Orchard water use relates to leaf area

Control vegetative growth to improve the water-use efficiency of apple orchards. By Engela Duvenage.

This is one of the conclusions in a recent report by a team of researchers from the Council for Scientific and Industrial Research, the Agricultural Research Council, Stellenbosch University, and the University of Pretoria.

The report followed the completion of a four-year project co-funded by the Water Research Commission and Hortgro. Research was conducted in high-yielding orchards of Golden Delicious, Cripps Pink, Cripps Red, and Rosy Glow on seven farms in the climatically diverse Koue Bokkeveld and Elgin-Grabouw-Vyeboom-Villiersdorp regions.

“To improve irrigation scheduling and decisions about water allocation, we needed to better understand the water use of orchards from which more than 100 tonnes of apples per hectare are harvested,” explained project leader Dr Sebinasi Dzikiti, senior researcher in the Department of Horticultural Science at Stellenbosch University.

The researchers found that the water use of mature Golden Delicious orchards was almost 20% more than that of Cripps Pink orchards in both regions, even though the Cripps Pink orchards were harvested more than a month later. This is because Cripps Pink trees have a relatively open canopy and shoot growth is restricted to allow light penetration that promotes red colour development of the fruit.

Despite having less dense canopies, the Cripps Pink orchards in the study were higher yielding than the Golden Delicious orchards, demonstrating that leaf area affects transpiration rate more than does crop load. For two orchards in the same region, the one with a greater leaf area per unit land area would be expected to use more water.

Low transpiration rates and higher yields resulted in better physical water productivity — kilograms of fruit produced per unit of water applied — of the Cripps Pink orchards compared to the Golden Delicious orchards. The top physical water productivity for a Cripps Pink orchard was 17 kilograms of fruit produced per cubic metre of water applied, compared to around 13 kilograms per cubic metre for Golden Delicious.

Since the Cripps Pink orchards generated higher income than the Golden Delicious orchards, their economic water productivity — income per cubic metre of water — was also much higher.

While it would be counterproductive to have very open Golden Delicious canopies due to the high risk of sunburn, excessive and unnecessary shoot growth increases water use. Producers with limited or unreliable access to water resources should aim to gradually replace low-value cultivars with high-value cultivars. This will ultimately increase farm-level economic water productivity and maximise profitability per unit of irrigation water.