In many countries cacao (Theobroma cacao L.) is invariably grown as an understory crop in
agroforestry types of cropping systems and subjected to low levels
photosynthetic photon flux density (PPFD) due to presence of large number of
upper story shade trees with poorly managed canopy structure. In recent years
carbon dioxide concentration 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 carbon dioxide (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 1117 and NO 81). Intraspecific variations were observed
in cacao genotypes for growth parameters at ambient to elevated carbon dioxide
and low to adequate levels of PPFD. With the exceptions of total root length
and leaf area, irrespective of carbon dioxide and PPFD levels, all 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 carbon dioxide 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 and net assimilation rate.
Irrespective of carbon dioxide and PPFD, invariably genotypes differed
significantly in macro-micronutrient uptake parameters such as concentration,
uptake, influx, transport and use efficiency. With few exceptions, raising PPFD
from 100 to 400 μmol·m-2
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