Water use of macadamia trees is dynamically regulated by the presence or absence of fruit

Abstract

• Stomatal conductance increases in the presence of macadamia nuts at a canopy level. • Canopy increases in stomatal conductance result in increased canopy transpiration. • Crop load should be considered in water use models for irrigation scheduling. Macadamia, an oil rich fruit producing tree, is characterized by a predominantly isohydric water management strategy, enforced by means of strict stomatal control. However, the presence of fruit, a significant sink, increases both the net assimilation rate ( A ) and stomatal conductance ( g s ) in macadamia and a range of other crops. It is, however, unclear if increases in A and g s at a leaf level in the presence of fruit would lead to increased canopy transpiration ( E c ) in macadamia. This study therefore aimed to demonstrate that the presence of fruit leads to an increase in A, g s and E c . In order to examine the effect of fruit on leaf gas exchange, the study used fruiting and non-fruiting branches, in combination with phloem-girdling to extrapolate possible effects of fruit on leaf gas exchange to a canopy level. Measurements of leaf gas exchange were made on four different treatments including non-fruiting (NF), fruiting (F), girdled non-fruiting (GNF), and girdled fruiting (GF) branches, over a 7-month period in a mature commercial macadamia orchard. Independent estimates of E c , using sap flow measurements, were made across two seasons in the same orchard. No significant differences in light saturated net assimilation rate ( A max ) and g s were observed between GF branches and non-girdled treatments approximately 2-months after girdling, whilst A max and g s of GNF were significantly reduced during the same period. Fruit removal on GF branches, resulted in a significant reduction in both A max and g s compared to F and NF treatments. There were also no significant differences in g s between F and NF branches throughout the trial, suggesting that increases in g s , in the presence of fruit can be scaled to a canopy level. The presence of fruit led to an ∼25% increase in g s at air vapour pressure deficits (VPD air ) > 1.50 kPa, which translated into ∼20% higher E c during fruiting compared to non-fruiting periods. This increase occurred irrespective of changes in canopy size and weather and was largely attributed to significantly higher E c in response to air vapour pressure deficit within the 0.0 – 3.0 kPa range. Crop load therefore needs to be considered when developing water use models for irrigation scheduling in macadamia orchards.