Genotypic differences in chlorophyll fluorescence of mango leaves. Relationship to carbon assimilation in Northern Australia

Abstract

Mango is the main horticultural industry in the wet-dry tropics of Northern Australia. At light saturating conditions, maximum C assimilation rate is observed at the end of the wet season (12-14 µmol m-2 s-1) but decreases by 70-80% at the onset of the dry season following changes in VPD and soil water availability. This reduction in C assimilation rate is associated with low stomatal conductance and high leaf temperature (>37ºC). Genotypic differences in C assimilation during the dry months indicate that the cultivar Irwin (IR) is able to maintain higher rates of photosynthesis than other commercial cultivars like Kensington Pride (KP) or Nam Dok Mai (NDM). In this study we explored the effect of light intensity on leaf photosynthetic parameters of these three commercial mango cultivars (IR, NDM and KP). Trees were grown in pots under three different light treatments (50%, 75% , and 100% of full incident PAR) during the dry months (April-August). Measurements of carbon assimilation and chlorophyll fluorescence of dark-adapted leaves were made on well-watered trees. In all cultivars, measurements at noon, demonstrated that chlorophyll fluorescence was most severely reduced in plants grown at full incident PAR and least affected in plants shaded to 75% PAR. Over all light treatments, comparisons between cultivars showed that while IR maintained high Fv/Fm chlorophyll fluorescence in KP was strongly reduced. This effect was greatest in the 100% PAR treatment in which Fv/Fm of IR was 0.76 compared with 0.64 in KP. In all cultivars Fv/Fm recovered at sundown, but the extent of recovery was lower in KP leaves, followed by NDM and maximum in IR.