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.
Highlights
The level of solar radiation is a factor that can limit some physiological processes on crops, such as photosynthesis [1,2]
specific leaf area (SLA) tended to increase with decreasing radiation and the change was significant except for ICS-95 and LUK-50 (Figure 2)
It has been reported that SLA is a trait related to the level of incident light [26] and in cocoa cultivation when increasing the level of radiation the SLA is reduced [59,60], a characteristic presented in crops under monoculture compared to that found under agroforestry systems [61,62]
Summary
The level of solar radiation is a factor that can limit some physiological processes on crops, such as photosynthesis [1,2]. Crops under shade conditions adjust a suite of anatomical, physiological and biochemical traits to increase the efficiency of carbon fixation [3] Among these traits are found the specific leaf area (SLA), the chlorophyll a/b ratio (chloroplast level) and the rate of light-saturated net photosynthesis (Amax) [4,5,6]. If the exposure process is more extensive, high irradiation can lead to irreversible oxidation of chlorophyll (Chl) and a loss of chloroplast function due to the formation of reactive oxygen species (ROS) [14] Faced with this stressful situation, the plant makes biochemical adjustments related to the content of photosynthetic pigments (a, b and carotenoids), protein content, antioxidant system, sugar content and malondialdehyde (MDA) [8,15,16,17]
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