Diurnal Leaf Net CO 2 Assimilation, Acid Content and Photochemistry of Pineapple (Ananas comosus L. Merr.) ‘Smooth Cayenne’

Abstract

Pineapple as a crassulacean acid metabolism (CAM)plant was reported to display full 4 carboxylation phases. To elucidate this process, we carried out 24 h monitoring of photosynthesis of pineapple ‘Smooth Cayenne’ in open field in all 3 seasons in Prachuap Khiri Khan Province. We measured leaf gas exchange, total acid content (H⁺), and photochemistry of PSII (chlorophyll fluorescence). The gas exchange data obtained, confirmed the presence of the 4 carboxylation phases. Phase I occurred during dark period, net CO² assimilation rate (A) increased in consistent with increase of stomatal conductance (g_s), having the maximal A in the low range of 2–2.6 μmolCO₂ m^–2 s ^–1, accompanied by the increase in H⁺ . The increment in A and H⁺ continued into Phase II during early morning which lasted about 2 h when radiation (PPF) was still low. In Phase II, photochemistry commenced showing an increase in electron transport rate (ETR), whereas H⁺ reached its maximum. Subsequent stronger PPF initiated Phase III, when A turned negative as the reducing g_s limiting external CO₂ intake. On the other hand, ETR continued to increase to its maximum, enabling the decarboxylation of the accumulated acid. The average acid decarboxylation rate (ADR), from the CO₂ carboxylation process was 6.7–7.8 µmolCO₂ m^–2 s ^–1. Phase III which lasted 6-8 h ended in late afternoon, as H⁺ was mostly spent and ADR dropped closed to zero. In pineapple, Phase IV clearly appeared when g_s increased (stomata reopening) and positive A was detected.