Irrigation water supply can often be less than crop water requirement during a drought—this leads to tree water stress and subsequent loss in fruit size and yield in apple orchards.
Growers must weigh up choices between purchasing scarce water, if available, at high cost; removing the entire crop and ‘parking’ trees on minimum irrigation to keep them alive; or using a combination of thinning and deficit irrigation to prioritise fruit size/quality over yield.
Understanding the relationship between water deficits, yield, fruit size, fruit quality and crop load is critical for water budgeting and growing fruit to market specifications in drought conditions.
Given the current forecasts and season outlook, it is timely for growers to review the established international and Agriculture Victoria research on these relationships, and to take them into consideration when evaluating irrigation plans for next season.
Yield
Apple yield increases with irrigation up to the point where irrigation (and rain) matches the transpiration from the trees and the evapotranspiration from the wetted understorey.
Applying additional irrigation will not increase yield. The trees cannot transpire the additional water and may become waterlogged if drainage is inadequate.
The response of apple yield to water (irrigation and rain) is shown in Figure 1: the response of yield to water shows a linear increase up to point A.
Point A corresponds to the maximum transpiration from the trees and evapotranspiration from the wetted understorey.
The water inputs at point A represent the crop water requirement of the orchard.
Yield has reached a biological upper limit at point A where increasing water inputs to point B has no effect on yield and the difference in water input between point A and point B is ineffective and lost from the orchard system as deep drainage and runoff.
Fruit size
Just like apple yield, fruit size increases with irrigation up to the point where irrigation (and rain) matches the transpiration from the trees and evapotranspiration from the understorey (Figure 2).
There is, however, an upper limit to how big fruit can grow for a given crop load (point A), and applying additional irrigation (point B) will not grow bigger fruit.
Fruit thinning
Fruit thinning is undertaken in apple orchards to increase fruit size.
The optimum fruit size is driven by market demand.
Generally, fruit thinning is set at a level so that fruit size is less than the maximum and there is no impact on yield.
Fruit thinning will shift the response of fruit size to water input to a higher fruit size threshold (see Figure 2).
Heavy thinning will not shift fruit size beyond the genetic limitation of the crop and yield will be reduced. Most importantly, the water input to reach the fruit size threshold will not change.
Fruit quality
The response of fruit sweetness, cracking and sunburn damage was explored in Agriculture Victoria experiments on Cripps Pink and Royal Gala.
The results from these experiments showed that fruit sweetness and the risk of fruit cracking in susceptible cultivars like Cripps Pink, decreases with higher amounts of irrigation.
Sunburn browning was not affected by irrigation amount.
Fruit sweetness is measured by the concentration of sugars in the fruit. As the water inputs to an apple orchard increases, the total amount of sugar in an individual fruit increases but this is diluted by the size of the fruit.
Hence the concentration of sugar in an individual fruit and its sweetness decreases up to the point where irrigation and rainfall matches the transpiration from the trees and the evapotranspiration from the understorey (Figure 3).
The response of fruit sweetness to water inputs is much less pronounced than yield and fruit size.
Fruit thinning strategies
The combination of heavy thinning and water deficits is a strategy that can be employed during water restrictions.
(continued next month)
See this article & graphs in Tree Fruit Oct 2019
