Horticultural News
       
        M. Elena Garcia,  Horticulturist

Apple trees and water relations

As the season progresses and the drought continues, the question in the mind of most growers in the state is; how will this drought affect my crop?   I will try to answer this question by reviewing some fundamental information about water relations in fruit trees. 
When apple trees are grown in areas that receive 30 to 50 inches of precipitation such as Vermont, they usually survive periodic droughts such as the one we are experiencing this year, but growers may lose money due to smaller fruit size, poorer fruit quality, and reduced yields. As a rule of thumb, 1.5 to 2.0 inches of precipitation per week are needed in an orchard because an individual tree may lose between 30 to 45 gallons of water in a day.  Even though fruit trees are made up of mostly water (table 1), only about 0.15 acre inches of water is harvested in the crop.  About 95 to 99% of the water taken up by a fruit tree is lost to the atmosphere through transpiration. 

Transpiration, the loss of water from the leaf tissue into the atmosphere, has two important functions in the plant.  First, the cells in the leaves are cooled because water is being evaporated.  This maintains the cells at an optimum temperature for metabolic processes such as photosynthesis to take place. If transpiration is limited due to water stress in the plant, photosynthesis will be limited and reductions in plant growth, fruit quality and yield will result.  Secondly, since water is moving from the roots to the different parts of the plant, this movement facilitates the passive movement of important minerals and dissolved chemicals throughout the plant.  Nutrient deficiencies are likely to be present under water stress situations.
During the winter months, apple trees use very little water. Water absorption from the soil by roots increases as leaf surface of the plant increases in the spring, and most of the water comes from the upper soil layer which is warmer and has greater feeder-root penetration.  The 'potential water' available to the plant depends on the root volume and the soil water holding capacity. 
The roots of deciduous trees such as apples are usually extensive and have been found at depths of 6 feet.  In sandy soil, roots have been found to extend laterally 2 to 3 times the branch spread and 1.5 times in loam or clay soils.  When roots are deep and extend well laterally and there is a good supply of winter precipitation, there is usually no need for irrigation.  However, where rooting is shallow such as the root systems of growth-controlling rootstocks of high-density plantings, the need for irrigation will be higher than for standard low-density plantings. Today, many orchards particularly those with the shallow-rooted dwarf trees provide some means of water application to protect the trees from water stress. 

The ability of the soil to retain water is called the water-holding capacity and this capacity to hold water varies with the different textures of the soil.  Table 2 shows the relationship of soil type to its water holding capacity and the moisture available to the plant.

During the growing season the water status within a tree changes from day to day and hour to hour, whereas the soil water changes slowly.  Tree water usage increases mainly with increased air temperature, air movement sunlight intensity and with lower relative humidity.  Table 3 which appeared in the March issue of Cornell Northeast Tree-Fruit Newsletter (K. Iungerman) gives you  an indication of how much water is needed when canopy size and evapotranspiration (evapotranspiration is the total loss of water by evaporation from the soil surface and transpiration from plants from a given area, and during a specified period of time ) are considered.