How Do Different Cocoa Genotypes Deal with Increased Radiation? An Analysis of Water Relation, Diffusive and Biochemical Components at the Leaf Level

Abstract

The cultivation of cocoa (Theobroma cacao L.) is traditionally managed under shade because of its photosynthetic characteristics; however, its behavior can vary according to the genotype and environmental conditions where it is grown. In this sense, here, we explore the possible mechanisms of protection against radiation stress and how these mechanisms are affected by variation between cocoa genotypes. Therefore, we evaluate the effect of the radiation level (HPAR, 2100 ± 46 mol m−2 s−1; MPAR, 1150 ± 42 mol m−2 s−1; LPAR, 636 ± 40 mol m−2 s−1) on the water status and gas exchange in plants of different cocoa genotypes (CCN-51, ICS-1, ICS-95, LUKER-40 and LUKER-50), and the occurrence of photoinhibition of PSII (as a marker of photodamage), followed by a characterization of the protection mechanisms, including the dynamics of photosynthetic pigments and enzymatic and non-enzymatic antioxidant systems. We found significant changes in the specific leaf area (SLA) and the water potential of the leaf (ΨL) due to the level of radiation, affecting the maximum quantum yield of PSII (Fv/Fm), which generated dynamic photoinhibition processes (PIDyn). Cocoa genotypes showed the lowest Light-saturated maximum net carbon assimilation rate (Amax) in HPAR. Moreover, the maximum carboxylation rate (Vcmax) was negatively affected in HPAR for most cocoa genotypes, indicating less RuBisCO activity except for the ICS-95 genotype. The ICS-95 showed the highest values of Vcmax and maximum rate of regeneration of ribulose-1,5-bisphosphate (RuBP) controlled by electron transport (Jmax) under HPAR. Hence, our results show that some genotypes were acclimated to full sun conditions, which translated into greater carbon use efficiency due to the maximization of photosynthetic rates accompanied by energy dissipation mechanisms.