Maize leaf development biases caused by air-apex temperature differences

Accurate determination of leaf appearance rate is required in crop simulati on models to estimate canopy development and ultimately crop yield. Most cr op simulation models use air temperature for thermal time calculations to e stimate leaf appearance rate, although the near soil temperature is more cl osely related to the growing apex temperature than air temperature before s tem elongation. A held experiment was conducted in 1996 at East Lansing, MI , to determine the effect of soil, air, and apex temperatures on maize (Zea mays L.) leaf development. Maize leaf tip appearance dates and leaf number s were observed on four sowing dates to provide variations in the thermal r egime of developing plants. Solar irradiance and temperature of the air (1. 5 m height), apex, and soil (1-, 3-, and 5-cm depths) were recorded on 0.5- h (half-hourly) intervals. The daily average soil temperature at the 3- to 5-cm depth was reasonably close (+0.6 degreesC in average) to the daily ave rage apex temperature for use as a surrogate for apex temperature to increa se the accuracy of maize development simulation in the sowing to ninth leaf tip stage. Thereafter, the air temperature was sufficiently accurate to es timate plant development. Using apex temperatures from leaf 3 to 9, this st udy indicated that the phyllochron was near 55 degreesC d (degree days) per leaf tip appearance. The consistent bias between air and apex temperature from sowing to V6 found in this study clearly indicates the necessity of us ing the right temperature in thermal time calculations for accurate maize d evelopment simulation.