No abstract

No abstract

Single attached leaves of sunflower (Helianthus annus L."Mennonite") were supplied "CO2 of constant specific radioactivity in gas mixtures containing various CO2 and 02 concentrations. The "CO2 and C02 fluxes were measured concurrently in an open system using an ionization chamber and infrared gas analyzer.The rate of photorespiration (5.7 + 0.3 mg CO2-dm 2 hr-') during photosynthesis in 21% 02 at 25 C and 3,500 footcandles was over three times the rate of dark respiration and was independent of C02 concentrations from 0 to 300 Al/l.The steady rate of C02 evolution into CO2-free air was about 30% lower. Low oxygen (1%) inhibited both "CO2 and C02 evolution, both during photosynthesis and in CO2-free air in the light.At 300 ul/l C02 apparent photosynthesis was inhibited 41% by 21% 02. Two-thirds of the inhibition was due to the inhibition of true photosynthesis by oxygen and one-third due to the stimulation of photorespiration. At 50 u1d/l C02, where the percentage inhibition of apparent photosynthesis by 21% oxygen was 92 %, photorespiration accounted for two-thirds of the total inhibition.The rate of 14CO2 uptake by the leaf decreased about 30 seconds after the introduction of 14CO2, indicating that "4CO2 was rapidly evolved from the leaf. The rate of "CO2 evolution increased rapidly with time, the kinetics depending on the C02 concentration. The high specific radioactivity of the "4CO2 evolved during photosynthesis or in the early period of flushing in C02-free air showed that the substrate for light respiration was an early product of photosynthesis. From the measurement of "CO2 and C02 evolution into C02-free air over a longer time period it was apparent that at least three compounds, each of deereased 1"C content, could supply the substrate for light respiration.Based on a consideration of the specific radioactivity of There is now much evidence to show that the rate of CO, evolution from green leaves in the light exceeds that of dark respiration and that the substrate and mechanism of CO2 evolution in the light are different from those of CO-evolution in the dark (24). All methods to date (23, 24, 38), however, have not measured the rate of photorespiration under conditions of steady state photosynthesis nor have they (18, 38) allowed a continuous measurement of the specific radioactivity of the 'CO. evolved from a leaf during or after a period of photosynthesis in 'CO2. We have recently described a system (26) which continuously measures the CO. or "CO2 fluxes from leaves. In this paper we present results on the rates of photorespiration duLring steady state photosynthesis and on the relationship of the "CO2 evolved to the products of "CO2 fixation. Brief accounts of this work have previously been presented (27,28). MATERIALS AND METHODSThe mnaterials and methods have been fully described in a preceding paper (26). Sunflower leaves (Helianthlls annulis L."Mennonite"), grown as previously described (26), were used for all experiments. RESULTSFrom the CO; and "CO2 uptake and the average specific ...

An open gas exchange system that uses an infrared gas analyzer and an ionization chamber to measure CO2 and 14CO2 is described. The system will continuously measure the CO2 and 14CO2 exchange from a leaf within 30 s after the 14CO2 gas mixture is supplied to the leaf. Measurements are obtained under steady rate conditions since the 14CO2 gas mixture is supplied at a constant composition from pressurized cylinders. Methods of determining true photosynthesis, apparent photosynthesis, CO2 evolution, and the specific activity of the 14CO2 evolved are clearly described. The rate of CO2 evolution in the light was not affected by the CO2 concentration and the evolved CO2 was derived from the early products of photosynthesis.

SummaryThe relation between the leaf area index (L.A.I.) of artificial communities of cotton plants and their rates of dark respiration and of net photosynthesis at three light intensities was examined. The L.A.I. was varied by the removal of successive layers of leaves, working from the base of the canopy upwards. Experiments were carried out at 20, 30, and 40°0 to vary the relative magnitudes of respiration and photosynthesis.At all temperatures and light intensities, net photosynthesis by the communities increased with increase in L.A.I. to values of 3-4. At 20°0, there was no change in net photosynthesis with further increase in L.A.I. to 7·6. At 30°0 there was a slight decrease in net photosynthesis as L.A.I. increased from 4 to 6· 5, and at 40°0 the decrease in net photosynthesis at the highest L.A.I. values was more marked. At 40°0 the maximum L.A.I. that could be maintained was, however, only 5·4. The maximum net photosynthetic rates by the communities, under a light intensity of 3300 f.c., were 32, 25, and 18 mg 002/dm2 of ground surface/hr at 20, 30, and 40°0 respectively.The lack of a marked decline in net photosynthetic rates of the cotton communities at high L.A.I. values is shown to be due to the low rates of respiration in the lower leaves of the canopy, the compensation point falling progressively with depth in the canopy.