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

Selecting plants with high harvest index (HI) can increase corm yield in we tland taro [Colocasia esculenta (L.) Schott]. Planting time, however, affec ts the response of III during the linear increase phase, with temperature b eing the most important factor affecting taro growth. The objectives of thi s research were to quantify the relationship of calendar days after plantin g (DAP) and growing degree days after planting (GDD) to HI in taro and to c ompare their ability to explain seasonal variation in the linear increase o f HI. Data from six planting months of field-grown taro across a 3-yr perio d were collated to investigate the linear increase in HI. The DAP and GDD ( with base temperature of 17 degreesC) were included as independent variable s to analyze the three-phase piecewise linear function with HI. Piecewise l inear functions based on either DAP or GDD fitted well. The responses durin g the linear increase phase of HI were, however, more stable across years f or 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 tha t the GDD model is a useful approach for determining weather-crop growth re lations in taro. Information gained in this study should help relate phenol ogical responses to seasonal variation in taro.