The Use of Pulse Amplitude Modulation (PAM) Fluorometry to Measure Photosynthesis in a CAM Orchid, Dendrobium spp. (D. cv. Viravuth Pink)

Pulse amplitude modulation (PAM) fluorometer techniques provide unique information on photosynthetic activity of CAM (crassulacean acid metabolism) plants such as the orchid Dendrobium spp. (cv. Viravuth Pink). CAM plants close their stomata for at least part of the day, creating a sealed compartment in the stems and leaves that precludes measurement of the light reactions of photosynthesis by any gas exchange–based method. PAM machines calculate photosynthesis as the electron transport rate (ETR) through PSII (four electrons per O2 produced) as mol m−2 s−1. Photosynthesis‐versus‐irradiance (P‐vs.‐E) curves fitted the waiting‐in‐line function (ETR\,=( ETR\,max\,× E/ Eopt\,) × e^1-E/ Eopt\,), allowing half‐saturating and optimal irradiances (Eopt) to be estimated. Effective quantum yield (Ymax), ETR (ETRmax), and the nonphotochemical quenching parameter NPQmax all vary on a diurnal (circadian) cycle, but the parameter qNmax does not show a systematic variation over a diurnal period. Dendrobium Viravuth is a “sun plant,” with Eopt between 404 and 539 μmol m−2 s−1 PAR but photosynthetic capacity is very low in the late afternoon. Total CO2 fixed nocturnally as C4‐dicarboxylic acids by leaves was only ≈120 mg C m−2 d−1. Titratable acid of leaves was depleted by ∼1500 hours and shows a classical CAM diurnal cycle. Dendrobium Viravuth stores only enough CO2 as C4 acids to account for ∼10% of the daily gross photosynthesis estimated with the PAM machine (≈1.4 g C m−2 d−1). Dendrobium Viravuth is an orchid cultivar with very limited capacity to fix and store C4 acids at night. It is probably a CAM‐cycling orchid.