Question

1) Choose one abiotic or biotic factor that affects primary productivity. Explain how it affects photosynthesis and respiration as well as Net Primary Productivity (NPP) and Gross Primary Productivity (GPP)

Answer 1

Abiotic factor - Temperature.

Effect of temperature on Photosynthesis-

• Temprature effect all biochemical reaction of photosynthesis,so the responses to temperature are complex. Temperature effect carboxylation reaction , as increase in carboxylation rate with increase in temperature and a decrease in the affinity of rubisco for CO₂ as temperature rises.These opposing effect dampen the temperature response of photosynthesis at ambient CO₂ concentration.
• The rate of photosynthesis highest at optimal temperature,when this temperature exceeds, rate of photosynthesis decrease again. Optimal temperature have strong genetic and physiological component. Plant of different species growing in habitats with different temperature have different optimal temperature for photosynthesis. Like plant growing at low temperature maintain higher photosynthetic rate at low temperature than plants grown at high temperature.
• The quantum yield of photosynthetic carbon fixation in a C₃ plant and in a C₄ plant as a function of leaf temperature. In normal air, photorespiration increases with temperature in C₃ plants, and the energy cost of net CO₂ fixation increases accordingly. This higher energy cost is expressed in lower quantum yields at higher temperatures. Because of the CO2 concentrating mechanisms of C4 plants, photorespiration is low in these plants, and the quantum yield does not show a temperature dependence.

Effect of Temperature on Respiration-

• Respiration typically increases with temperature. Between 0 and 30^oC, the increase in respiration rate for every 10^oC increase in temperature ,commonly referred as temperature coefficient Q₁₀ which is about 2. Above 30^oC the respiration rate often increases more slowly, reaches a plateau at 40 to 50^oC and decreases at even higher temperatures. High night temperatures are account for the high respiratory rates of tropical plants.

Effect of temperature on Gross primary productivity (GPP) and Net primary production (NPP)-

• With increase in temperature Gross primary productivity increase as rate of photosynthesis increases.Gross primary production (GPP) is equivalent to the carbon fixed during a given time period of photosynthesis. This increase seen up to optimal temperature.
• Net primary production (NPP) is gross primary production minus the energy lost in plant cellular respiration.Temperature affects production primarily by slowing or accelerating plant metabolic rates.For each 1^oC increase in mean annual temperature, total net primary production increases.However, at extremely high temperatures,above 30^oC and beyond, NPP can decline as leaf stomata close and plants conserve water.