Estimation of leaf number of eggplant using thermal time model

A two-year investigation was carried out during 2006 (on three cultivars) and 2007 (on one cultivar) under open-field conditions, respectively, to test whether a model based on the thermal time approach could be developed to estimate the leaf number of eggplant (Solanum melongena L.) across cultivars. Thermal time (°Cd) was calculated using a base temperature of 11°C and a ceiling temperature of 32°C. Leaves that were beginning to unfold with a width of 2 cm or greater were counted every 3 to 4 days. A relationship between the leaf number and the thermal time was established in the calibration experiment (2006) on three cultivars, and data from another cultivar in 2007 growing season was used for model validation. Results indicated that a single equation could be used to predict leaf number of all cultivars in response to thermal time. The response of the leaf numbers to thermal time was curvilinear, with a lag over the first 450°Cd. After 450°Cd, the increase in the leaf number per plant was linearly related to the thermal time. These results would be useful in developing a model of leaf area development and eventually a crop growth model for eggplant.