This article will guide you about how to determine stomatal frequency and stomatal index.
Stomatal frequency can be defined as the number of stomata present per unit area of a leaf. Determination of stomatal frequency and the total area of stomata covered in a leaf are the essential prerequisite to assess the rate of water loss through stomata. Several environmental and genetical factors affect stomatal frequency.
The following environmental factors change the stomatal frequency- water availability, carbon dioxide concentration, temperature and light intensity. Water stress results in a greater stomatal frequency. Plants growing in wet soil with high humidity have lower frequency than the plants growing in dry soil with low humidity. Stomatal frequency is reduced in polluted atmosphere, e.g. Trifolium partense.
Light intensity has marked effect on stomatal frequency. In low light intensity frequency reduces. Stomatal frequency becomes higher when a plant grows in full sunlight.
In an experiment it is observed that tomato plants grown in controlled condition have hypostomatic leaf and when grown at high light intensity the leaves are amphistomatous. The degree of ploidy also affects stomatal frequency. Polyploid plants have less stomatal frequency and larger stomata. Generally stomatal frequency decreases where the stomata are large.
Stomatal frequency is not constant within a plant. Highest frequencies are often found in those leaves that occur on the top. Even within a leaf there exists variation in frequency. As for example the grass leaf has lowest frequency at the tip. The highest frequency is observed at the point of insertion of the leaf where the cells are still developing and smallest.
Because the stomatal frequencies often vary Salisbury (1928) proposed the term ‘stomatal index’. Stomatal index relates the number of stomata to number of epidermal cells.
Stomatal index is calculated in the following way:
This value is found to be reasonably constant for any particular species.