Photosynthetic Rate Measurements

The subject of marine photosynthesis and its measurement is becoming highly complicated and cannot be discussed in any detail in the present manual. The topic has been reviewed fully by Strickland (Measuring the Production of Marine Phytoplankton, Bull. Fish. Res. Bd. Canada, No. 122, 1960) or, more recently, the chapter by Strickland (Riley and Skirrow [ed.] Chemical Oceanography, Academic Press, 1965) and the review of Eppley and Strickland (M. Droop [ed.] Advances in Microbiology of the Sea, Vol. I, Academic Press, 1968). 

All these subjects are dealt with by Strickland in the references mentioned above and in the present manual we will give methods only for measuring growth and the photosynthesis that has occurred in a BOD bottle during a period of illumination. The origin of the sample, the nature of its illumination, and the interpretation 

Rates of photosynthesis can be expressed in many ways. In the two methods which follow the unit used will be milligrams of carbon taken up (as a result of photosynthetic processes) in a cubic meter of sea water in 1 hr mg C/m per hr. 

The terms gross and net are used in conjunction with photosynthesis measurements to distinguish between the gross true synthesis of organic matter resulting from exposure to light and the net formation of organic matter that is found after allowance has been made for the respiration and other losses that occur in a plant cell simultaneously with the photosynthetic processes. 

The photosynthetic quotient (PQ) and respiratory quotient (RQ) are dimensionless numbers indicating the relative amounts of oxygen and carbon involved in the processes of photosynthesis and respiration. 

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Carpenter, E.J. and Roenneberg, T., The marine planktonic cyanobacteria Trichodesmium spp. photosynthetic rate measurements in the SW Atlantic Ocean, Mar. Ecol. Prog. Ser., 118, 267, 1995. 

The average photosynthetic rates for each of the three experiments performed, increased with UV exclusion and decreased with increased UV exposure (Table I). However, primary production rates of phytoplankton (collected on 17 November and 3 December 1987) measured under near-ambient light conditions were low, but not statistically different from those measured under either UV-reduced or UV-enhanced conditions. Small but statistically significant increases in... 

The oxygen evolution rate was measured by using the photosynthetic activity measurement system (Fig. 1). When light was illuminated to the reaction vessel, algal cells began to evolve oxygen and the linearity between dissolved oxygen and time was observed just after a few... 

To measure the dark respiration rates of photosynthetic tissue, measurements have to be performed in complete darkness, or with apparatus specially designed for this purpose. Long term rates of photosynthesis can be evaluated indirectly by dry matter accumulation. However, this is a somewhat inaccurate measure, because the measured amount of dry matter accumulated under stress conditions is less than it would be under non-stressed ones. 

At least six major phytotoxic air pollutants have been shown to reversibly inhibit apparent photosynthetic rates in plants (1 - ). Studies indicate that these phytotoxicants ranked in the following order according to the relative amount of inhibition effected after several hours of exposure to equal pollutant concentrations HF>Cl2-03>S02>N02>N0. A summary of the experimental results which compares measured depressions in CO2 uptake rates of barley and oat canopies after 2-hr pollutant exposures in environmental chambers appears in Figure Typical inhibition and recovery rate curves for exposures that reduced CO2 absorption rates by 20 percent at the end of the 2-hr fumigations are also shown. Similar data have been obtained for alfalfa, another important crop species which was cultured and exposed under identical conditions In contrast, equivalent... 

Table II includes supporting data for greater-than-additive inhibition of alfalfa apparent photosynthetic rates induced by SO2+NO2 mixtures. The enhanced effects were most marked at the lower concentrations applied, becoming less pronounced as pollutant levels were raised. At 50 pphm of each gas no synergism was evident. At this SO2 exposure concentration, sulfur dioxide appeared to regulate the observed plant responses. Significant amounts of inhibition resulted from the lowest bipollutant concentrations used (15 pphm of each gas) these concentrations were well below those required for the individual pollutants to measurably suppress apparent photosynthesis rates. At these exposure levels where no tissue necrosis occurred, the plants recovered completely within 2 hr after fumigation. The manner by which this inhibiting interaction occurred is not well understood. This pollutant combination is also known to act in a synergistic fashion to cause visible injury to plants, and further study of this mixture may be warranted.

It should be noted that NPP and photosynthesis are not synonymous. On a planetary scale, the former includes chemoautrophy, the latter does not. Moreover, photosynthesis per se does not include the integrated respiratory term for the photoautotrophs themselves (Williams, 1993). In reality, that term is extremely difficult to measure directly, hence NPP is generally approximated from measurements of photosynthetic rates integrated over some appropriate length of time (a day, month, season, or a year) and respiratory costs are either assumed or neglected. 

In summary, plants produce leaves with a continuum of photosynthetic characteristics, ranging from short-lived thin leaves with a high nitrogen concentration and high photosynthetic rate to long-lived dense leaves with a low nitrogen concentration and low photosynthetic rate. These correlations among traits are so consistent that specific leaf area (leaf area per unit leaf mass) is often used in ecosystem comparisons as an easily measured index of photosynthetic capacity. 

S. Beer, B. Vilenkin A. Weil, M Vete, L. Susel, A. Eshel (1998). Measuring photosynthetic rates in seagrasses by pulse amplitude modulated (PAM) fluorometry. Mar. Ecol. Prog. Ser., 174, 293-300. 

Identify the control in the following experiment. A student had four plants grown under the following conditions and was measuring photosynthetic rate by measuring mass. 2 plants in 50% light and 2 plants in 100% light. 

Photosynthetic rates determined on replicate samples from the same moss cushion were generally highly reproducible (standard error of mean <10%). Comparative measurements on Tradescantia were made under identical conditions. 

FIGURE 1. Average hourly values of chamber measurements on third stress day. (A) Solar radiation. (B) Canopy gross photosynthetic rate. 

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). 

Photosynthetic rates have been measured in the field since about 20 years. There is a fairly good understanding of the general photosynthtic response under most environmental conditions (e.g. Schulze and Hall, 1982, Lange, 1986). 

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