Photosynthesis and Photorespiration in Whole ... - Plant Physiology

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736 [180 V. d[ 180 = [180 . dv - 2 u. dt 2 2 [16Q is I 02] + I 0 ] dl[ 180 -dv u.dt/V [180] V 1[1602] + [180 ] By subtracting equation I from equation 3: GERBAUD AND ANDRE Plant Physiol. Vol. 64, 1979 The comparison of the P, derived from this formula with the value (2) already known is a test of the validity of our measures. PHYSIOLOGICAL VALUE OF THE MEASUREMENTS The measure of the true rate of 02 uptake can be biased by two (3) causes. (a) If the 02 from photosynthesis is taken up as well as the external 02 in the process of photorespiration, then our measurement will be less than the true rate. This sort of recycling seems to be neligible, due to the high relative pressure of the outer 02 (22). d[ 80 1 d[Ne] - u.dt/V 1 2] [Ne] 160 ] + [ 180 During the course of the day, the total 02 concentration [02] stays in the range 21 to 22%, so that we may consider it as a constant in the integration of equation 4: (4) z 0 z 8 Trd[Ne] d[ 1802 2 1 T =Fu.dt INe 118 O Jo [Ne] [ °2]- [02].V 0 Hence: [02] *V I[0210 [Ne]1 U = in TJ [[118021 [NeIT (5) (6) The net 02 evolution P0 was calculated from the increase of 02 concentration in the chamber. In the unit time the plant assimilates a volume Pc of CO2, (k+ 1) Pc of the mixture (CO2 + kN2) is injected into the chamber and the plant produced P0 of 02; therefore the following volume must be expelled from the chamber: Lv = (k+1)Pc + P0 - Pc or Av = k.P c +P (7) containing an amount 11602] Av of 1602. The plant produces only 1602, and in our case 1802 is about 50 times less concentrated in the cell than 1602, so that we shall equate here the uptake of 1602 with the total 02 uptake U. Hence the increase of 1602 in the chamber is: V A[ 16 ] 1 T 0 2 1 [ + k p [16 T 1160 L 2 'r 64 (8) .f -E z (9) o w A HOURS C02 0 x D a a k. - 2C From P. we calculate the photosynthetic ratio po/PC. The gross 02 evolution E was calculated from the net evolution plus the uptake: E = PO + U. Test of Photosynthesis. There is another way of measuring photosynthesis than counting the pulses of injected CO2: the decrease of neon concentration is related to volume dv, (eq. 1), which is proportional to the photosynthesis, because AV = k Pc+P0 in equation 7 can be replaced within a good approximation by (k+ 1) Pc, that is, set in the differential form: dv = (k+ 1) Pcdt, hence: d[Ne] / [Ne] = - (k+1) pC.dt / V (10) 1 T dN 1 [me]0 = 1T kV1 d{Ne] = 1 V n (11) P = _ln --- c T k+1 o [Ne] T k+1 [HeiT 0 - y 2( to -10DO Y. R I- - T n n n of I I 1- 1 a I I I I I I 0 Q, I - - - - - " 2 4 6 8 10 12 1x 16 18 20 22 24 HOURS FIG. 1. Recorded data during a typical day of a 46-day-old plant of wheat (day/night temperature 20.3 C/18.4 C, PAR irradiance 175 w m-2, 02= 21%, CO2 = 335 ,l/1). The two parts show in parallel the time course of the gas exchanges (A), and the isotopic concentrations (B). PR: net photosynthesis (C02); R: night respiration; T: transpiration. The decrease of 1802 traces the 02 traces the 02 uptake by the plant, using Ne as an internal standard. The slight variations of 1602 correspond to the photosynthesis or respiration. Initial concentrations were [18021 = 2970,l/1, [Ne] = 3,160 ul/l; isotopes were added before the beginning of the light periods when their concentrations went under 0.5%. Downloaded from www.plantphysiol.org on January 11, 2014 - Published by www.plant.org Copyright © 1979 American Society of Plant Biologists. All rights reserved.
(b) If the absorbed 02 is recycled through the plant into the atmosphere, our measure will also be underestimated. Dimon (10) has shown in short term experiments (1 h) that plants do not discriminate between isotopes of 02 and the photosynthetic 02 is not enriched in 1802. We have checked that no 1802 is released from the plant even after several days of 1802 consumption. As the evolved 02 comes from water, which has only one atom of 02, any evolved 180 would be predominantly in 18016O molecules. As we detect no enrichment of the evolved 02 in 18016O, we consider that there is no recycling of the photorespiratory 02- From these two points we conclude that the disappearance of 1802 from the atmosphere is a good measure of the true flux of 02 in photorespiration. RESULTS AND DISCUSSION Normal CO2 Level. Figure 1 shows the data obtained with a plant of wheat during the course of a typical day at normal CO2 level (average 335 t,ll). Net photosynthesis P, calculated from the number of pulses of injected CO2 is 43 ml/h. As a verification, net photosynthesis, as seen by neon (equation 11) is 45 ml/h. From the curves of Ne and 1802 we calculate U = 43 ml/h, that is, as much as Pc. From the increase of 1602 concentration (from 20.6 to 21.5%), we calculate the net 02 evolution PO = 46 ml/h and the gross 02 evolution E = 88 ml/h. The photosynthetic ratio P./PC is 1.07. Other experiments in the same conditions give U/P, = 1.02 ± 0.05 (four measures) and P./P, = 1.05 ± 0.02. We also found the same P./P, at other CO2 concentrations and the same U/Pc at half-light intensity (experiment not described here). PHOTORESPIRATION IN WHEAT Plant Physiol. Vol. 64, 1979 737 Our estimation of the importance of photorespiration is much larger than the most frequently found values: many authors estimate the CO2 evolution in the light from one-sixth to one-half of the apparent photosynthesis (8, 12, 25). These differences arise primarily from the difficulty in measuring the CO2 evolution (12). It is known that all measures using '3C or "C, even corrected for the biochemical recycling of the assimilated carbon, are still underestimated because photorespired CO2 is recycled into photosynthesis, to an extent depending on the different diffusion resistances in the pathway of CO2 (21). This recycling could even be complete, preventing any release of CO2 (4) at normal CO2 level. On the other hand, the present results are in agreement with the observed stimulating effect of low 02 pressures on plant growth: + 90%o for dry weight in Mimulus cardinalis (7), + 103% in soybean (18), + 90 to 106% in Phaseolus vulgaris (17). Parkinson et al. (17) observed that such an effect corresponds to an increase of the photosynthetic efficiency of up to 100/o at 5% 02 concentration. The order of magnitude of the flux of 02 raises the problem of its pathway(s). We know that atmospheric 1802 labels glycolate and further metabolites of the glycolate pathway, as well as the evolved CO2 (5, 6, 11), but it is not established if all of the absorbed 02 goes into the glycolate pathway, which should then be as active as the Calvin cycle (13). Otherwise, other reactions, such as the pure Mehler reaction (pseudocycic electron flow), have to occur concurrently (10). Response to Various CO2 Levels. For 10 days we subjected the plant to low (200 A1/1), normal (300-400 1il/l), and high (680- 12,000 p1/l) CO2 levels; each level was maintained for I or 2 days -1 I I I I I 41 42 43 44 45 46 47 48 49 50 51 52 DAYS AFTER SOWING FIG. 2. Gas exchange of a plant of wheat at various CO2 levels. U: photorespiration (02 uptake in light); E: gross 02 evolution: Pc + U: photosynthesis + photorespiration. Other symbols and conditions, except CO2, are as in Figure 1. Downloaded from www.plantphysiol.org on January 11, 2014 - Published by www.plant.org Copyright © 1979 American Society of Plant Biologists. All rights reserved.
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