Rate, net photosynthesis

Figure I. (left) Relationship betwen net photosynthesis rate (Ph)and internal C02 concentration (CD in the tissue of leaves from the Greenhouse (G) and from the Cabinet C).

Net photosynthesis rate of the medial part of the leaf above the ear was measured with a portable C02 analyser (ADC). Net C02 fixation was not affected during the first 20 days, then declined more rapidly in plants without ear (fig.Sa). This corresponded to the onset of visible senescence in the plants. 

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. 

The effect of zearalenone on crop development may be connected to its influence on the status and functioning of the photosynthetic apparatus (Koscielniak et al. 2008). The after-effects of zearalenone on the growth of soybean and wheat plants, net photosynthesis and transpiration rates, stomatal conductance, photochemical efficiency of photosystem 2 and on final seeds yield were determined. Modifications in leaf area were more pronounced in soybean than in wheat, and this tendency increases in successive developmental phases. The net photosynthesis was stimulated during the juvenile phase and during that of the final one by about 13.6% (average) in soybean plants. Stimulation of transpiration was also observed after... 

Todd and Todd and Probst also measured the effects of ozone (at 4 ppm for 40 min) on photosynthesis and found that development of symptoms was associated with inhibition of carbon dioxide fixation. This effect was also confirmed by Macdowall, who reported that the inhibition of photosynthesis was greater than that which could be accounted for by chlorophyll destruction. Hill and Littlefield associated decreased net photosynthesis caused by ozone (at 0.06 ppm for 1 h) with both stomatal opening and rates of transpiration. These studies have generally shown that net photosynthesis can decrease without visible injury. 

Although photosynthesis is the ultimate source of O2 to the atmosphere, in reality photosynthesis and aerobic respiration rates are very closely coupled. If they were not, major imbalances in atmospheric CO2, O2, and carbon isotopes would result. Only a small fraction of primary production (from photosynthesis) escapes respiration in the water column or sediment to become buried in deep sediments and ultimately sedimentary rocks. This flux of buried organic matter is in elfect net photosynthesis , or total photosynthesis minus respiration. Thus, while over timescales of days to months, dissolved and atmospheric O2 may respond to relative rates of photosynthesis or respiration, on longer timescales it is burial of organic matter in sediments (the net photosynthesis ) that matters. Averaged over hundreds of years or longer, burial of organic matter equates to release of O2 into the atmosphere - - ocean system ... 

Fermentation and reduction of secondary electron acceptors are temporary expediencies for soil microbes. The resulting products are unstable in the presence of oxygen and eventually oxidize further when oxygen becomes available. SOM is a beneficial result of incomplete oxidation and fermentation. The SOM content reflects the difference of the rates of organic matter addition vs. oxidation rates. The rate of addition is essentially (lie rate of net photosynthesis. The oxidation rate is governed by temperature and by the rate of oxygen supply. 

Respiration is essentially the converse of the process of photosynthesis. If the two processes occur concurrently, then the rate of photosynthesis as actually observed (net photosynthesis) will be less than the total rate (gross photosynthesis) by an amount equal to the rate of consumption of the products of photosynthesis in respiration. At certain low light intensities or carbon dioxide concentrations the two processes balance so that there is no net gas exchange and the photosynthetic organism is then said to be at its compensation point. It has usually been assumed, according to Fogg... 

Fig. 22. Data of Hanson et al. (H3) on net specific rate of photosynthesis in continuous propagation of algae. This establishes the stoichiometric relation between growth and photosynthesis.

In controlled laboratory mesocosms, Spartina patens, a dominant brackish marsh species found along the U.S. Gulf coast, and rice (0. sativd) showed a decrease in net photosynthesis in response to reduced soil redox potentials. Net photosynthesis decreased when soil redox potential or Eh was below -100 mV (Kludze and DeLaune, 1995b). A similar reduction in photosynthetic rates was observed in 0. sativa with increase in intensity of reduction (Figure 7.31). However, wetland plants... 

Most recent, fully expanded leaves at the peak of the vegetative growth stage were included in this study. Net photosynthesis (Pn) rates were measured as described by Lawlor et al. (1). Leaf area was measured by Li-Cor LI-3050A leaf area meter. 

On both a fresh weight and a chlorophyll basis leaflets and stipules have similar phqtosynthetic rates (219 pmol CO, g FW.sec and 133 UTlol COg mg CHLh Y Net photosynthesis of tendrils is 40% lower, but expressed on a surface area basis, tendrils are only 10-15% less efficient photosynthetically (Fig.1 C). In tendrils transpiration rate and dark respiration are higher than in the... 

These biochemical changes were accompanied by a decrease of net photosynthesis and a progressive increase of the respiratory rate recorded before illumination (fig. 1). The KCN insensitive respiration, suppressed by SHAM, was comparable to the uncoupled rate after 24 hours of glucose feeding (fig. 2). 

Over a x riod of 3-7 days total chlorophyll showed a sharp decline in infected leaves compared to uninfected controls [Fig. 1]. A small decrease in total carotenoid concentration occurred over the same period. The chlorophyll a b ratio was unaffected by the pathogen. The light saturated rate of photosynthesis per unit area was initially increased, over the first three days, and then decreased over the next four days [Fig. 2A]. However when expressed per unit chlorophyll the rate of O2 evolution was greater in mildewed than control leaves [Fig. 2B]. The numbers of haustoria increased during the p>eriod of decline in total chlorophyll concentration and net photosynthesis (leaf area basis). 

Susceptible Isoline Starting with the fifth day post inoculation net photosynthetic rate of the infected leaves falls below that of the noninfected control. In the last few days of the measuring interval, net photosynthesis stabilizes to a final level at 40-45 % of the controls (Fig. 1). Linked to the drop in photosynthesis, the chlorophyll content declines to 50-60 % of the control leaves (Fig. 2). Hill activity from H O to ferricyanide (PSII + PSI) of thylakoid suspensions isolated from infected leaves decreases to 60-70 % of that of healthy leaves (Fig. 3). DMMDBQ stimulated and DBMIB inhibited thylakoid suspensions (PSII solely) only show a small infection related change in activity (Fig. 3). 

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