Effect of growth irradiance on the maximum photosynthetic capacity of water hyacinth [Eichhornia crassipes (Mart.) Solms]

We grew water hyacinth Eichhornia crassipes (Mart.) Solms]for 60 days in a greenhouse under natural light and in a controlledenvironment room at 31/25?C day/night temperatures and 90, 320and 750/µEm^–2sec^–1. We then determined maximumphotosynthetic rates in 21% and 1% oxygen, stomatal diffusionresistances, contents of chlorophyll and soluble protein, andthe size and density of the photosynthetic units (PSU) in representativeleaves from the four treatments. In air containing 21% oxygen,maximum photosynthetic rates were 14, 27 and 29 mg CO₂ dm^–2hr^–1for plants grown in artificial light at 90, 320 and 750µEm^–2sec^–1,respectively. Plants grown in natural light (maximum of 2000µEm^–2sec^–1) had maximum photosynthetic ratesof 34 mg CO₂ dm^–2hr^–1. In all treatments, photosyntheticrates in 1% oxygen were about 50% greater than rates in normalair, indicating the presence of photorespiration in water hyacinth.There was no apparent relationship between maximum photosyntheticrate per unit leaf area and stomatal conductance, chlorophyllcontent per unit area, or PSU density per unit area. However,the higher maximum photosynthetic rates were associated withgreater mesophyll conductances, specific leaf weights and proteincontents per unit area. When plants grown at 90µEm^–2sec^–1for 120 days were transferred to 750µEm^–2sec^–1for 5 days, only young leaves that were just beginning to expandat the time of transfer exhibited adaptation to the higher irradiance.The 40% increase in light-saturated photosynthetic rate in theseyoung leaves was associated with increases in mesophyll conductance,soluble protein content per unit area, and specific leaf weight. ¹ Mississippi Agricultural and Forestry Experiment Station cooperating. (Received July 19, 1978; )