Effects of environment, season and crop load on gas exchange and plant water status of Annona spp. hybrid cultivar, African Pride

Diurnal changes in net carbon dioxide assimilation (A), leaf conductance (g_s) and leaf water potential (ψ_L) were investigated for custard apple (Annona cherimola × Annona squamosa) trees of cultivar ‘African Pride’ in subtropical Queensland, Australia. Most of the variation in A and g_s could be attributed to changes in r.h. (r²0.69, P < 0.05). The addition of other variables besides r.h. into multiple linear regressions analysis added little to explaining the variation in A or g_s. Leaf water potential (ψ_L) was highly responsive to air temperature (AT) (r² = 0.56, P < 0.05). The marked sensitivity of custard apple stomata to low r.h. may be one of the reasons for poor fruit set and size of custard apple fruit under subtropical conditions because of carbohydrate source limitations. The fruit set and early fruit growth of custard apple in the field could be improved by techniques which raise r.h., such as the use of windbreaks and overhead intermittent misting.