Photochemical responses to light in sun and shade leaves of Theobroma cacao L. (West African Amelonado)

Theobroma cacao is a tropical understory tree that is traditionally grown in an agroforestry system under a shade canopy; but is increasingly being grown in an orchard system with limited shade or no shade. This study investigated the light response of sun and shade leaves from mature trees of one of the most cultivated cacao accession (West African Amelonado), grown under full sun conditions. Chlorophyll fluorescence was used to measure F-v/F-m, and to generate light response curves for photosynthetic and non-photosynthetic quenching parameters. The light response curves indicated an increased capacity for photosynthesis in the sun leaves however the F-v/F-m and NPQ data showed that the sun leaves were subject to photoinhibition. Biochemical assays were used to compare sun and shade leaves for photosynthetic pigment content and a range of parameters related to reactive oxygen species scavenging and cellular damage. The sun and shade leaves demonstrated typical pigment plasticity however the chlorophyll carotenoid ratio was low for both types of leaves. There was no observed plasticity in superoxide dismutase activity between the sun and shade leaves however the plant was able to respond to the increased photosyntheically active radiation by increasing the activity of ascorbate peroxidase and to a lesser extent catalase. Increased electrolyte leakage in the sun leaves however suggests that the ability to mitigate reactive oxygen species was limited. The results point to limitations in the ability of currently cultivated cacao to acclimate to full sun conditions.

Automated summary

The study found that while sun leaves have higher photosynthetic capacity, they are subject to photoinhibition, resulting in lower chlorophyll pigment content and chlorophyll carotenoid ratio. The plant responds to increased photosynthetically active radiation by increasing the activity of ascorbate peroxidase to mitigate reactive oxygen species.