Estimating the temperature of a maize apex during early growth stages

When the leaf area index is low at the early stages of growth, the temperature of a maize apex can be much higher than the air temperature measured at screen level. In order to account for temperature effects in plant growth simulation models, it would be better to use plant temperature rather than air temperature. We propose a model to estimate the apex temperature for both day-time and night-time averages from a small number of readily available meteorological data: solar radiation, wind speed, air temperature and humidity. It is based on an energy balance of a maize apex under field conditions. It performs a radiation balance that separates diffuse and direct components, and assumes a similarity between the apex and soil surface temperature evolutions. In the absence of any references, the apex stomatal resistance was parameterized as a simple linear function of water vapour deficit, deduced from experimental data. The calculated temperatures were compared with those measured for two sets of experimental data collected in 1989 and 1990. The agreement was quite satisfactory, the average absolute error being in all cases less than 1.0°C. Furthermore, the empirical relationship between stomatal resistance and water vapour deficit was shown to be valid for both sets of data. We should now confirm this relation under different soil or climatic conditions, as well as the similarity between the apex and soil surface temperatures.