Measurement of Drought Resistance in Plant Breeding
Various procedures are used for measuring drought resistance in crop plants. The most commonly used procedures include: 1) Leaf water retention, 2) Photosynthesis, 3) Yield performance, and 4) Root lengths of seedlings. These techniques can be used for large scale screening of segregating populations in breeding programme.
1. Leaf Water Retention:
In this method leaves are excited from the plant and are allowed to dry. The slower drying genotypes are considered as drought tolerant. In other words, the high water retainer genotypes are considered as drought tolerant. The cut leaf method was used in wheat, barley and oats. In wheat, two varieties Pelissier and Pitic 62 were significantly better water retainers when leaves were excised from three week old plants. These two varieties are highly resistant to drought. Some investigators use tissue water potential as an index of water stress under drought conditions. The tissue water potential is measured with the help of thermocouple psychrometer. The portable field psychrometer is widely use for measuring drought resistance in segregating populations.
2. Rate of Photosynthesis:
The rate of photosynthesis during and after moisture stress is an important index of drought resistance. In wheat, Pitic 62 (a drought resistant variety) exhibited high photosynthetic rate under drought conditions in several tests. Now portable non-destructive photosynthesis analyzers are available which can be used in the fields for large scale screening of germplasm as well as segregating populations in standing crops. The genotypes which have high photosynthetic rate under moisture stress are considered as drought resistant, because such genotypes give higher yield than those having low photosynthetic rate. A simple portable photosynthesis analyser made it possible to measure photosynthesis of many plants within a short time. Now photosynthesis is used as a criterion to select for drought resistance.
3. Yield Performance:
Superior yield performance under moisture stress conditions is an important and reliable index of drought tolerance. The yield tests should be conducted in drought prone areas at several locations or for several years. These will help in identification of genotypes with drought resistance and also in the elimination of drought susceptible lines. The yield test should be conducted under both field as well as glass house conditions. Moreover, large number of populations should be grown. This will enhance chances of obtaining superior drought resistant genotype.
4. Root Length of Seedlings:
The root length during seedling stage is also used a measure of drought resistance. In wheat during root length of 5-7 days old seedlings grown in sand was related to root mass at maturity. In a more recent study it was observed that root mass after 30 days was reliable index of root mass at maturity. Thus those genotypes which have longest root during seedlings stage also exhibit extensive root system at maturity. This is a simple and quick method of measuring drought resistance in each crop season. Moreover, after screening superior plants can be replanted and grown to maturity. Some workers use hydroponics tank to measure the root growth of seedlings. ( Sullivan and Rose, 1977).
In rice, the force required to pull out the seedlings from paddy soil is used as a measures of drought tolerance. The force required is correlated with root weight, branching and number. This technique may be used for large scale screening of rice populations for drought resistance (O Toole and Soemartono, 1981).