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The objective of this analysis was to use a simple, mechanistic crop growth model to examine the effects of variation in solar radiation and temperature on potential maize (Zea mays L.) yield among locations. Crop phenology and leaf growth were calculated from daily mean temperature data obtained at the five locations studied. Daily biomass accumulation was calculated by estimating the amount of radiation intercepted and assuming maximum crop radiation use efficiency of 1.6 g MJ−1. Grain yield accumulation was simulated using a linear increase in harvest index during grain filling. Observed and simulated grain yields were compared for several sowings at each of five localions ranging from latitude 14°S to 40°N lat. Averaged across sowings, respective observed and simulated oven‐dry grain yields (g m−2) were 816 and 830 at Katherine, Australia; 953 and 908 at Gainesville, FL; 1059 and 1106 at Quincy, FL; 1091 and 1119 at Champaign, IL; and 1580 and 1626 at Grand Junction, CO. Temperature primarily affected growth duration with lower temperature increasing the length of time that the crop could intercept radiation. The solar radiation response was related to the amount of incident radiation and to the fraction of radiation intercepted by the crop. In the tropics (Katherine), high temperature decreased the duration of growth and grain yield, despite high levels of radiation. Only at locations with low temperature and consequent long growth duration. and with high radiation were maize yields simulated to be more than 1600 g m−2 (300 bushels per acre at 15.5% moisture).
Evidence of differential sensitivity to drought between symbiotic N fixation and leaf photosynthesis in legumes has been measured under greenhouse conditions. If the greater sensitivity of N accumulation to drought is substantiated under field conditions, then important questions concerning legume production under water‐limited conditions must be considered. Consequently, the relative sensitivity of N and biomass accumulation to drought was examined in a 2‐yr study on ‘Biloxi’ soybean [Glycine max (L.) Merr.] grown in the field on Arredondo fine sand soil (loamy, siliceous, hyperthermic Grossarenic Paleudults). Various irrigation treatments were imposed during vegetative growth after canopy closure when solar radiation was almost fully intercepted. Shoot and root plus nodule biomass and N accumulation were determined from periodic harvests. Solar radiation use‐efficiencies were computed from regressions of biomass against cumulative solar radiation, and were found to range from 0.53 g MJ−1 under daily irrigation, to 0.23 g MJ−1 under the most severe drought treatment. Nitrogen accumulation rates were computed by regression of N accumulation against time, and were found to decline from 0.31 g N m−2 day−1 for the daily irrigation to 0.003 g N m−2 day−1 for the most severe drought treatment. In both years and all three drought treatments, the N accumulation rate was decreased relatively more than the solar radiation use‐efficiency for biomass accumulation. These results indicate that a high sensitivity of N accumulation to soil dehydration may be an important constraint on soybean productivity.
Mycorrhizae improve plant nutrient uptake and are known to affect the water relations of plants grown in growth chambers and greenhouses. This paper summarizes a 3‐yr field study that tested the effects of mycorrhizae and water management on the growth and grain yield of maize (Zea mays L.). In each year, two inoculation treatments (inoculated or not with Glomus etunicatum Becker and Gerdemann) and three water‐management treatments (fully irrigated, moderate stress, and severe stress) were applied to fumigated and fertilized Millhopper fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult). Inoculum was placed in a furrow 10 cm deep at an average rate of 1500 propagules per meter of row. Six to 7 wk after planting, colonization ranged from 0 to 6% of total root length on noninoculated plants and from 10 to 61% on inoculated plants. Twelve to 13 wk after planting, colonization ranged from 2 to 30% on noninoculated plants and from 21 to 56% on inoculated plants. Water stress had little effect on root colonization. By 52 d after planting, one more leaf had appeared and one additional leaf had formed a collar on inoculated plants. Inoculation increased the concentrations of P and Cu in both shoots and grain on all measurement dates. Overall, grain yields (0.306) and total above‐ground biomass yields (0.458 Mg ha‐1 cm‐1 of water) increased linearly with irrigation. A positive response to mycorrhizal inoculation was constant across irrigation levels (0.802 for grain and 1.170 Mg ha−1 for biomass). Therefore, the proportional response of maize to inoculation with G. etunicatum increased with increasing drought stress.
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