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

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Abstract
Single clover plants were grown in the vegetative state, at 20 [plus or minus] 1o, 85 [plus or minus] 5% relative humidity, 320 [plus or minus] 10 ppm CO2, 12-hr. day, with Hoagland nutrient in Perlite, and 100 w [middot] m-2 of photosynthetically active radiation (0.4-0.7/u) from mercury-fluorescent lamps. The CO2, exchange rate of the whole plant was measured every 2nd day for 3 mo. 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 of 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 CO2, 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 decrease in the gross photosynthesis rate caused by climate changes of lack of nutrient, for example. The difference between production rate and growth rate or yield is emphasized.