Growth regulators ABA and paclobutrazol were used at different concentrations to induce hardiness in blueberry flower buds and floral parts. Critical freezing temperatures and the effectiveness of the treatments were determined by differential thermal analysis (DTA), electrolyte leakage, visual browning, and tetrazolium staining. Treatment effects of growth regulators were nonsignificant on whole flower buds, but treatments induced hardiness in floral parts on the second flush of flowers at stage six produced in April. Induction of cold hardiness by ABA and paclobutrazol was concentration dependent. The higher the concentration, the greater the response. Viability test results on each floral part showed a close relationship with the critical freezing temperatures recorded by DTA. Control treatments showed that floral parts at stage six developed in April were more prone to freezing injury compared to floral parts at stage six developed in early March.
Air temperature on crop productivity is a complex topic in environmental physiology. Blueberry growers in the Southeastern United States experience major crop losses due to late spring frosts. A 2-year study was conducted on `Tifblue' rabbiteye blueberry flower flushes to determine the effect of abscisic acid (ABA) at 0, 20, 40, and 60 mg·L–1 and paclobutrazol at 0, 100, 200, and 300 mg·L–1 on cold hardiness of floral parts. Two types of flower flushes were identified in `Tifblue'. Critical freezing temperatures and the effectiveness of the treatments were determined by differential thermal analysis (DTA), electrolyte leakage (EL), oxidative browning and tetrazolium staining. Floral parts frozen to –40 °C produced only one exotherm, confirming that rabbiteye floral parts do not supercool. Both growth regulators were more efective in the induction of hardiness in floral parts at second flush than at first flush. Floral parts developed in April were more prone to freezing injury than the floral parts of March. Ovaries were the hardiest, followed by calyx, stamen, style, and corolla. Air temperature had a profound influence on cold hardiness as influenced by ABA and paclobutrazol. The sequence of exotherms of DTA and the LT50 of the viability tests were air temperature-dependent.