Breeding for Drought Tolerance by Combining Shoot Dehydration Tolerance with Deep and Dense Root System.

Success in breeding for drought tolerance has not been as pronounced as for other traits. This is because several factors and mechanisms operate in shoot and root to enable plants to cope with the drought stress making it appear as a complex trait. These include i). morpho-phenological traits such as reduced leaf area, leaf rolling, leaf abscission, wax content, efficient rooting system, reduced tillering,  earliness, and awns, etc., ii).physiological traits such as reduced transpiration, high water-use efficiency, stomatal closure and osmotic adjustment and iii). biochemical traits such as accumulation of proline, polyamine, trehalose etc. and increased storage of carbohydrates. Thus, when drought tolerance is taken as one trait, it appears very complex and difficult to breed for. The occurrence of drought stress is also unpredictable and heterogeneous   in time and space making it difficult to develop simple, cheap and reliable screening methods. Screening under field condition is not always possible because of unpredictability of rain and   variable residual moisture. Also, rain out shelters for in-season screening are expensive and screening for drought tolerance in off-season using controlled watering is often not relevant to the real growing season environment particularly when temperature and photosensitivity play important role in crop growth and productivity. Therefore, if the factors responsible for drought tolerance can be screened individually in shoot and root which is easier to do, the components leading to drought tolerance will appear less complex and may be easy to manipulate by breeders. This paper presents an example of successful breeding for drought tolerance in cowpea by separately screening for relevant traits in shoot and root using simple methods under controlled conditions and combining them together in promising varieties.