INFLUENCE OF ALUMINUM STRESS ON SHOOT AND ROOT-GROWTH OF CONTRASTING GENOTYPES OF COLEUS

Two cultivars of Coleus blumei Benth., found previously to differ in s tress tolerance to UV-B radiation, water stress, and air pollutants-st ress-tolerant 'Marty' ('M') and stress-sensitive 'Buckley Supreme' ('B S')-were grown in a greenhouse at Beltsville, MD in nutrient solutions containing 0 to 24 mg aluminum (Al) L-1 (Experiments I and II) and on an acid Al-toxic Tatum subsoil (clayey, mixed, thermic, Typic Hapludu lt) (Experiment III) at six lime levels, from 0 to 6,000 mu g calcium carbonate (CaCO3) g(-1) (pH 4.47 to 7.30 and percent Al saturations fr om 60 to 0) to characterize their possible genotypic differences in re sponse to Al toxicity. Significant inhibitory effects of Al stress on shoot growth were generally observed in solution culture at 8 mg Al L- 1 or higher, while inhibition of root growth in solution culture was g enerally observed at 16 mg L-1 or higher. At 16 and 24 mg Al L-1 (Expe riment II), the relative dry weights of shoots and roots were reduced to about 60% of the control values in both cultivars. Leaf tissue conc entrations of phosphorus (P) and Al were increased and calcium (Ca) wa s decreased at 24 mg Al L-1. Both cultivars of Coleus were extremely t olerant to Al stress, both in nutrient solution and in an acid Al-toxi c Tatum subsoil. Thus; Coleus appears to be a calcifuge type plant, re sembling azalea in its behavior. The lack of differential sensitivity to Al is in contrast to the responses of these cultivars to other envi ronmental stresses such as drought, UV-B radiation, and air pollutants .