Photosynthesis of Submerged and Surface Leaves of the Dwarf Water Lily (Nymphoides aquatica) Using PAM Fluorometry

Abstract

Dwarf Water Lilies (Nymphoides aquatica (J.F. Gmel) Kuntze have floating and submerged leaves. Some submerged aquatic vascular plants have a form of CAM (Crassulacean Acid Metabolism) called Submerged Aquatic Macrophyte (SAM) metabolism. Blue-diode based PAM technology was used to measure the Photosynthetic Oxygen Evolution Rate (POER: 1O₂ ≡ 4e^– ). Optimum Irradiance (E_opt), maximum POER (POER_max) and quantum efficiency (α₀) all vary on a diurnal cycle. The shape of the POER vs. E curves is different in seedling, submerged and surface leaves. Both E_opt and POER_max are very low in seedling leaves (E_opt ≈ 104 μmol photon m^–2 s^–1, PPFD; POER_max ≈ 4.95 µmol O₂ g–1 Chl a s^–1), intermediate in mature submerged leaves (E_opt ≈ 419 µmol photon m^–2 s^–1 PPFD, POER_max ≈ 38.1 µmol O₂ g^–1 Chl a s^–1) and very high in surface leaves (E_opt ≈ 923 µmol photon m^–2 s^–1 PPFD, POER_max ≈ 76.1 µmol O₂ g^–1 Chl a s^–1). Leaf titratable acid (C4 acid pool) is too small (≈20 to 50 mol H⁺ m^–3) to support substantial SAM metabolism. Gross daily photosynthesis of surface leaves is ≈3.71 g C m^–2 d^–1 in full sun and as much as 1.4 gC m^–2 d^–1 in shaded submerged leaves. There is midday inhibition of photosynthesis.