Photosynthesis in Pineapple (Ananas comosus comosus [L.] Merr) Measured Using PAM (Pulse Amplitude Modulation) Fluorometry

PAM (Pulse Amplitude Modulation) fluorometer techniques directly measure the light reactions of photosynthesis that are otherwise difficult to estimate in CAM (Crassulacean Acid metabolism) plants such as pineapple (Ananas comosus comosus cv. Phuket). PAM machines calculate photosynthesis as the Electron Transport Rate (ETR) through PSII (4 electrons per O₂ produced) as mol m^?2 s^?1. P vs. E curves fitted the waiting-in-line function (an equation of the form $ {hbox{ETR}} = left( {{hbox{ET}}{{hbox{R}}_{{ max }}} times {hbox{E}}/{{hbox{E}}_{rm{opt}}}} right).{{hbox{e}}^{{1} - {rm{E}}/{rm{Eopt}}}} $ ) allowing half-saturating and optimal irradiances (E_opt) to be estimated. Effective Quantum Yield (Y_max), Electron Transport Rate (ETR_max) and the Non-Photochemical Quenching parameter, NPQ_max all vary on a diurnal cycle but the parameter qN_max does not show a systematic variation over a diurnal period. Phuket pineapple is a “sun plant” with Optimum Irradiance (E_opt) from 755 to 1,130 μmol m^?2 s^?1 (400–700 nm) PAR but photosynthetic capacity is very low in the late afternoon even though light conditions are favourable for rapid photosynthesis. Total CO₂ fixed nocturnally as C4-dicarboxylic acids by leaves of the Phuket pineapple was only ≈0.14 gC m^?2 d^?1 (0.012 mol C m^?2 d^?1). Titratable acid of leaves was depleted about 3 pm (15:00) and shows a classical CAM diurnal cycle. The Phuket pineapple variety only stored enough CO₂ as C4 acids to account for only about 2.5% of photosynthesis (P_g) estimated using the PAM machine (≈5.6 gC m^?2 d^?1). Phuket pineapples are classifiable as CAM-Cycling plants but nocturnal fixation of CO₂ is so low compared to the more familiar Smooth Cayenne variety that it probably recycles only a small proportion of the respiratory CO₂ produced in leaves at night and so even CAM-cycling is only of minor importance to the carbon economy of the plant. Unlike the Smooth Cayenne pineapple variety, which fixes large amounts of CO₂ nocturnally, the Phuket pineapple is for practical purposes a C3 plant.