Effects of environmental variables and cropping on leaf conductance of custard apple (Annona cherimola x Annona squamosa) 'African Pride'

Diurnal variations in leaf conductance (g) were investigated in bearing and non-bearing custard apple (Annona cherimola x Annona squamosa) trees, cultivar 'African Pride', in subtropical Queensland, Australia (latitude 27°S). Most of the variation in log g could be attributed to changes in relative humidity (RH) (R² = 0.86, P < 0.01). At RH < 60%, g was extremely low (< 0.4 cm s^-1). The marked sensitivity of custard apple stomata to RH suggests that CO₂ assimilation could be dramatically reduced at low RH, and this may be one of the explanations for the poor fruit size of this cultivar. Although there were no significant differences in g on non-fruiting terminals of either bearing or non-bearing trees, the g of leaves adjacent to fruit was ∼ 20% lower than leaves on non-fruiting terminals at similar nodal positions. The productivity of custard apple in the field could be improved by techniques which raise RH, such as the use of windbreaks and overhead intermittent misting.