Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/30/1999
Publication Date: N/A
Citation: Interpretive Summary: Frost damage to fruit crops remains a critical problem and each year millions of dollars are lost due to losses in yield and quality. If the frost damage occurs in the spring, whole crops can be lost due to killing of flower buds. Besides the actual impact on yield, millions of dollars are also spent in trying to provide frost protection. Unfortunately, these emethods are often too costly to implement, have only a limited effectiveness, or are environmentally unfriendly. In order to develop new, effective methods of frost protection a better understanding of ice formation and ice propagation in plants is needed. Recently, ARS has taken the lead in demonstrating the utility of using high resolution, infrared thermography to study freezing. Using this technology, one can visually observe how, when, and where ice is initiated. Additionally, one can determine how ice spreads. This information can be determined in a non- invasive manner. In the present study, infrared thermography was used to study the freezing process in cranberry plants. We determined that freezing of stems could only be initiated through the lower surface of leaves and that a barrier was present that prevented ice from propagating through the pedicel into the fruit. Freezing of the fruit could only be initiated through the calyx-end of the fruit. Therefore, for frost protection measures to work they must protect these two critical areas (the undersurface of the leaf and the calyx-end of the fruit). This information will be used to evaluate frost protection technologies.
Technical Abstract: Infrared video thermography has recently been used to visualize ice nucleation and propagation in plants. Using this method, the formation of ice in various parts (leaves, stems, fruits) of cranberry (Vaccinium macrocarpon Ait. Cv. Stevens) uprights was studied at the UW-Madison Biotron facility. Ice formed on the plant surface at -1 or -2 degrees C by yfreezing of a droplet of water containing ice-nucleating-active bacteria (Pseudomonas syringae). Samples were then cooled to a minimum of -8 degrees C. Observations on the initiation and propagation of ice were recorded. Leaves froze only when ice was presnet on the abaxial surface. Once initiated, ice propagated to the stem and the redily to other leaves. Under our experimental conditions we did not observe ice propagation from the stem to the fruit via the pedicel, in both unripe and ripe fruits. Fruit remained supercooled for periods up to one hour after ice was present tin the stem. Fruits could only be nucleated when ice was present at their calyx (distal) ends. Red (ripe) berries supercooled to colder temperatures and for longer durations than blush (unripe) berries before an apparent intrinsic nucleation event occurred. Our observations provide evidence that leaves are nucleated by ice penetration via stomata. The ability of fruits to supercool appears to be related to the presence of barriers to extrinsic ice propagation at both the pedicel and fruit surface. Stomata at the calyx end of the fruit in the remnant nectary may provide avenues for extrinsic ice nucleation.