Seasonal variation in linear increase of taro harvest index explained by growing degree days

Abstract

Selecting plants with high harvest index (HI) can increase corm yield in wetland taro [Colocasia esculenta (L.) Schott]. Planting time, however, affects the response of HI during the linear increase phase, with temperature being the most important factor affecting taro growth. The objectives of this research were to quantify the relationship of calendar days after planting (DAP) and growing degree days after planting (GDD) to HI in taro and to compare their ability to explain seasonal variation in the linear increase of HI. Data from six planting months of field-grown taro across a 3-yr period were collated to investigate the linear increase in HI. The DAP and GDD (with base temperature of 17°C) were included as independent variables to analyze the three-phase piecewise linear function with HI. Piecewise linear functions based on either DAP or GDD fitted well. The responses during the linear increase phase of HI were, however, more stable across years for the GDD model. Moreover, the model based on GDD was superior to DAP for explaining the seasonal variation of HI in taro. These results indicate that the GDD model is a useful approach for determining weather–crop growth relations in taro. Information gained in this study should help relate phenological responses to seasonal variation in taro.