**3. What characters are associated with drought resistance?**

All parts of the plant are affected by drought stress. Conversely, all parts of the plant can potentially contribute to augment/enhance stress resistance. Among the plant species that inhabit the earth a wide range of traits enable plants to either tolerate excess water (submer‐ gence) or very low-water (drought). Each species has a wide range of characters accumulated during the course of evolution to adapt to the habitat. Crops have evolved over centuries in a particular habitat and have, over time, adapted to the adverse conditions. All the crops occupying a habitat share the same/similar set of adaptive traits as per the law of homologous series of variation [10]. For example, in rice there are accessions, which tolerate short span to long periods of submergence. They outgrow the water level and produce panicles above (like lotus flowers) or kneel down when water recedes, all the time keeping panicles facing upwards. On the other hand, there are accessions of rice, which tolerate prolonged periods of drought. Referred to as upland rice, these accessions grow like other arable crops with no need for standing water, ever. In the middle of this wide range are genotypes, which grow under submerged soils (irrigated rice).

Extensive studies of gross morphology and hard pan penetration ability of roots among traditional and improved accessions of rice had brought out the fact that the most deep-rooted accessions were traditional accessions and improved varieties had relatively shallow roots. The traditional accessions had greater hard pan penetration ability compared to improved accessions [11]. Most likely, this was due to the natural selection over decades/centuries or crop improvement efforts in well-endowed habitats. Most traditional accessions manifested low grain yield, but had high degree of tolerance to drought [12] but improved varieties were susceptible to drought, shallow rooted and were high yielders. Based on the germplasm study, it almost appeared that the drought resistance and grain yield were under such a genetic control that bringing them together was impossible. This would have been the case if the two sets of traits were pleiotropic. Extensive QTL mapping studies for root traits and grain yield in the same mapping population in similar habitats contradicted this [13].

Considerable amount of investment has been made by the scientific community in studies associated with drought resistance. Several crops have been studied. A wide range of traits is found to be associated with drought resistance (Table 1). Ironically, the only trait that the farmer is interested in is that which fetches him/her an income from what constitutes a marketable produce [6]. Thus, finally, for practical breeding purposes, manifestation of drought resistance can be summarized as increase in the economic product(s). In food crops, it would be the grain, in fruit crops it would be fruit and in vegetable crops it would be the edible part, whatever it is root, leaves or stem. Thus, yield is one single trait that could be considered as a manifestation of drought resistance. As yield is a result of a well-grown plant, biomass could be the cause or a prerequisite to high yield. Further, good biomass is a result of adequate quantity of water and nutrients made available at appropriate growth of the plant. This implies a well-endowed root system. As a breeding objective, the plant traits that could be selected among segregants are robust and efficient root systems, well-endowed shoot system and finally grain yield. This means selection for the entire plant characters and not only for shoots that are easy to see, measure and select. Every part of the plant, above-ground or below-ground should be subjected to selection pressure.


**Table 1.** Development of drought tolerant quality rice varieties

**3. What characters are associated with drought resistance?**

submerged soils (irrigated rice).

154 Plant Breeding from Laboratories to Fields

All parts of the plant are affected by drought stress. Conversely, all parts of the plant can potentially contribute to augment/enhance stress resistance. Among the plant species that inhabit the earth a wide range of traits enable plants to either tolerate excess water (submer‐ gence) or very low-water (drought). Each species has a wide range of characters accumulated during the course of evolution to adapt to the habitat. Crops have evolved over centuries in a particular habitat and have, over time, adapted to the adverse conditions. All the crops occupying a habitat share the same/similar set of adaptive traits as per the law of homologous series of variation [10]. For example, in rice there are accessions, which tolerate short span to long periods of submergence. They outgrow the water level and produce panicles above (like lotus flowers) or kneel down when water recedes, all the time keeping panicles facing upwards. On the other hand, there are accessions of rice, which tolerate prolonged periods of drought. Referred to as upland rice, these accessions grow like other arable crops with no need for standing water, ever. In the middle of this wide range are genotypes, which grow under

Extensive studies of gross morphology and hard pan penetration ability of roots among traditional and improved accessions of rice had brought out the fact that the most deep-rooted accessions were traditional accessions and improved varieties had relatively shallow roots. The traditional accessions had greater hard pan penetration ability compared to improved accessions [11]. Most likely, this was due to the natural selection over decades/centuries or crop improvement efforts in well-endowed habitats. Most traditional accessions manifested low grain yield, but had high degree of tolerance to drought [12] but improved varieties were susceptible to drought, shallow rooted and were high yielders. Based on the germplasm study, it almost appeared that the drought resistance and grain yield were under such a genetic control that bringing them together was impossible. This would have been the case if the two sets of traits were pleiotropic. Extensive QTL mapping studies for root traits and grain yield

Considerable amount of investment has been made by the scientific community in studies associated with drought resistance. Several crops have been studied. A wide range of traits is found to be associated with drought resistance (Table 1). Ironically, the only trait that the farmer is interested in is that which fetches him/her an income from what constitutes a marketable produce [6]. Thus, finally, for practical breeding purposes, manifestation of drought resistance can be summarized as increase in the economic product(s). In food crops, it would be the grain, in fruit crops it would be fruit and in vegetable crops it would be the edible part, whatever it is root, leaves or stem. Thus, yield is one single trait that could be considered as a manifestation of drought resistance. As yield is a result of a well-grown plant, biomass could be the cause or a prerequisite to high yield. Further, good biomass is a result of adequate quantity of water and nutrients made available at appropriate growth of the plant. This implies a well-endowed root system. As a breeding objective, the plant traits that could be selected among segregants are robust and efficient root systems, well-endowed shoot system and finally grain yield. This means selection for the entire plant characters and not only

in the same mapping population in similar habitats contradicted this [13].

All the traits fit well in the well-known formulae given below

Grain Yield – Transpiration x Water Use Efficiency

Finally, for a trait(s) to be of use, it should be correlated with or causally related to yield, easy to select for and should not be a drain on the energy reserves of the plant. For a breeder to use such a trait there should be ample genetic variability for the trait in the germplasm accessions. The heritability of the trait must be greater than heritability of the grain yield per se.
