Predicting the Final Fruit Size of Kiwifruit cv. 'Hayward' Using Mathematical Equations

Fruit size is an important attribute of all fruit crops and harvest fruit weight has a major influence on market acceptance and returns for many fruit crops like kiwifruit (Actinidia deliciosa 'Hayward'. The purpose of this study was to develop a non-destructive technique by measuring fruit dimensions and fruit volume by water displacement to measure fruit growth and using this information to develop a method for predicting fruit weight at different stages of its development.

This experiment was conducted using the mature vines of ‘Hayward’ kiwifruit. The kiwifruit vines were grown in an orchard located in Astara, Guilan province, Iran (latitude 38.22°N and longitude 48.51°E), trained to a T-bar system with plants spaced of 4×5 m. Fruits were destructively harvested at one-two weeks after fruit set during the season to obtain measurements of individual fruit volume by water displacement, fruit fresh weight, circumference of fruit and three linear measurements of fruit size, from which a correlation relation was derived to enable estimation of fruit volume, fruit fresh weight from the fruit measurements (r> 0.995). This enabled the growth of tagged fruit on the vine to be followed by non destructive measurements of the fruit dimensions and the growth curves of each mean were estimated at each measurement step.

The increase in the fresh weight or volume of 'Hayward' fruit over the season was a double sigmoid in shape. The fruit density was less than one in about three months after the fruit set and then increased to more than one unit. Correlation analysis between the data of different stages of measurement with fruit harvesting data showed that with the progress of fruit growth, the correlation between fruit size and its final size increased, and about 55 days after fruit set reached to more than 0.95. The standard deviation of fruit size at this stage was 11.55 cm3. The correlation coefficient above 0.95 between fruit weight and its length with fruit weight and length at harvest time was calculated 72 and 89 days after fruit set, respectively. Fruit size in kiwifruit is affected by various factors and the final size and weight of the fruit is likely to be determined by the rapid fruit growth period. This period that results from cell division and cell size enlargement took about 50-60 days after fruit set for Hayward cultivar when the fruit reaches approximately half of its volume in this period. Applying any stress at the cell division stage will affect the final fruit size. Given the high correlation between the dimensions of the fruit with the final fruit size at the end of the rapid growth period, it is possible to predict the final fruit size at the end of this period with great accuracy. Therefore, using the obtained mathematical equations, the final size of the fruit can be estimated by the measurement of the length and circumference of fruit in one and a half months to two after the fruit set.

Based on the results of this study, with a simple and practical method and with a high and significant correlation coefficient, by measuring the length and circumference of the fruit about one and a half months to two after fruit formation, the final fruit size at harvest can be predicted in order to take the necessary measures to export, sell and/or store the fruit.