Non-Existence of an Optimum Leaf Area Index for the Production Rate of White Clover Grown Under Constant Conditions

Single clover plants were grown in the vegetative state, at 20 +/- 1 degrees , 85 +/- 5% relative humidity, 320 +/- 10 ppm CO(2), 12-hour day, with Hoagland nutrient in Perlite, and 100 w . m(-2) of photosynthetically active radiation (0.4-0.7 mu) from mercury-fluorescent lamps. Each plant was confined within a circle 18 cm in diameter by means of a wire framework. The CO(2) exchange rate of the whole plant was measured every second day for 3 months. There was no optimum leaf area index for the net photosynthesis rate. The respiration rate was determined mainly by the gross photosynthesis rate and only partly by the amount of non-photosynthetic or heavily shaded tissue. At the maximum leaf area index, when leaves were dying as fast as they were being produced, both photosynthesis and respiration remained at or near their maximum rates. At the end of 3 months, the whole plant was harvested and the dry weight and carbon content determined. The measured dry weight was close to that calculated from the total CO(2) uptake and a constant ratio of carbon content to dry weight of 39%. Optimum leaf area indices observed in field experiments are attributed to the failure to include the material which dies between harvests, and to decreases in the gross photosynthesis rate caused by climate changes or lack of nutrient, for example. The difference between production rate and growth rate or yield is emphasized.