Maize leaf development biases caused by air-apex temperature differences

Accurate determination of leaf appearance rate is required in crop simulation models to estimate canopy development and ultimately crop yield. Most crop simulation models use air temperature for thermal time calculations to estimate leaf appearance rate, although the near soil temperature is more closely related to the growing apex temperature than air temperature before stem 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 numbers were observed on four sowing dates to provide variations in the thermal regime 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 average apex temperature for use as a surrogate for apex temperature to increase the accuracy of maize development simulation in the sowing to ninth leaf tip stage. Thereafter, the air temperature was sufficiently accurate to estimate plant development. Using apex temperatures from leaf 3 to 9, this study 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 using the right temperature in thermal time calculations for accurate maize development simulation.