Photosynthesis and Principles of Pasture Growth

Hello :) In this post we'll take a look at the process of photosynthesis and the differences between the C3 and C4 photosynthetic pathways. We'll also discuss the impacts of water, light intensity, temperature and leaf area on photosynthesis. We'll finish off by going over the principles of pasture growth and managing leaf area. Enjoy!

Photosynthesis

Through a process called photosynthesis, plants trap light energy in a form useful to the plant by combining carbon dioxide with water to form simple carbohydrates. This is summarised in the equation below:

6CO2 + 12 H2O →→→ C6H12O6 + 6O2 + 6H2O

The carbon dioxide comes from the air while the water comes from the moisture of the soil. The simple carbohydrates are used for plant growth and maintenance or are stored.

Photosynthesis occurs in chloroplasts which are located in the mesophyll cells in the leaves of C3 plants. In C4 plants it takes place in these cells as well as the bundle sheath cells.  The process takes place on the surface of thylakoids and in the stroma of the chloroplasts. Chlorophyll pigments are found in the chloroplasts and they are light absorbing pigments which capture solar energy.

Two different reactions occur during photosynthesis. These are:

1. Light dependent reaction: chlorophyll absorbs energy from sunlight and uses it to oxidise water and produce oxygen and energy (in the form of ATP and NADPH).
2. Light Independent Reaction (aka Dark Reaction): this involved carbon fixation using energy from the light reactions to produce carbohydrates (this is the Calvin Cycle).

C3 and C4 Photosynthetic Pathways

In C3 plants carbon dioxide is fixed to a 5 carbon sugar known as RuBP in a reaction catalysed by the enzyme Rubisco. The 3 carbon compounds are rearranged into sugar phosphates which are used for the synthesis of carbohydrates and new RuBP. The Rubisco enzyme is also able to oxygenate RuBP to carbon dioxide by photorespiration (especially when there is a high temperature and light intensity for the plant). This is a distinguishing feature of C3 plants as this doesn't occur in C4 plants.

With C4 plants, carbon fixing and carbohydrate synthesis occur in different cells. Mespophyll chloroplasts release energy from light dependent reactions but do not have the enzyme Rubisco. Carbon dioxide is fixed as a 4 carbon compound and transferred to bundle sheath cells where carbon dioxide is concentrated. In these plants, the Calvin cycle occurs in the absence of oxygen and no photorespiration occurs. This makes C4 plants more efficient at fixing carbon than C3 plants.

In addition, C3 plants grow in low temperatures (optimum is 20-25° C) while C4 plants prefer high temperatures (optimum: 25-30°C). C4 plants are also more efficient at using water and achieve higher photosynthetic rates than C3 plants.

The efficiency of photosynthetic processes increases at high light intensity and at the optimum temperature for the plant. Efficiency also increases with an increase in atmospheric carbon dioxide and when optimum levels of water are available to the plant. The amount of light intercepted by the plant's leaves also has an impact on photosynthesis.

Pasture Growth

 

There are two important principles when it comes to understanding pasture growth and management.

1. The Optimum Growth Phase: there are three phases:
1. Slow growth after grazing (low yield)
2. Rapid growth because of an increase in leaf surface area (increasing yield)
3. Slow growth due to the shading by other plants which are now tall. (yield reaches a maximum but then starts to decline).
   The optimum yield is reached between phase 2 and 3. 
2. The Need to Rest Grasses after Grazing: some species need to be rested. This is because there is a certain number of leaves present per tiller and the time it takes for each leaf to appear may change depending on the season. In winter, a new leaf is likely to appear every 10-15 days while in summer this period is reduced to 5-7 days. 

We can illustrate the second principle by using ryegrass (which has 3 leaves per tiller) as an example. If the grass plant has just been grazed and there are no leaves left on the plant, the plant must use sugars from its energy stores in order to create a new leaf. This new leaf will allow photosynthesis to occur and the plant will be able to replenish its sugar stores. Once two to three leaves are present the sugar stores in the plant are quite high and this will allow the plant to regrow if it is grazed at this point in time. Thus, animals should only be allowed to graze the grass after the three-leaf stage. If the grass is grazed at an earlier stage more than once, the plant will not have enough sugar reserves to generate a new leaf and the plant will die. This is because the plant hasn't been given enough time to push up a new leaf in order to photosynthesise. Grasses should be rested for a period of time to allow a sufficient amount of leaves to grow. It has also been shown that the duration of grazing has an effect on pasture production. It seems that the optimum grazing duration is one day as this will result in the most pasture growth. 

However, some pastures don't need to be rested. Examples include sub-clover and broad leafed weeds as these species can maintain some leaf area for photosynthesis and replenishment of carbohydrates even when grazed low to the ground. Thus these species can tolerate continuous grazing.

That's it for this post, see you next time :)