Photosynthesis, maximum rate

The effect of temperature fluctuations on net carbon dioxide uptake is ikustrated by the curves in Figure 18. As the temperature increases, net photosynthesis increases for cotton and sorghum to a maximum value and then rapidly declines. Ideally, the biomass species grown in an area should have a maximum rate of net photosynthesis as close as possible to the average temperature during the growing season in that area. 

At low irradiances, photosynthesis uses virtually 100% of the quanta, but in full sunlight, about 2000 imol quanta s , more quanta are available than can be used in photochemistry. Maximum rates of photosynthesis by Populus or Spinacia leaves of 15 and 70 jumol O2 m s , respectively, would require only 15 x 9 = 135 to 630 jumol quanta m s , or 10-40%. Leaves, therefore, need to be able to dissipate 60-90% of the quanta at high irradiance in an orderly manner such as non-radiative decay if they are to avoid the potentially damaging formation of oxygen radicals from reduced ferredoxin (Asada Takahashi, 1987). When plants are under a stress that restricts CO2 assimilation, excessive light will be reached at even lower irradiances. 

Illumination affects the rate of photosynthesis Rp. The R parameter as a function of E has a maximum at some optimal value of Ernax, which drifts from this critical value when illumination increases or decreases. The maximum Rp at various latitudes ip is located at depths that vary as a function of season (i.e., sun elevation). Thus, in tropical zones this variability with depth is most pronounced. On average, the photosynthesis maximum is located at depths of 10m-30m, and in open water bodies it can be observed at depths below 30 m. Here Emax = 65 cal cm 2 da 1 85 cal cm-2 da-1. At depths where E = 20 cal cm-2 da- -25 cal cm-2 da-1, photosynthesis decreases in proportion to E. An apparent suppression of phytoplankton by light is observed at E > 100 cal cm 2 da 1. These estimates are quite different in northern latitudes, where the photosynthesis maximum is located, as a rule, at the surface. 

According to modern observations chemosynthesis has a maximum rate in the 15-20-m layer below the sulfide onset [78,79] and its value is comparable with the rate of photosynthesis [79]. That should lead to the significant consumption of inorganic carbon, phosphate, and ammonia. 

As we have just calculated, each chlorophyll molecule in an unshaded chloroplast can absorb a photon about once every 0.1 s. When there are 250 chlorophylls per reaction center, 12.5 of these molecules are excited every 5 ms (250 chlorophylls x 10 excitations per chlorophyll/1 s x 0.005 s). However, because the average processing time per reaction center is about 5 ms, only one of these 12.5 excitations can be used photochemically — the others are dissipated by nonphotochemical deexcitation reactions. Consequently, although the biochemical reactions leading to CO2 fixation operate at their maximum rates under such conditions of high PPF, over 90% of the electronic excitations caused by light absorption are not used for photosynthesis (Fig. 5-12). 

Fmax at light saturation and at the optimal temperature for photosynthesis varies with plant species but is usually from 2 to 10 mol m-3 s-1. We can also estimate Vmax from measurements of the maximum rates of CO2 fixation by isolated chloroplasts. These maximum rates—which are sustained for short periods and are for optimal conditions—can be 100 mmol of CO2 fixed (kg chlorophyll)-1 s-1 [360 pmol (mg chlorophyll)-1 hour-1 in another common unit], which is approximately 3 mol m-3 s-1 (1 kg chlorophyll is contained in about 0.035 m3 of chloroplasts in vivo). In vitro, the key enzyme for CO2 fixation, ribulose-l,5-bisphosphate carboxylase/oxygenase, can have rates equivalent to 200 mmol (kg chlorophyll)-1 s-1. The estimates of Vmax using isolated chloroplasts or enzymes usually are somewhat lower than its values determined for a leaf Measurements using leaves generally indicate that KqOz is 5 to 20 mmol m-3. For instance, Kcch can be 9 mmol m-3 at 25°C with a Q10 of 1.8 (Woodrow and Berry, 1988 Q10 is defined in Chapter 3, Section 3.3B). 

Some of the most productive crops are C4, including maize and sugarcane as are some of the worst weeds, such as Amaranthus (love-lies-bleeding) and Trihuliis terrestris. Provided with a high light intensity, their maximum rates of photosynthesis may be double those of C3 plants. 

Diurnal cycles of in situ primary production follow in general the diurnal curve of PAR, except for surface samples, where the maximum rates are already found in the morning whereas high PAR intensity may inhibit photosynthesis at noontime (Behrends et al., 1994). The record of diurnal cycles of primary production was not the aim of our work, but was mainly taken as a tool for estimating the integrated daily production. 

Apparent Maximum Rates of Photosynthesis under Natural Conditions... 

The impact of light is represented by the lumped yield coefEcient. This represents the first linear part in the so-caUed PI curve, see below. Of course this does not go at infinitum, but ends at a specific maximum rate represented by the second more or less constant part of the PI curve. This maximum value can be determined either by maximum capacity of the light reaction in photosynthesis or by another limiting step, may be capacity of RuBisCo for CO2 fixation or nutrient availability. Also other intracellular botdenecks in metabolism cannot be excluded a priori. 

Ozone fumigation decreased the initial slope of the A/c curve which is dependent on the efficiency of ribulose bisphosphate carboxylase. Carboxylation efficiency declined to 20% of the control value after 16 hours at 400 nmol mol O3 (Fig. 2). Conversely, regeneration of the primary CO2 acceptor, i.e. RubP, was not severely affected by O3 because at elevated levels of c the CO2 saturated rate of photosynthesis was inhibited by no more than 30%. The capacity for regeneration of RubP relates to the maximum rate of coupled electron transport in vivo providing it is not restricted by the balance of chloroplast sugar phosphate export and inorganic phosphate import. 

The result is interpreted, that plants do not optimize water use at the whole plant level, but rather use water when available at maximum rate. Annual production is eventually determined by the maximum rate of photosynthesis and the length of time for open stomata at high water consumption. The physiological responses of stomatal closure and restricted assimilation are rather a response towards ensuring survival rather than increasing water use and productivity. 

Maximum rate of net photosynthesis 1- 4 (mg CO2 dm leaf surface h ) (highest reported values 11-13)... 

The rate of maximum cation uptake has been shown to be dependent upon maximum levels of organic acids in the root cells (119,120). The organic acids, such as citric and malic acids, are transported from the sites of photosynthesis by the phloem while the cations are transported to the leaves by the xylem. Brown and Chaney (120) have suggested that iron uptake into the plant will be dependent upon the citric acid content of the root cells. 

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