Irrigation water supply can often be less than crop water requirement during a drought. This inevitably 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 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. In Figure 1, the response of yield to water shows a linear increase up to point A corresponding 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.

Fig. 1. The response of apple yield to water inputs. Yield is expressed relative to the maximum possible yield in the orchard. Water input is expressed relative to the maximum transpiration from the trees and the evapotranspiration from the wetted understorey. Point A represents the maximum yield and the corresponding crop water requirement of the orchard. Point B shows that additional water has no impact on yield.

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 is undertaken in apple orchards to increase fruit size. The optimum fruit size is driven by the 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.

Fig. 2. The response of apple fruit size to water inputs at a standard crop load (solid line) and a low crop load (dashed line). Fruit size is expressed relative to the maximum possible fruit size in the orchard. Water input is expressed relative to the maximum transpiration from the trees and the evapotranspiration from the wetted understorey. Point A represents the fruit size upper threshold for a given crop load and the crop water requirement of the orchard. Point B shows that additional water has no impact on fruit size.

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.

Fig. 3. The response of fruit sweetness to water inputs at a standard crop load. Fruit sweetness is expressed relative to the maximum concentration of sugars in the fruit. Water input is expressed relative to the maximum transpiration from the trees and the evapotranspiration from the wetted understorey. Point A represents the fruit sweetness corresponding to the crop water requirement of the orchard. Point B shows that additional water has no impact on fruit sweetness.

Fruit thinning strategies

The combination of heavy thinning and water deficits is a strategy that can be employed during water restrictions. Market fruit size can be maintained, but yield will be substantially reduced. For example, a market fruit size (e.g. fruit weight at 80 per cent of maximum size) with 50 per cent less irrigation is possible if trees are thinned to 20 – 25 per cent of the standard crop load.

Overseas data published in Food and Agriculture Organisation of the United Nations (FAO); Irrigation and drainage paper 66 (FAO 66) – Crop yield response to water http://www.fao.org/docrep/016/i2800e/i2800e00.htm  was used to construct the response of fruit size and yield to a range of water deficits and crop load shown in Figure 4. The important points in this figure are:

• The relationship between fruit size and yield is determined by crop load.
• Yield increases with crop load up to the biological maximum in an orchard. Further increases in crop load have no impact on yield.
• Fruit size rapidly declines with increases in crop load when the maximum yield is reached.
• Fruit size and yield decrease with increasing water deficits irrespective of crop load.
• The effects of water deficits on fruit size are much less with heavy thinning.

Fig. 4. The response of fruit size and yield to combinations of water inputs and crop load. Fruit size and yield are expressed relative to the maximum possible fruit size and yield in the orchard. The response curves for three levels of irrigation (50 per cent (red), 75 per cent (orange), 100 per cent (blue) of crop water requirement) and five of crop load (very low, low, reduced, standard and high) are shown. Source: FAO 66.

Thinning early in the season is recommended. Base the level of thinning on fruit set and the outlook for irrigation water allocations. Regularly review irrigation strategies for different cultivars when low allocations are expected. A further follow-up thinning may be needed, although the longer thinning is delayed, the smaller the fruit will be at harvest. There are two reasons for this. Firstly, cell division dominates fruit growth at the start of the season. Any restriction on cell division cannot be regained later in the season. Conditions must be favourable for cell division so any competition between fruits needs to be minimal. Secondly, dry weight accumulates in developing fruits but there is a limited total pool of dry weight to go around. Too many fruits means that the tree can only supply each fruit with a limited amount of dry weight. At the start of the season there is usually enough dry weight for all the fruit. However, as fruit grows its demand for dry weight increases rapidly. Thinning well before the demand by the fruit exceeds the supply available from the tree is critical to maximise final fruit size.

Contact/Services available from Agriculture Victoria

Ian Goodwin, Research Leader, Tatura, Email: ian.goodwin@agriculture.vic.gov.au, Phone: 03 5833 5240

Acknowledgements

Content written by Ian Goodwin, Mark O’Connell, and Lexie Mcclymont.

This is an updated version of an article originally compiled as part of the apple and pear industry Productivity Irrigation Pests and Soils (PIPS) program and funded by Hort Innovation using the apple and pear industry levy and matched funds from the Australian Government. Additional financial support was provided by Agriculture Victoria. Thank you to Maurice and Rein Silverstein for allowing experiments to be conducted on their orchard.