Modeling pollen tube growth of 'Gala' and 'Fuji' apples

Environmental factors like air temperature directly affect the pollen tube growth which in turn can regulate fruit set and crop load in apple. The estimation of the pollen tube length and the time when the pollen tube reaches the stylar base is useful information for managing fruit load, site selection and the improvement of technologies such as mechanical pollination. Therefore, the objective of this study was to develop a model to predict pollen tube length based on air temperature and style penetration (%). Flowers of two apple cultivars, 'Fuji' and 'Gala', were emasculated, hand pollinated with 'Red Delicious' commercial pollen and placed in growth chambers at one of eight constant temperatures (4.5, 7, 11.5, 14.5, 21.5, 22.6, 28 and 30 °C). Pollen tube lengths of three flowers were measured at different five times after pollination for each evaluated temperature using a fluorescent microscope and measurements were adjusted to logistic and logarithmic curves as a function of time after pollination. Derivatives of the adjusted curves allowed the estimation of the hourly pollen tube growth rates, which in turn were fitted to a response surface as a function of temperature and style penetration. Finally, predicted pollen tube length was obtained by integrating hourly growth rates from the adjusted response surface. The assessment statistics showed an adequate performance of the model with a root mean square error (RMSE) of 0.95 mm for 'Fuji' and 1.87 mm for 'Gala'. However, predicted pollen tube lengths were underestimated when the observed pollen tube lengths were short, mainly related to growth at low temperatures. For this reason, future upgrades should adjust the hourly pollen tube growth rates at low temperatures as well as set apart the temperature effects on fully and semi-compatible crosses.