Fruit tree growers in many areas are concerned with orchards that have been damaged by extremely cold weather. Others are faced with severe cut backs on irrigation water and are worried about maintaining groves under drought conditions. A proper nutritional spray program can assist the grower in coping with both these problems.
Environmental Stress on Crops
Freeze damage and drought are obvious forms of environmental stress. Furthermore, the two conditions share a common stress: dehydration. In freeze injury, loss of cellular water to extracellular ice also results in a mechanical stress as the cell walls collapse and the cells die. Dehydration by water stress under drought conditions is similar except that the extracellular space is filled with air rather than ice and the cell walls do not necessarily collapse(1). Plants can avoid drought stress and maintain turgor by minimizing water loss through transpiration and by osmotic adjustment; i.e., a reduction in cell osmotic pressure (or increase in solutes) greater than can be explained simply by cellular water loss or reduced cell volume(6).
Properly used, foliar nutrients have the ability to prevent or to alleviate a wide variety of stress conditions. Nutritional sprays are normally timed to anticipate and relieve physiological stress, specifically reproductive stress associated with the blooming and fruiting periods. But foliar nutrients also are effective in dealing with environmental and mechanical stresses. The mechanisms are not well understood and are considered to be extremely complex(2), but the most basic explanation is simply that injury from severe environmental conditions leads to stress-induced nutrient deficiencies, either by interfering with normal uptake of nutrients from the roots or by the loss of carbohydrate reserves from cell loss. Superimposed on this is the influence of micronutrients on plant growth regulators (hormones) which are ultimately responsible for proper plant response. Freeze damage is likely to cause delayed leafing-out and reduced production on fruit trees since the regulator systems that are responsible for normal spring growth and development have been disrupted(3).
The Role of Nutrients for Freeze Recovery & Drought Resistance
The following nutrients are considered to play major roles in recovery from freeze damage or in hardening-off for drought resistance. The program to be applied will vary depending on the desired response.
Zinc is important for its ability to influence auxin levels. Zn has long been known to be a co-enzyme for production of tryptophane, a precursor to the formation of auxin(4). In most trees, the general sequence of activity in early spring is auxin production, bud swell, auxin movement, auxin-directed transport, root growth, followed by phloem and xylem differentiation(3). As indicated above, normal auxin functions are likely to be disrupted in freeze-damaged trees. Maintaining adequate hormone levels gives the tree a competitive advantage to withstand adverse conditions of all kinds.
Manganese, foliarly applied in early season, tends to give a vegetative response less pronounced than Nitrogen but longer lasting in effect. The result is a boost to early growth to promote recovery from freeze damage. Mn is not recommended in a drought resistance program.
Phosphate is the principal element involved in plant energy processes. Its need is critical because of the role of ATP (adenosine triphosphate) in recovery(1). Phosphate levels may be low due to dry soil conditions or impaired root uptake and should be reinforced for both types of stress.
Potassium is the major nutrient lost in the cell leakage that results from freeze injury(1) and needs to be replaced. Potassium also promotes root development. It is a critical part of a program for both freeze recovery and drought hardiness. Potassium is very active in plant-water relations where it serves an osmoregulatory function. It has been well documented that K plays a role in stomatal opening and closing(7). Increasing K fertilization is believed to promote osmotic adjustment, helping to maintain positive turgor pressures at low leaf water potentials and improving the ability of plants to avoid drought stress(6).
Calcium was once considered important only for cell-wall structure, but since the recent discovery of Calmodulin it has become clear that Calcium is not just a macronutrient but a major controller of plant metabolism and development(5). Calcium is considered to play a role in mediating stress response during injury, recovery from injury, and acclimation to stress(1). It has been suggested that Ca++ is necessary for recovery from freeze injury by activating the plasma membrane enzyme ATPase which is required to pump back the nutrients that were lost in cell damage(1). Since dehydration is the common denominator, Calcium also has a role to play in drought tolerance.
The contributions of other elements—Boron, Iron, Copper, Magnesium, and Sulfur are not as well-defined. Nitrogen, of course, plays the key role in stimulating vegetative growth but only moderate amounts are suggested in a freeze recovery program. Copper promotes freeze-hardiness due to its ability to inhibit ice-nucleating bacteria; it should be applied before a freeze, but its role in freeze recovery is considered less important. In the Pacific Northwest, Boron is important in any regular nutritional spray program to influence fruiting.
SUGGESTED PROGRAMS FOR FREEZE RECOVERY:
|2 quarts per acre.
1 quarts per acre.
1 quarts per acre.
Alpha may be substituted for CAL-BOR.
Sigma may be substituted for Hi-K
Deciduous fruits: Delayed dormant up to pre-bloom.
Citrus, Avocados: As soon as possible following the freeze.
This program supplies: Zinc, Manganese, Phosphate, Potash, Calcium, Boron.
For additional FREEZE RECOVERY spray programs, read more.
SUGGESTED PROGRAMS FOR DROUGHT RESISTANCE:
|1-2 quarts per acre.
1-2 quarts per acre.
Timing: Early to mid-summer, prior to onset of highest temperatures.
This program supplies: Zinc, Potassium, Phosphate, and Calcium.
- Palta, J. P. Stress Interactions at the Cellular and Membrane Levels. HortScience 25(11):1377.
- Lakso, A. N. Interactions of Physiology with Multiple Environmental Stresses in Horticultural Crops. HortScience 25(11):1365.
- Seeley, S. Hormonal Transduction of Environmental Stresses. HortScience 25(11):13
- Bennett, J. P. and F. Skoog. Preliminary Experiments on the Relation of Growth-promoting Substances to the Rest Period in Fruit Trees. Plant Physiol. 13:219-225.
- Poovaiah, B. W. and A. S. N. Reddy. Calcium Messenger Systems in Plants. CRC Crit. Rev. Plant Sci. 6:47-102.
- Eakes, D. J., R. D. Wright and J. R. Seiler, J. Am. Soc. Hort. Sci. 116(4):712-715 (1991).
- Meidner, H. and C. Willmer. Mechanics and Metabolism of Guard Cells. Current Adv. Plant Sci. 17:1-15 (1975).