Photosynthetic efficiency and relationship to mesocarp dry matter content of ‘Carmen’ avocado (Persea americana Mill.) fruit in a cool subtropical climate

Physiological changes associated with leaf chlorophyll fluorescence and gas exchange are the key components towards understanding performance of fruit trees. In this study, the photosynthetic efficiency of leaves from inside and outside canopy positions of avocado trees was assessed during the fruit growing season. The aim was to determine to what extent photosynthesis and gas exchange of the leaves correlates with the position of the leaf inside canopy and outside canopy of an avocado tree, and how does that affect fruit maturity. The study was conducted in a commercial orchard at Everdon Estates in the KwaZulu-Natal, South Africa during 2017/18 season. A total of fifteen 8-year old avocado trees (cv. ‘Carmen’) were selected in a completely randomised design with three replicates and each replicate consisting of five trees. Data was collected bi-weekly from full bloom to fruit physiological maturity (25% dry matter content). Measured variables included gs, Ci, T, A, F0, Fm, Fv/Fm, ΦPS II, qP, qN, ETR, 1-qP and DM content, were estimated within and outside canopy. Results showed that leaves from the outside position had higher A (22.28 mol CO2 m−2 s−1), gs (0.22 mol CO2 m−2 s−1), ΦPS II (0.35), and qP (0.57) compared to those on the inside position of the canopy with lower values of 14.19 mol CO2 m−2 s−1, 0.14 mol CO2 m−2 s−1, 0.047 and 0.071, for A, gs, ΦPS II and qP respectively. Conversely, Fo’, Fm’, Fv’, Fv’/Fm’, and 1-qP were about two times higher on the inside canopy than on the outside. The high photosynthetic efficiency of outside fruit was linked fruit maturity with fruit from the outside maturing earlier with an average mesocarp dry matter content of 27.6% compared to 26.5% on fruit from the inside canopy. The findings suggest that the high sunlight interception by leaves from the outside position improved photosynthetic efficiency, increased rate of dry matter accumulation and maturation of avocado fruit. There is a strong sink demand within the outside canopy demanding more carbohydrates, and thus commanding leaves from the outside canopy to photosynthesize more efficiently leading to fast maturity.