Photorespiration lowers efficiency of photosynthesis from 90% ..
When pH increased, all three macrophytes displayed continuously decreasing photosynthetic rates, while only the two seagrasses exhibited photorespiratory activity. The decrease in photosynthesis at high pH was likely due to the decreasing availability of usable forms of DIC that occurs at higher pH values, while the total DIC level was sinking due to consumption , , , . Together with decreasing gross photosynthetic rates, an O2-level effect was observed in the two seagrasses. This indicates that part of the loss in photosynthetic efficiency seen when pH increased can be explained by photorespiration. The O2 sensitivity effect was exhibited when pH exceeded 8.1, in line with the commonly accepted notion that when available CO2 at the active site of Rubisco decreases relative to O2 levels, the oxygenation of ribulose bisphosphate (RuBP) increases . Unlike under “normal” O2 conditions, at which the macroalga performed better than did the two seagrasses, all three macrophytes responded similarly to increasing pH under non-photorespiratory conditions. This further supports the suggestion that photorespiration is the underlying cause of lower photosynthetic efficiency in the observed seagrasses, and that such a susceptibility to photorespiration is likely a result of limited capacity of their carbon acquisition mechanisms . The O2 sensitivity of photosynthesis has been observed in other submerged aquatic macrophytes, including some seagrasses , , , , , and photorespiration has been proposed to cause a decline in photosynthesis in Halophila stipulacea when O2 accumulates under flow-restricted conditions . The present study demonstrates that seagrass photorespiratory activity is induced by natural variations in the surrounding water caused by the primary productivity of other plants in the system. Moreover, as photosynthesis became more suppressed by low carbon availability, photorespiration increased greatly, particularly in Z. marina. Photorespiration could, under such conditions, reduce photosynthetic capacity with up to 40%. This highlights the ecological significance of photorespiration, especially in productive shallow coastal areas where DIC is usually limiting and the O2:DIC ratio is high. This was confirmed by the experiments conducted with field-collected seawater. The lower gross photosynthetic rates under natural O2 versus O2-depleted conditions indicated that photorespiration was taking place in seagrasses in their natural settings where DIC was limited. Although no difference in O2 concentration was observed between baymouth open water and bay water, DIC in the shallow bay was insufficient to eliminate the competitive effect of O2. We did not find any significant O2 effect on gross photosynthesis in U. intestinalis. However, this was to be expected, as various Ulva species are reportedly able to suppress photorespiration , by maintaining an efficient carbon-concentrating mechanism (CCM) that supplies the active site of Rubisco with high CO2 levels and consequently suppresses the oxygenase activity , .
Essay Photorespiration lowers the efficiency of photosynthesis by ..
Meaning of "photorespiration" in the English dictionary
Photorespiration competes with carbon assimilation, resulting in a lowered photosynthetic rate. However, the process also consumes oxygen, so decreased gross oxygen evolution rates are expected under photorespiratory conditions. For these reasons, a lower gross photosynthetic rate under normal conditions (i.e., natural O2) than under O2-depleted conditions was used as a measure of photorespiration in these studies. The relative level of photorespiration at each pH was defined as the percentage reduction of the gross photosynthetic rate under photorespiratory conditions (natural O2 concentration) versus non-photorespiratory conditions (low O2).
Photorespiration lowers the efficiency of photosynthesis by.
Regression analysis was conducted to describe the relationships between total DIC and O2 concentrations and pH value, respectively. In further analyses, pH was used as a proxy for water chemistry variables. The effect of O2 condition (normal or low O2) and pH on the photosynthetic rates of the three macrophytes was tested using repeated-measures ANOVA (O2 condition as the within-group factor and pH as the categorical factor). Fisher's least significant difference (LSD) test was used to determine the pH value at which the gross photosynthesis differed between the normal and low-O2 conditions. Functional relationships between pH and relative gross photosynthetic rates (percentage of the rates at the reference pH 8.1, i.e., the pH of the seawater well equilibrated with air) in all three macrophytes were calculated using linear regression, and significant differences between the linear relationships were tested using an analysis of covariance (ANCOVA). The relationship between photorespiration level in both seagrasses (% reduction of photosynthetic rate under photorespiratory conditions, dependent variable) and pH (independent variable) were assessed using regression analysis. The relationships between dark respiration rates and the pH and initial O2 concentration of Ulva-incubated seawater were also assessed using regression analysis. The effect of the low-O2 treatment on the dark respiration rates of the three macrophytes was tested using repeated-measures ANOVA. Before conducting ANOVAs, the assumption of homogeneity of variances was tested using Cochran's Test.