Impacts of cold stress and temperature fluctuations on bud abortion and malformed flower development on Hayward kiwifruit

This study investigated the effects of cold stress and fluctuating temperatures on bud abortion and flower malformation in one-year-old dormant canes of 'Hayward' kiwifruit (Actinidia deliciosa). Cuttings were collected from a commercial orchard in Rasht, Iran, and standardized prior to exposure. Following 950 chilling hours at 4 °C, cuttings were subjected to either cold stress (- 2 °C for 40 h) or fluctuating temperature treatments (17 °C → 30 °C → -1 °C) at three phenological stages: chilled dormancy (CD), woolly bud (WB), and bud break (BB). Bud development was assessed morphologically and histologically across six developmental stages. Floral buds were classified as normal, aborted, or misshapen, and percentages were calculated per treatment. Cold stress during WB and BB stages caused severe bud damage, including incomplete floral organ differentiation, vacuolization, protoplast shrinkage, and phloem necrosis. Bud abortion rates increased with phenological advancement, reaching 51% in BB-stage buds, while abnormal flower formation peaked at 29% in WB-treated buds. Fluctuating temperatures also induced developmental instability, with abortion rates of 39%, 31%, and 58% at CD, WB, and BB stages, respectively, and histological analyses revealed widespread necrosis and organ malformation. Control buds under constant growth conditions (24/17°C day/night) showed normal organogenesis. These findings demonstrate that advanced developmental stages are particularly sensitive to low or fluctuating temperatures, which disrupt floral organ differentiation and increase the incidence of abortion and malformation. Cold stress at critical stages (WB and BB) caused the highest cellular damage, while temperature fluctuations accelerated developmental instability. These results emphasize the importance of understanding the impact of sudden climatic fluctuations on kiwifruit bud development, highlighting the need for growers to implement appropriate strategies to mitigate the negative effects of such environmental stressors.