Plants' Physiology- Photosynthesis

Plants' Physiology I. Photosynthetic pathways

 Just like humans eat food to maintain life, plants also consume food to grow, reproduce, and sustain their life. Although in plants' case, their food source is carbon dioxide(CO2) gas. Not only the plants consume food like humans, but plants also have their own mechanisms to convert their meals into stored energy inside their body. It is called photosynthesis.

 There are mainly two different kinds of photosynthetic pathways: C3 pathway and C4 pathway

 C3 pathway is the most abundant photosynthetic pathway in land plants including wheat, rice and soybeans. When C3 plants take in CO2 gas from the atmosphere, the C3 pathway works in a way that the CO2 gas to react with an enzyme inside the plants to be converted to energy source like sugar. However, C3 pathway is not always efficient because the enzyme also allows oxygen(O2) gas which is not a main food source for plants to be attached and produce CO2, and this is called photorespiration(2). Photorespiration involves with assimilation of nitrogen in plants which helps plants to grow and reproduce(14). Although Nitrogen is necessary nutrient for plants to grow and reproduce better, competition between O2 and CO2 in plant cells will not be efficient for plants to store life energy in low CO2 environment.

C3 pathway and photorespiration
the enzyme that convert CO2 into glucose is called Runisco.
BLOOM 2009

 So, some plants in low CO2 environment have adapted to have more efficient photosynthetic pathway-C4 pathway. The main C4 plants include maize and sugarcane. To avoid photorespiration, these plants developed a system to increase concentration of CO2 at the site of the enzyme by separating location of the enzyme from where oxygen exists. Although this process requires plants to spend some more extra energy, plants become more efficient in producing sugar. Because they hardly do photorespiration, C4 plants are likely to survive and grow better in low water and low nitrogen environment than C3 plants normally do(1).

C4 pathway.
Copyright © Gary E. Kaiser

Plants' Physiology II. Thermal optima

Another factor that influence plants physiology is plants' thermal optima. Plants have different thermal optimal rage depend on the location that they have evolved. If the temperature is too low compared to plants' thermal optimal rage, the reaction to maintain plants' life is too slow. If the temperature is too high for their thermal optima, enzymes that catalyze chemical reaction in plants denature, or plants die of drought even before the enzymes denature. Outside the plants' thermal optima, plants tend to dramatically reduce their fitness as photosynthetic rate reduces or seed production reduces(13). Reduced fitness of plants cause decrease in plants' biomass, which further affects other organisms that feeds on plants. 

Corn and Soybean Temperature Response
ARS USDA

 Because of their physiology that is greatly affected by atmospheric CO2 level and temperature, plants tend to evolve or to make adaptations to survive and grow better when these environmental sources change. Plants' evolution involves picking up genetic feature that are more suitable for them to survive from various genes. But there are always trade-offs in the nature. Some favorable traits for natural selections are negatively correlated to each other, leading to "inhibited selection responses"(5). So it is hard to just expect plants to make adaptations to the new environmental conditions made by human forces. That is why we need to pay more attention to the climatic change and its impacts. 

 Now we know that plants need CO2 and appropriate temperature to sustain their lives. How are these factors differentiated upon climate change? How are these related to our diet?

 Let's dig in!