A phenological study of snap bean (Phaseolus vulgaris L.) for predicting harvest

Field studies were conducted in 1989 and 1990 at Knoxville and in 1990 at Crossville, Tennessee to observe the phenology of snap bean (Phaseolus vulgaris L.) and to develop heat unit models for the cultivars 'Eagle' and 'Provider'. The best models were selected to predict bean plant development, especially the time to harvest maturity.

The experimental design was a randomized complete block with sub treatments in a split plot arrangement. The main treatments were cultivars 'Eagle' and 'Provider'. Sub plot treatments consisted of 10 planting dates at Knoxville and 6 planting dates at Crossville. Phenological observations were taken on a weekly basis and analyzed using ten methods of heat unit summation. Data on pod yield, pod distribution, and fiber content were recorded.

Since none of the ten methods of heat unit summation utilized resulted in consistently low coefficients of variation (c.v.), a model of growing degree days using the conventional method in which daily mean temperatures minus the base temperature 10°C (50°F) were summed was performed for snap bean in this study. A greater number of heat units was required at Knoxville than at Crossville to obtain harvest maturity for both cultivars. Results from multivariate regression analysis indicated the highest was obtained in 1989 at Knoxville and in 1990 at Crossville for the model including five variables; growing degree days (GDD), solar radiation (RAD), air relative humidity (ARH), wind speed (WSP), and precipitation (PPT). Solar radiation (RAD) resulted in a greater R² than growing degree days in 1990 at Knoxville and Crossville when canopy height temperature and height relative humidity, and soil temperatures at 2.5 cm and 10.2 cm (1 and 4 inch) depth were added to the regression models.

Pod yield was significantly influenced by planting date over all experiments although it was not influenced by cultivars. Pod fiber content was greatest from plants at the latest planting date in 1989 at Knoxville. The pod fiber content, however, was affected only by pod size in 1990 at Knoxville and at Crossville.

Results of these experiments in arriving at the best heat accumulation models for predicting the time of harvest maturity suggested that further research is needed since the best model was different for each experiment. A more precise description of each phenological stage and more frequent observations would improve the model. Other possibilities might include using base temperatures other than 10°C (50°F) and using different base temperatures for vegetative stages and reproductive stages although this approach would change the current assumption of heat unit summation.