Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis

We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, H_M, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PH_L ^–H_M state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, H_L, is blocked and the yield of transient P⁺H_M ^– can be estimated with respect to the lifetime of P^*. Under these conditions flash induced absorbance decreases of the bacteriochlorophyll dimer 990 nm band suggest that a transient P⁺ state is formed with a quantum yield of 0.09±0.06 compared to that formed during normal photochemistry. These transient measurements provide an upper limited on the yield of a transient P⁺ H_M ^– state. An estimate of 0.09 as the yield of the P⁺ H_M ^– state is consistent with all current observations. This estimate and the lifetime of P^* suggest that the electron transfer rate from P^* to H_M, k_M, is about 5 × 10⁹ sec^–1 (_M = 200ps). These measurements suggest that the a branching ratio k_L/k_M is on the order of 200. The large value of the branching ratio is remarkable in view of the structural symmetry of the reaction center. This measurement should be useful for electron transfer calculations based upon the reaction center structure.     

Influence of temperature and nitrogen concentration on photosynthesis ofDunaliella viridis Teodoresco

The photosynthetic behaviour ofDunaliella viridis has been studied under a combination of three variables: irradiance (0–900 mol m^–2 s^–1), temperature (15, 23, 31, 38, 42 °C) and nitrogen concentration (0.05, 0.5, 1.5, 5, 10 mM NO ₃ ^- ) at a salinity of 2 M NaCl.The highest rates of photosynthesis have been found at 31 °C and a nitrate concentration of 10 mM. There exists a synergistic effect between temperature and nitrogen availability on the photosynthesis ofD. viridis; under nitrogen deficiency oxygen evolution is low, even null at high temperature. The interaction between these two variables of control occurs in a multiplicative way. There is also a general increase in photosynthetic pigments following the increase in nitrogen concentration in the culture medium. The normalization of net photosynthesis data in relation to chlorophylla shows that nitrogen concentration makes an indirect control of the photosynthetic rate ofD. viridis through the variation of pigment concentration.     

Seasonal patterns of water relations and enzyme activity of the facultative CAM plant Portulacaria afra (L.) Jacq.

Abstract. Portulacaria afra (L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C₃ to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from -0.1 to -0.5 MPa, predawn and midday, when the C₃ pathway of photosynthesis was utilized. Osmotic potentials were constant, from -0.7 to - 0.8 MPa, indicating very little change in turgor. P. afra shifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m⁻²) despite C₃-like predawn water potentials. Midday water potentials usually decreased 0.2-0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three-to five-fold over C₃ levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other inducible CAM species.     

Soybean nodule gas permeability,nitrogen fixation and dirunal cycles in soil temperature

While diurnal cycles in nitrogen fixation rates are sometimes assumed to result from diurnal variation in photosynthetically active radiation, contradicting evidence exists that indicate soil temperature is the primary environmental influence. These studies assessed the significance of temperature on soybean nitrogen fixation under field conditions. Two groups of intact field-grown soybean plants, one at ambient and the other exposed to a 10°C diurnal variation in soil temperature, were nondestructively assayed for acetylene reduction rates. Activity was closely associated with soil temperature (R²=0.85), even when temperature was 12 h out of phase with ambient. Data were also obtained to determine if the effects of rhizosphere temperature on nitrogen fixation are mediated through an effect on the nodule oxygen permeability. Nodule oxygen permeability of intact, aeroponically grown soybean was closely correlated with the diurnal changes in temperature (R²=0.90).     

Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment

A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed.     

A simple model relating photoinhibitory fluorescence quenching in chloroplasts to a population of altered Photosystem II reaction centers

A model is presented describing the relationship between chlorophyll fluorescence quenching and photoinhibition of Photosystem (PS) II-dependent electron transport in chloroplasts. The model is based on the hypothesis that excess light creates a population of inhibited PS II units in the thylakoids. Those units are supposed to posses photochemically inactive reaction centers which convert excitation energy to heat and thereby quench variable fluorescence. If predominant photoinhibition of PS II and cooperativity in energy transfer between inhibited and active units are presumed, a quasi-linear correlation between PS II activity and the ratio of variable to maximum fluorescence, F_VF_M, is obtained. However, the simulation does not result in an inherent linearity of the relationship between quantum yield of PS II and F_VF_M ratio. The model is used to fit experimental data on photoinhibited isolated chloroplasts. Results are discussed in view of current hypotheses of photoinhibition.Abbreviations F_M maximum total fluorescence - F₀ initial fluorescence - F_V maximum variable fluorescence - PS Photosystem - Q_A, Q_B primary and secondary electron acceptors of Photosystem II     

Sediment addition enhances transpiration and growth of Spartina alterniflora in deteriorating Louisiana Gulf Coast salt marshes

Transpiration, leaf conductance, net photosynthesis, leaf growth, above-ground biomass and regeneration of new culms were studied in a rapidly subsiding Spartina alterniflora Lois. salt marsh following the addition at 47 and 94 Kg m^–2 of new sediment. Plant growth was enhanced in response to sediment addition as was evident by a significant increase in leaf area, above-ground biomass production and regeneration of new culms (p 0.05). Leaf conductance and transpiration rates were significantly greater in sediment treated plants than in control plants (p 0.05). Enhanced production of culms per unit area of marsh resulted in increased leaf area which allowed a greater capacity for net photosynthesis and contributed to increases in above-ground biomass of sediment treated plots.