CO2 and Light Intensity on Growth and Macronutrient Uptake Traits of Cacao Genotypes.
Invariably cacao(Theobroma cacao L)is grown as an understory plant and is subjected to various levels of shade. In recent years, concentration of [CO2] in the atmosphere has risen to 370 μmol ּmol -l with levels expected to double by the end of 21st century. A climatically controlled greenhouse experiment was undertaken to assess the influence of two levels of [CO2] concentrations (400, 700 μmol ּmol -l) and three levels of photosynthetic photon flux density (PPFD, 50, 150, 450 μmolּm-2ּs-1) on the growth, and uptake, and use efficiency of macronutrients by two cacao genotypes (SCA-6 and TSH-516) in their early growth stage. At all the levels of PPFD, with few exceptions, increasing [CO2] concentrations tended to increase shoot and root biomass accumulations, shoot/root ratio, relative growth rates and net assimilation rates. At both [CO2] concentrations, increasing PPFD from 50 to 450 μmolּm-2ּs-1 increased shoot and root growth parameters and net assimilation rates of both the genotypes. At 400 μmol ּmol –l [CO2], increasing PPFD enhanced growth of SCA-6 higher than TSH-516, however at 700 μmol ּmol -l[CO2], THS-516 performed better than SCA-6 at all levels of PPFD. With some exceptions, in both genotypes increasing [CO2] and PPFD increased the uptake of all the macronutrients (N, K, Ca, Mg, P, S). With few exceptions in both genotypes increasing [CO2] concentrations and PPFD overall enhanced the macro nutrient use efficiency. Shade management, in increasing atmospheric concentrations of [CO2] appears to be a critical management strategy to maintain high sustainable cacao production in the tropical regions.
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