PHOTOSYNTHESIS IN THE BLUE WATER LILY (NYMPHAEA CAERULEA SALIGNY) USING PULSE AMPLITUDE MODULATION FLUOROMETRY

摘要

Water lilies, such as the blue Egyptian water lily (Nymphaea caerulea Savigny), are ubiquitous aquatic plants. Leaves of mature plants normally unfold at the surface and are floating or emergent. Some aquatic vascular plants have a form of CAM (crassulacean acid metabolism) known as submerged aquatic metabolism (SAM). The presence of aerenchyma in water lily leaves and petioles makes it very difficult to measure photosynthesis in water lily plants by gas exchange. Pulse amplitude modulation (PAM) fluorometer techniques provide direct information on the light reactions of plants. PAM technology calculates photosynthesis as the electron transport rate (ETR) through PSII (4 electrons per O_2 produced) in mol m~(-2) s~(-1). Here, relative ETR (rETR) was based on an assumed leaf absorptance factor of 0.84. Photosynthesis-versus-irradiance (P vs. E) curves were fitted with the waiting-in-line function (rETR = (rETR_(max) × E/W_(opt)) × e~(1-E/E_(opt)). Maximum effective quantum yield, maximum relative ETR (rETR_(max)), and quantum efficiency all vary on a diurnal cycle. The nonphotochemical quenching parameters qN_(max) and NPQ_(max) are highly correlated with each other (r = 0.7476, p 0.001) but do not show a systematic variation over a diurnal cycle. Nymphaea is a "sun plant" with optimum irradiance (E_(opt)) of 1000 μmol m~(-2) s~(-1) PPFD or higher. Titratable acid of leaves varied from ～50 to 70 mol H~1 m~(-3) (leaf water basis) and was depleted at -0600 and 1800 hours each day, a diel pattern inconsistent with SAM/CAM physiology. The N. caerulea C4 acid pool is too small to support substantial SAM-type metabolism. Gross photosynthesis of Nymphaea leaves had a high value of -5.3 g C m~(-2) d~(-1). For a daily irradiance of 56 mol m~(-2) PPFD, this works out to a 3.3% conversion efficiency in terms of moles of carbon.