Microsprinkler irrigation has proved effective in providing frost protection for young citrus trees, but there can be a risk of damaging trees during advective or windy freezes. The objectives of this study were to monitor evaporative cooling and to determine if height of young tree protection cold be increased to include the major scaffold branches. Microsprinklers were evaluated during a severely damaging advective freeze that occurred in central Florida in Jan. 1985. Trunk temperatures were measured at 15-, 30-, 45-, and 60-cm heights. By positioning the sprinkler at a 45° or higher angle, microsprinkler spray was aimed up into the young tree. This treatment was compared to the normal 15° low-angle spray pattern. Application rates on the wetted area were estimated to be 7 mm·hr−1 for high-angle treatment and 8.8 mm·hr−1 for low-angle treatment. With high-angle spray, trees were protected to a height of 85 cm, which was significantly higher than low-angle spray. Evaporative cooling below air temperature was not seen on irrigated trees. Continuous microsprinkler spray helped reduce evaporative cooling, but cooling did occur on a nearby tree that received only intermittent spray. Microsprinkler irrigation directed at an upward angle protected the trunk and scaffold branches of young trees in this particularly severe freeze and enhanced tree recovery.
Mature Florida ‘Valencia’ oranges [Citrus sinensis (L.) Osbeck] go through a period of reduced response to abscission chemicals. This nonresponsive period is characterized by both a reduction in the amount of ethylene induced by abscission chemicals and by decreased sensitivity of the abscission process to exogenous ethylene. During the nonresponsive period, application of the abscission chemical 5-chloro-3-methyl-4-nitro-1H-pyrazole (Release) induced less ethylene formation and less reduction in fruit removal force than at times either prior to, or following, the nonresponsive period. Experiments with radioactive Release showed that uptake was not a factor in the reduced effectiveness of this material during the nonresponsive period. Abscission of explants induced by exogenous ethylene was slower during the nonresponsive period. Mature ‘Valencias’ also go through a period of regreening. Regreening and the nonresponsive period may begin at about the same time, but regreening continues after the nonresponsive period is over. In addition, color changes induced by exogenous ethylene remain similar throughout the regreening and nonresponsive periods, but abscission of explants induced by exogenous ethylene is reduced only during the nonresponsive period. These differences in time-course and ethylene sensitivity between regreening and the nonresponsive period allow differentiation of these two processes.