Citrus Irrigation: Why Is It Important To You?

Article reproduced with permission
Citrus Industry Magazine, April 1995

By Larry Parsons and T. Adair Wheaton

Most growers accept that irrigation is a necessary part of their operation, but many may not realize just how important irrigation is. Water, or the lack of it, has a major impact on both ultimate growth and production. The purpose of this article is to discuss why water is important and to answer some questions related to irrigation.

Does it pay to irrigate in rainy Florida?
Early reports in the 1950s suggested that, given the normal rainfall in Florida, it did not pay economically to irrigate. This conclusion was reached because growers in the 1950s didn't have the knowledge or technology they have today to properly manage irrigation systems. Also, citrus in Lake and Orange counties was often grown on soils with better water holding capacities than most of the southern Ridge soils. These better soils made the benefits of irrigation less apparent. In the 1950s, growers used high volume overhead sprinklers, volume guns, flood irrigation systems or systems that required labor to move pipe around the field. Growers would often wait until trees wilted before starting to irrigate. Irrigation was practiced to save the trees, and yields generally were not sufficiently improved to justify the cost of the irrigation system. In his studies from the 1960s through the 1980s, Robert Koo showed that irrigation definitely paid off even in Florida. By irrigating trees before they wilted, Koo showed that yields could be increased significantly compared to the non-irrigated controls even in a year of above normal rainfall. Not all varieties responded equally to irrigation. For example, in his experiments, Marsh grapefruit yield increased by 52 percent; Valencia and Hamlin yield increased 35 to 38 percent, but Pineapple orange yield increased only 14 percent. In most cases, it definitely pays to irrigate. If Florida had soils with better water holding capacity and if rainfall came when the trees needed it, then irrigation would not be necessary except for frost protection. But periodic droughts and coarse-textured soils have convinced most growers that irrigation is worthwhile.

What causes water stress and what process is most sensitive to water stress?
Water stress can be caused by many factors, but the simplest cause of water stress is that water loss from the plant is greater than water uptake. Hot dry winds can desiccate trees by removing water from the leaves faster than they can take water up. Cold soil can promote wilting by slowing root water uptake. Ironically, flooding can also cause wilting. The excess standing water can damage roots and thus reduce water uptake, particularly if toxic hydrogen sulfide builds up in the soil. Usually, wilting is caused by inadequate water. When the soil is dried to the point where the tree cannot recover from a wilt by the next morning, the soil is said to be at the permanent wilting point. Growth, or cell enlargement, is the process that is most sensitive to water stress. Without adequate water, the cellular turgor pressure (or turgor potential) is reduced and then cell enlargement is reduced. Growth can be slowed down at relatively mild levels of water stress. Leaf enlargement or shoot elongation can be reduced slightly by a few days of water stress. This can translate into reduced canopy growth in a few seasons. In a test where trees were not allowed to experience even mild water stress, 100 inches of reclaimed water were applied to trees in a well-drained soil. These trees grew faster and their canopies were significantly larger than trees that received only a moderate amount of water. The trees that received 100 inches of water experienced virtually no water stress and hence showed the greatest growth rate. Trees that reached the larger canopy size produced more fruit. This 100-inch rate was not intended to be a recommended irrigation rate but was applied to see how excessively high rates of reclaimed water would affect citrus trees. This rate would not be economically practical or allowed unless one were trying to dispose of excess quantities of reclaimed water in a well-drained location. While irrigation can increase yield, effects on fruit size can be variable. In some cases, irrigation will increase fruit size. However, if the irrigation treatment increases total fruit set, the greater number of fruit per tree can lead to smaller individual fruit size.

What other processes are affected by water stress?
Many processes are affected by water stress. Some enzyme levels are lowered while the levels of some enzymes involved in substrate degradation can be increased. Certain growth regulators, such as abscissic acid, increase during water stress. Free amino acid levels often increase with water stress. Proline, one of the amino acids, can rise noticeably in some species after several days of water stress. From the grower's point of view, one of the more important points is that water stress can promote stomatal closure and this can reduce photosynthesis. Since sugar is manufactured by photosynthesis, it is important that photosynthesis be kept near optimum levels to achieve sufficient sugar production for growth and development. Water stress can reduce overall photosynthesis in two ways: 1) by reducing leaf growth and total tree leaf area, and 2) by closing stomata and reducing carbon dioxide uptake. Stomata are influenced by several factors including humidity, carbon dioxide levels, and light intensity, but water stress is one of the major factors that promotes stomatal closure. Both low temperature and water stress play a role in flower induction. In the hot and humid lowland tropics, there is little seasonal variation in temperature. In these areas, flowering is usually rather continuous throughout the year. Greater flowering intensity usually occurs when rainfall comes after a period of dry weather. Water stress has been used for more than a century in Mediterranean countries to stimulate flowering in lemons. By withholding water during certain periods, the timing of flowering can be altered so fruit can be harvested at a time of higher prices (this is the case with 'verdelli' lemons). It is important to determine the optimum level of stress at which irrigation should be started after the dry period. Too little stress can promote an inadequate flowering response. Too severe a stress can cause excessive leaf and fruit drop. Moderate water stress is recommended to induce sufficient flowering without excessive fruit drop. The lengthening of water stress (up to 45 days) increased flower density in Valencia oranges in studies carried out in Cuba in the 1970s. Water stress can also influence cold hardiness. Mild water stress has been shown to temporarily increase cold hardiness in greenhouse tests, but excessive water stress can weaken the trees and reduce cold hardiness. The old idea that fall irrigation should only be applied when trees are severely wilted appears to be ill-advised. Koo found that after freezes, non-irrigated trees showed more leaf and fruit loss than trees irrigated one to five weeks before freezes. He suggested that temporary leaf wilt at midday could be used as a guide for fall irrigation. With variable fall and winter rainfall, it is difficult to maintain cold hardiness, particularly if the winter daytime temperatures are above normal.

Are there disadvantages to too much irrigation?
Yes. As suggested above, over-irrigation in the winter can potentially reduce flower induction and decrease cold hardiness. Water restrictions are another reason for not applying too much water. Excessive irrigation can promote greater weed growth and cause other problems. In the example given above, the trees that received 100 inches of water had excessive weed growth, particularly when the trees were young. This could be managed with extra mowings, but the caretaking costs were higher. This excessive irrigation also caused dilution of the soluble solids in the fruit. Due to the greater fruit production, total pounds solids per acre were greater, but pounds solids per box declined with the high irrigation rate. Phytophthora propagule counts also increased to higher levels after several years of the high irrigation treatment. Another problem with excessive irrigation is the increased risk of degrading groundwater quality by leaching of nutrients and pesticides. The sandy soils of the Ridge are very susceptible to leaching. Years ago, no one worried about leaching of pesticides or nitrates into the groundwater, but now nitrate-N levels above 10 mg/liter are of concern in some areas. Over-irrigation that moves materials below the root zone represents an economic loss as well as a potential for movement into the groundwater. In terms of groundwater issues, regulatory pressure is likely to increase, so growers need to keep refining their irrigation management practices.

What about the present economics of irrigation?
While irrigation can pay off, at some point the increase in yield with greater water application levels off. Fruit prices and the cost of irrigation will determine if higher rates of irrigation are justified. Advantages of higher irrigation in terms of yield must be compared with the disadvantages and risks associated with too much irrigation. With high fruit prices, additional water application could give some additional yield and income, but with low fruit prices, additional irrigation may not be economically justified.

CONCLUSIONS
This article answers a few of the basic questions on why irrigation (or water stress) is important. With a background knowledge of how water stress and irrigation can affect trees, growers can better understand how to manage irrigation. Irrigation is economically worthwhile, and proper water management can pay off in terms of improved tree health, growth, and yield. Parsons and Wheaton work at the University of Florida/IFAS Citrus Research and Education Center at Lake Alfred.

Back