Stomatal regulation of transpiration and photosynthesis in macadamias

Macadamia is an increasingly important crop in South Africa and many other countries, with planted area increasing rapidly each year. As many of these orchards are planted in areas where irrigation is required to maximise production, the industry has a significant water requirement. In order to improve water management in these orchards, a thorough understanding of the ecophysiology and the water use of this crop is required. Studies of transpiration and plant water relations in a mature 'Beaumont' macadamia orchard revealed that macadamias exhibit typical isohydric behaviour, with stomata closing in response to high vapour pressure deficits (VPD) to prevent leaf water potential dropping below critical levels. The closing of stomata as VPD increased above 2.0 kPa was also associated with a decrease in net CO₂ assimilation. This indicated that stomata play an important role in controlling both transpiration and net CO₂ assimilation in macadamia. The need to close stomata in response to high VPD is thought to be due to higher resistances in the stem to leaf hydraulic pathway, than the root to stem pathway. This creates an imbalance between the potential loss of water out of the leaf due to the VPD gradient and the ability of the plant to supply water to the evaporative surfaces in the leaf. This is likely a result of adaptations in the leaf, which allows for the maintenance of plant water status under very dry conditions, such as those occurring during winter in macadamias' native habitat. This has consequences for overall carbon gain and growth.