowered leaf A, E, gs, as well as leaf SPAD values and leaf N concentrations measured simultaneously. Leaf A and E were highly correlated, and with the same amount of transpiration increase, the increase of leaf photosynthesis was lower in high N rate treatment than in low N treatments. The intrinsic water use efficiency (WUEi) varied substantially among sampling times, but was unaffected by plant density or by N rate. Leaf photosynthetic rates declined faster with plant development, and responded more to plant density stress and N deficiency, than was apparent for SPAD values that were measured from the identical leaves used for photosynthesis measurements. SPAD values, therefore, were not a good indicator of photosynthetic rate. Strong correlations were observed of A with plant growth rate (PGR), A with per plant kernel weight (Kwp), A with per plant kernel number (Knp), and A with grain yield per unit area. Leaf photosynthesis was better correlated with PGR when the selected growth period was longer (more than 20 days) and the reproductive stage was included. In addition, correlations between leaf photosynthesis and PGR were higher when using the mean leaf photosynthetic rates for the beginning, middle (if applicable), and end of the growth period being examined. Maize grain yield tended to peak at the intermediate plant density, and increased in response to N rate in all six site years. Grain yield, Kwp, and Knp were more strongly correlated with leaf photosynthesis during reproductive stages rather than that of vegetative stages. Even though photosynthesis values represent instantaneous values from an individual leaf, they were meaningful indicators of maize plant response to management treatments. Our core photosynthesis measurement procedures were very effective in arriving at meaningful data, with minimal interference from non-treatment factors and plant-to-plant variation, by using a consistent time of daily measurement, controlled light intensity, multiple plants and consistent leaf sampling positions, and a specific isolation method just prior to measurement.