Boron dynamics related to fruit growth and seed production in kiwifruit (Actinidia deliciosa, cv. Hayward)
The effect of foliar boron treatments on fruit growth and seed development in Actinidia deliciosa, cv. Hayward, was studied. The distal leaves (BH) or flowers (BF) of a shoot were sprayed with 500 mg L-1 boric acid during the flowering period. The mobility of boron from the leaves to the flowers through the phloem was determined by spraying only the leaves. From 0 to 96 h after treatment, leaves and flowers were sampled and total boron was assessed. At harvest time, fruit weight and diameter plus seed weight and number were measured for the different treatments. The weight of fruits growing from shoots with boron-treated leaves were 14.1% higher than the control, while the weight of fruits derived from boron-treated flowers was 17% higher than that of fruits from untreated flowers. Regarding fruit length, significant differences between boron treatments and the control were observed, with an 8.2% and 8.5% increase for BH and BF treatments, respectively. No significant differences were found in fruit diameter. Fruits grown from shoots with boron-treated leaves had 43% more seeds than fruit grown from untreated control shoots, while fruits resulting from boron-treated flowers had 44% more seeds. This demonstrates the positive effect of boron treatment in seed production. No differences were observed in seed weight between treatments. The correlation coefficient between seed number and fruit weight was r2 = 0.1294 for BF, r2 = 0.1298 for BH and r2 =0.0002 for the control. There was no variation in non-treated leaves and flowers with respect to boron concentration for a 96 h time period. On the contrary, in sprayed leaves boron levels reached up to 16 mg kg-1 between 24 and 48 h and slightly decreased after 96 h, demonstrating the boron absorption capacity of leaves. In the flowers from the shoots with boron-treated leaves, there was a significant 14 mg kg-1 increase in boron concentration from 24 to 96 h, showing boron mobility from leaves to flowers.