Frost Avoidance: Sodium Alginate + CaCl2 Can Postpone Flowering of ‘Kawanakajima Hakuto’ Peach Trees

The objective of this study was to investigate whether treatment with sodium alginate and CaCl 2 treatment in the 2 nd (swollen bud) blooming stage could delay peach flowers as a strategy to avoid frost. Regarding the expression rate at the 3 rd (calyx green) stage, the control treatment (CT, distilled water) showed a sharp increase from 8 days after treatments (DAT), reaching the maximum at 14 DAT. However, the 5AG group (5% sodium alginate + 100 mM CaCl 2 ) and the 7AG group (7% sodium alginate + 100 mM CaCl 2 ) recorded the maximum expression rate at 17 DAT. After the maximum expression, CT showed a sharply diminishing trend, whereas 5AG and 7AG showed gentle decreases. At the 4 th (calyx red) stage, CT, 5AG, and 7AG reached the peak expression at 20 DAT (43.5%, 31.9%, and 40.2%, respectively). However, maximum expression levels of all treatments were below ~50%. The 5th (first pink) stage was first expressed in all treatments when the expression rate at the 4 th stage peaked in all groups at 20 DAT. Maximum expression was 50.7% in CT, 33.7% in 5AG, and 49.2% in 7AG. This tendency was similar to that at the 4 th stage. Particularly, expression in 5AG diminished more slowly than that in CT and 7AG at 3 rd , 4 th , and 5 th stages. CT group bloomed first at 20 DAT, reaching full bloom at 81.4%, ahead of AG treatments at 28 DAT, while 7AG and 5AG groups bloomed fully at 34 and 37 DAT, respectively. Our results suggest that 5AG treatment is suitable to delay peach flowering when phenological development of the flower buds reaches the 2 nd stage.