STUDIES ON THE GROWTH OF EXCISED ROOTS

Abstract

SummaryExcised tomato roots supplied their nitrogen as nitrate in media of pH 4–5 or pH 7 have an essential requirement for molybdenum. Their growth is markedly stimulated and their continued culture made possible by the addition of 0.000005 p.p.m. Mo. The minimum addition which fully satisfies the molybdenum requirement is 0.001 p.p.m. The depression of growth resulting from a deficiency of molybdenum is not due to the toxicity of accumulating nitrate.Excised roots supplied their nitrogen as nitrite, ammonium, urea, casein hydrolysate or as a mixture of L(+)— arginine and glycine are capable of growth in molybdenum‐omitted medium. Their growth is stimulated by addition of molybdenum and a similar addition is required for optimum growth whether nitrogen is supplied as nitrate or as ammonium.The tolerance of excised tomato roots to excess manganese at pH 7.0 is greater in ammonium than in nitrate media. The optimum manganese addition is increased by supraoptimal concentrations of ammonium. Optimum growth is only achieved when the medium has a satisfactory magnesium: manganese ratio. The deleterious effects of excess manganese can be counteracted by increasing the concentration of magnesium. At low manganese concentration, growth is depressed by enhancing the magnesium concentration.Using a medium buffered with calcium carbonate (pH 7) and with a manganese addition of 0.01 p.p.m. the continuous culture of excised tomato roots supplied their nitrogen solely as ammonium has been achieved. No improvement of the very poor growth occurring in ammonium media at pH 4–5 has been achieved by altering the absolute and relative concentrations of a number of macronutrient elements.