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

Abstract

Summary. Single clover plants were grown in the vegetative state, at 20 ± 10, 85 + 5 % relative humidity, 320 + 10 ppm CO, 12-hour day, with Hoagland nutrient in Perlite, and 100 w * m-2 of photosynthetically active radiation (0.4-0.7 ,u) from mercury-fluorescent lamps. Each plant was confined within a circle 18 cm in diameter by means of a wire framework. The COO 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) 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.We have shown previously (14) that young clover plants adapt their rates of respiration (R) when subjected to a change in the incident light level. The adaptation is virtually complete within 24 hours, the final respiration rate being proportional to the photosynthesis rate (P) over a wide range of light levels.In that experiment, each leaf received the same amount of light. In a plant communitv, the lower leaves are shaded by the upper ones. If the same adaptation occurs, the respiration rate of the whole community will be proportional to its photosynthesis rate. Both P and R will increase with leaf area until there are sufficient leaves to absorb all the incident radiation, and then remain constant ( fig la). This picture is quite different from the one given by most models of the net P of plant communities (6,11,15,16,18,19,23,24). In these models, R is assumed to be proportional to leaf area and independent of P, with the result that net P passes through a maximum at a point known as the optimum leaf area index1 (f g lb).

Prior Illumination and the Respiration of Maize Leaves in the Dark

“…The course of respiration was similar in air and C02-free air, and for leaves from the field and growth room. 0 Others found that dark respiration of leaves (4,6) and whole plants (7,8) varied with prior illumination in air, but my experiments reveal that respiration is quantitatively related to prior illumination and not to net CO2 fixation. The stimulation of respiration in maize leaves differs in onset and duration from the postillumination burst of photorespiration (1,10), but resembles the enhancement by blue light of respiratory 02 uptake in starved cells of Chlorella (5,9).…”

Development of the Monsi-Saeki Theory on Canopy Structure and Function

“…Sun and shade leaves differentiate within a stand and they have different photosynthetic capacities from each other (Boardman, 1977;Björkman, 1981). [McCree and Troughton (1966) suggested that the optimal LAI as assumed by Monsi and Saeki (1953) would not exist because shade leaves have a lower light compensation point.] However, Saeki (1959) noted that as the contribution of shaded leaves to total canopy photosynthesis was small, canopy photosynthesis calculated based on the photosynthesis of sun leaves gave virtually the same result as that calculated based on different photosynthetic capacities between sun and shade leaves.…”

Leaf death and decomposition during pasture regrowth

“…Past and current opinion favours leaf death as the major cause of reduced net growth rate at high Leaf Area Index (L.A.I.) values, so that at yield ceilings leaf production is offset by leaf death (Donald 1956;Barnard 1964;Bean 1964;Hunt 1965;Brougham 1966;McCree and Troughton 1966). Willoughby (1965) suggests that even in a seemingly highly utilised sward the amount of pasture lost through death and decay is generally underestimated.…”