DIFFERENTIAL RESPONSE OF CITRUS ROOTSTOCKS TO ALUMINUM LEVELS IN NUTRIENT SOLUTIONS .1. PLANT-GROWTH

The objectives of this study were to determine and compare the Al tolerance of selected citrus rootstocks. Six-month-old seedlings of five citrus rootstocks were grown for 60 days in nutrient solutions. The solutions contained 7 levels of Al ranging from 4 to 1655-mu-M and similar P concentration of 28-mu-M. The nutrient solution pH was maintained at 4.0 +/- 0.1 and the temperature at 25 +/- 1-degrees-C. At high Al treatment levels, plants had thickened root tips and root caps covered with black gelatinous material. At high levels of Al treatments, seedlings of some rootstocks had yellow, mottled, and withered new leaves near end of experiment. New-growth root lengths and shoot height responded differently to Al concentrations in the nutrient solution. New-growth fresh weight of whole plants appeared to be the most sensitive indicator of Al tolerance. Based on response of fresh weight of whole plants to Al concentrations, relative Al tolerances of the rootstocks were Cleopatra mandarin > rough lemon > sour orange > Swingle citramelo > Carrizo citrange. The neutral or dividing Al concentrations in solution between beneficial and toxic effects were 371, 193, 189, 178, and < 100-mu-M Al, respectively, for the above rootstocks. Concentrations below or above the neutral Al levels caused either beneficial or toxic effects, respectively. The apparent optimum Al concentrations for the growth of whole plants were 163, 93, 89, 85, and < 50-mu-M, respectively.