An analysis of ash and isotopic carbon discrimination (Δ13C) methods to evaluate water use efficiency in apple

• Fruit load affected seasonal and daily water use efficiency (WUE) of apple trees.
• Apple WUE was negatively correlated with leaf ash and leaf 13C discrimination.
• Leaf 13C discrimination was useful in assessing WUE in apple.
• Apple leaf ash is useful in identifying WUE extremes at a lower cost.

Apple cultivars are selected for fruit quality, disease and insect resistance, not water use efficiency (WUE), however, the need for more water use efficient crops is accelerating due to climate change and increased competition for water resources. WUE be calculated from short-term gas exchange measurements of photosynthesis (A) and transpiration (Ec) and expressed as A/Ec (instantaneous water use efficiency; WUEInst), or on a whole plant basis, measuring leaf, fruit and wood dry matter (DM) together with a measurement of water use (WU), and expressed as DM/WU. Plant ash and carbon isotope discrimination (Δ13C) have been correlated with water use efficiency in herbaceous plants. The purpose of this study was to measure WUEInst and seasonal WUE of mature and productive apple trees in the field and evaluate WUE correlations with ash and Δ13C. WUEInst was measured in whole tree gas exchange studies and from seasonal dry matter yield studies in ‘Empire’ apple for 2005, 2007 and 2009. Leaf ash and Δ13C were positively correlated with mean tree photosynthetic rate (A) and transpiration (Ec) but only leaf Δ13C was negatively correlated with WUEInst. In seasonal dry matter yield studies, leaf ash and Δ13C responses were highly negatively correlated with WUE based on the annual fruit yield/seasonal potential transpiration (Ef), termed WUEFrt. There were also highly significant correlations between leaf and shoot ash with leaf and shoot Δ13C. Both leaf ash and Δ13C were useful in identifying extremes in apple WUE and leaf ash can be used at a much lower cost with a high through-put potential.