The comparative drought resistance of landraces of sorghum and millet from dry and humid regions

It may be that land-races of sorghum (Sorghum sp.) and millet [Pennisetum americanum (L.) Leeke] which evolved along geographical gradients of rainfall in Africa and India, differ in their drought resistance. Any physiological attributes found to be correlated with low rainfall might be important and effective characteristics for crop production in dry regions. Twenty land-races were chosen which evolved along geographical gradients of rainfall, seven millets from India, six sorghums from Mali, and seven sorghums from the Sudan. Races were evaluated for their growth potential and plant water relations under hydroponics conditions in a growth chamber. A water stress treatment was imposed by adding polyethylene glycol-8000 to the nutrient solution, giving a solute water potential of -0-5 MPa, compared with a control solution at 003 MPa. Drought resistance, in terms of relatively less growth inhibition under stress, was higher in races from dry regions than in races from humid regions. Of all the physiological variables measured [carbon exchange rate, (CER), transpiration, transpiration ratio (CER/transpiration), leaf diffusive resistance, leaf water potential and osmotic adjustment], only osmotic adjustment under stress was generally correlated with average rainfall at each race's origin, indicating greater osmotic adjustment in land-races from drier regions. Races with a greater capacity for osmotic adjustment were characterized by smaller plants with high rates of transpiration and low rates of leaf senescence under stress. The carbon exchange rate per unit leaf area increased as live leaf area decreased under stress due to leaf senescence. Thus, drought resistant races under stress tended to have lower CER per unit live leaf area (but not per plant) than susceptible races. Transpiration ratios under stress were lower in resistant than in susceptible races, mainly because resistant races had higher transpiration. The results for the measured variables showed a general trend for greater drought resistance in sorghum than in millet, indicating that the commonly observed adapation of the millets to dry environments may be due to other factors, such as drought escape or heat tolerance.