Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.11 Photosynthetic Indices

Photosynthesis is characterized by its rate or intensity. Photosynthetic intensity is measured by The amount of milligrams of CO₂ absorbed per square decimeter of leaf surface area per hour. This is determined by measuring the decrease in carbon dioxide concentration within a closed chamber containing the leaf. Simultaneously, along with measuring photosynthetic intensity, the Respiration rate of these Cells is determined in the dark. The amount of CO₂ released during respiration is added to the amount absorbed under illumination. It is generally assumed that the cellular respiration rate is the same in both light and dark conditions. However, these corrections cannot accurately estimate true photosynthesis because shading the leaf not only halts photosynthesis but also suppresses the glycolate cycle, and furthermore, respiration is actually influenced to some extent by light.

The intensity of photosynthesis can also be measured by the amount of oxygen released per square decimeter of leaf surface area per hour. The values of photosynthetic intensity calculated from the amount of absorbed carbon dioxide and the amount of released oxygen may not match, as CO₂ is involved in the dark reactions, whereas oxygen is produced in the light reactions of photosynthesis. The ratio of the volume of released oxygen to the volume of absorbed Carbon dioxide is termed the photosynthetic quotient. During carbohydrate synthesis, the photosynthetic quotient equals one (6O₂ : 6СО₂ = 1). During Protein Synthesis, it equals 1.25, and during lipid synthesis, it is 1.44. The average photosynthetic quotient across various plants is 1.04.

The value of the photosynthetic quotient depends on lighting conditions and mineral Nutrition. Illuminating plants with blue light and providing an adequate supply of nitrogen enhances protein synthesis, thereby increasing the photosynthetic quotient.

To characterize photosynthesis, indicators such as quantum requirement, quantum yield, and the assimilation number are used. The quantum requirement of photosynthesis is the ratio of the number of absorbed light quanta to the number of assimilated carbon dioxide molecules. The reciprocal of this value is known as the quantum yield. The assimilation number is the amount of carbon dioxide assimilated by a leaf per hour, calculated per unit of chlorophyll contained within the leaf.

The amount of dry matter (in grams) accumulated in a plant per 24 hours per 1 m² of leaf surface area characterizes the net assimilation rate (NAR).