The rate of photosynthesis is measured by one of the following methods: By measuring (I) the volume of CO, consumed (2) volume of oxygen liberated (3) amount of carbohydrate accumulated.
1. OXYGEN EVOLVED
It is the simplest method. It this case, number of bubbles of oxygen are counted given out in unit time by green aquatic plants like Hydrilla. But there is objection on this method.
• The gas evolved is not pure oxygen. It also contains some carbondioxide and nitrogen.
• Another objection is that bubbles are not always of uniform size. Wilmott passes the bubbles through the nozzle of a tube immersed in distilled water. Now the bubbles escape from the nozzle is of uniform size.
In another method the evolved oxygen is combined catalytically with hydrogen to form water in the presence of palladium. It can give exact amount of oxygen evolved during photosynthesis.
(b) CARBON DIOXIDE ABSORBED
Air of known carbon dioxide content is passed through a chamber. This chamber contains photosynthetic material. Some of the carbon dioxide is used up in the process. The unused carbon dioxide is passed through a vessels containing soda lime. It gives the percentage of unused CO2. This amount is deducted from the total CO2 supplied. It givecthe total amount of CO, used during photosynthesis. Following methods are used to estimate the exact amount of CO2 and 02 released.
I. Eudiometer Method: It gives the amount of carbon dioxide absorbed and oxygen evolved simultaneously. A leaf is placed in an eudiometer tube. This tube contains air of known carbon dioxide content. The tube is sealed off by inverting its open end over mercury. It is placed in light. Then air inside the eudiometer is analyzed for its carbon dioxide and oxygen content.
2. Photosynthometer: It was developed by Ganong. It is a convenient laboratory method of analyzing the gas contents. The ,photosynthometer consists of three parts: (a) a glass bulb A. (b) a graduated tube C, and (c) a connecting link.
(c) INCREASE IN THE DRY WEIGHT
Dry weight method was adapted by Sachs. Photosynthesis increases the dry weight of plant. It can be measured in the form of dry weight. This method has following steps:
- A plant is placed in darkness. Cuttings from its leaves are taken. They are cut into discs of uniform sizes. These discs are then heated in oven. Their dry weight is estimated.
- Another plant is illuminated in light. Its leaves are taken. These leaves are divided into discs. Their dry weight is determined.
- Dry weight of both the leaves is estimated.
Some of the products of photosynthesis are used during photosynthesis. So it is not an accurate method.