Genetic and environmental influence on foliar carbon isotope composition, nitrogen availability and fruit yield of 5-year-old mango plantation in tropical Australia

Purpose: The aim of this study was to quantify the effect of different varieties, planting densities, tree training systems and canopy aspect (north and south) on tree water use efficiency and nitrogen (N) availability in relation to mango fruit yield and fruit size as well as soil fertility (particularly total carbon (C) and total N as well as C and N isotope compositions) in a 5-year-old mango plantation of tropical Australia. Material and Methods: Mango foliar samples were collected from a 5-year-old, factorial field experiment, where we tested the effects of two mango varieties (Calypso vs Keitt); two planting densities (medium vs high); two training systems (single leader vs conventional); and two sampling canopy aspects (north vs south) on foliar total C (%), total N (%), and stable C and N isotope compositions (δ13C and δ15N). The 0–10 cm of surface soils were also assessed for total C, total N, and δ13C and δ15N of different varieties, planting densities, and training systems. In addition, mango fruit yield, and fruit size were measured. Results: There were significant genetic effects on foliar total N, tree water use efficiency (WUE) as reflected by foliar δ13C, N availability as indicated by foliar total N and δ15N, mango fruit yield, and fruit size in 5-year-old mango trees in tropical Australia. Overall, Keitt had higher foliar δ13C and N availability (higher total N and lower δ15N) as well as higher mango yield and greater fruit size, compared with those of Calypso. There were also significant environmental influences on foliar δ13C and N availability. In particular, high planting density had higher N availability (higher total N), and lower foliar δ13C as well as higher N loss (higher δ15N), compared with those of medium planting density. High planting density treatment also had higher soil total N, compared with that of medium planting density treatment. The conventional training system had higher N availability and foliar δ13C, compared with those of the single leader training system. The northern side of tree canopy (sunny side) had lower fruit number, compared with that of the southern side (shady side) of tree canopy. Conclusion: There were significant genetic and environmental influences on tree WUE and N availability as well as mango fruit yield and fruit size in the 5-year-old mango trees. Among them, Keitt’s fruit was significantly larger than Calypso and had higher foliar δ13C (higher WUE). Although high planting density increased soil total N, it also resulted in lower foliar δ13C and more N loss. Our results highlighted the significant potential for improvement of foliar δ13C and N availability as well as fruit yield and soil fertility with both genetic selection and site management regimes.