Carbon dioxide concentration and light levels on growth and mineral nutrition of juvenile cacao genotypes

Authors: Baligar, Virupax; Elson, Marshall; ALMEIDA, A-A - Universidade Estadual De Santa Cruz; ARAUJO, D - Universidade Estadual De Santa Cruz; AHNERT, D - Universidade Estadual De Santa Cruz; HE, Z - University Of Florida

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2021
Publication Date: 5/28/2021
Citation: Baligar, V.C., Elson, M.K., Almeida, A., Araujo, D., Ahnert, D., He, Z. 2021. Carbon dioxide concentration and light levels on growth and mineral nutrition of juvenile cacao genotypes. American Journal of Plant Sciences. 12:818-839.

Interpretive Summary: In agroforestry types of cropping systems, cacao is grown as an understory plant and under such situations it is subjected to low levels of light due to the presence of to upper story shade trees with dense and poorly managed canopy structures. Young cacao trees grow better under moderate shade; however, heavy shading is detrimental to growth, production, and nutrition of mature and old trees. It is not well understood how increasing concentrations of carbon dioxide in the atmosphere and high levels of shade affects cacao growth and nutrition. In this paper, we report our findings on three genetically contrasting cacao genotypes that showed the presence of intraspecific differences in growth and nutrient use efficiency traits at ambient and elevated levels of carbon dioxide under low to adequate levels of light intensities. This information could be useful to cacao breeder to breed superior cacao cultivars that have high nutrient use efficiencies under increasing levels of atmospheric carbon dioxide that are adaptable to heavily shaded cacao agroforesty systems.

Technical Abstract: In many countries cacao (Theobroma cacao L.) is invariably grown as an understory crop in an agroforestry types of cropping systems and subjected to low levels photosynthetic photon flux density (PPFD) due to presence of large number of upperstory shade trees with poorly managed canopy structure. In recent years carbon dioxide concentration [CO2] in the atmosphere is steadily increasing and it is unclear what impact this will have on performance of cacao grown under shade of upper story shade trees. A climatically controlled greenhouse experiment was undertaken to evaluate the effects of ambient and elevated [CO2] (400 and 700 µmol mol-1) and three levels of PPFD (100, 200, and 400 µmol m-2 s-1) on growth, and macro-and micronutrient use efficiency of three genetically contrasting cacao genotypes (CCN 51, VB 117 and NO 81). Intraspecific variations were observed in cacao genotypes for growth parameters at ambient to elevated [CO2] and low to adequate levels of PPFD. With the exceptions of total root length and leaf area, irrespective of [CO2] and PPFD levels, all the three genotypes showed significant differences in all the growth parameters. For all the cacao genotypes, increasing PPFD from 100 to 400 µmol m-2 s-1 and [CO2] from 400 to 700 µmol mol-1 increased overall growth parameters such as leaf, shoot and root biomass accumulation, stem height, leaf area, relative growth rate (RGR) and net assimilation rate (NAR). Irrespective of [CO2] and PPFD, invariably genotypes differed significantly in macro-micronutrient uptake parameters such as concentration, uptake, influx, transport and use efficiency. With few exceptions, rising PPFD from 100 to 400 µmol m-2 s-1 and [CO2] from 400 to 700 µmol mol-1 nutrient use efficiency increased for all the cacao genotypes. Elevated [CO2] and adequate PPFD are beneficial in improving cacao growth and mineral nutrient uptake and use efficiency.