Freezing and chilling injury in plants has been categorized into three distinct groups according to the threshold below which injury is observed. Chilling sensitive species of plants exhibit injury between 10° C. and 15° C. Chilling resistant but freeze sensitive species of plants exhibit damage from 0° C. to −10° C. Freeze tolerant species of plants are able to survive long periods of subzero temperatures via cold acclimation.
The existing technology to mitigate chill and freeze damage includes:
1) materials that change the freezing point of the plant tissue or water. Patents related to this technology include: Barr, et al U.S. Pat. No. 5,133,891 which relates to Treatment of plants for frost protection by the application of an organic chemical; Shin et al U.S. Pat. No. 5,276,006 which relates to a Cryoprotectant composition that increases the tissue resistance to freeze damage; Savignano et al U.S. Pat. No. 5,653,054 which relates to a Process for preventing frost formation on plants that lowers the freezing point of water; Lengyel U.S. Pat. No. 4,597,883 which relates to a Composition and method for minimizing frost damage to plants that uses a salt-based solution to lower the freezing point of water and resist cell damage to freezing temperature; Artozon U.S. Pat. No. 5,618,330 which relates to Plant treatment compositions and process that uses high concentrations of salts to protect against frost damage; and Suslow et al U.S. Pat. No. 5,633,450 which relates to Chitinase-producing plants that are resistant to cold damage.
2) reduce the populations of ice nucleating bacteria on the crop surface, thereby inhibiting ice formation on the crop surface. Patents related to this technology include: Lindow U.S. Pat. No. 4,432,160 which relates to microorganism inhibition of frost damage to plants which is a method describing the selection and use of ice nucleating deficient bacteria to prevent freezing damage; and Orser et al U.S. Pat. No. 4,766,077 which relates to Ice nucleation deficient microorganisms by genetic manipulation which is a method to produce ice nucleating deficient organisms to be applied to plants as a frost protectant.
3) delay dehardening. This technology does not directly prevent frost damage, but delays the development of frost-sensitive reproductive tissues in the early growing season so that frost does not occur when frost-sensitive tissues are exposed.
4) non-chemical approaches. Muscatell U.S. Pat. No. 4,434,345 which relates to a Microwave system for frost protection of fruit trees that generates heat to prevent freezing, and Donohue et al, which relates to a Method and apparatus for the protection of citrus trees from frost damage that is an insulating pad for the trunk of the tree. Wind machines that mix the air over a field and prevent inversion layers of freezing air at ground level are a common method of frost control. Burning material to generate heat and smoke to disrupt the inversion layer at ground level was used in the past but environmental concerns preclude its present day usage. The most common non-chemical strategy is to apply water to plant surfaces to utilize the heat of fusion and keep the plant temperature at or near freezing.
5) preventing ice nucleation. Wisniewski, M. and M. Fuller (Ice nucleation and deep supercooling: new insights using infrared thermography in: Cold Adapted Organisms: Fundamentals and Applications. Eds. R. Margesin and F. Schinner. Landes BioScience, Austin, Tex.) have shown that the application of a silicone grease to plant surfaces makes a water repellant film that prevents ice from propagating into the plant and allows the plant to supercool, thus preventing frost damage. However the application of silicon grease to plants is phytotoxic by also preventing the exchange of gases from the leaf. Glenn et al. U.S. Pat. No. 6,235,683 “Method for enhanced supercooling of plants to provide frost protection” is a method that prevents ice nucleation within the plant by coating the plant with a hydrophobic particle film that prevents water from contacting the plant surface and transferring ice nucleation into the plant.