CALCIUM AND TEMPERATURE EFFECTS ON SEEDLING EXUDATION AND ROOT-ROT INFECTION OF COMMON BEAN ON AN ACID SANDY SOIL

Soil born fungi such as Phytium ultimum, Fusarium ssp., and Rhizoctonia solani (Kuhn) severely restrict stand establishment of common bean (Phaseolus vulgaris L.) on acid soils of the Tropics. Calcium application is known to alleviate fungal infection in many legumes but the causes are still unclear. To investigate environmental factors and physiological mechanisms involved, growth chamber experiments were conducted with an acid sandy soil from Mexico. Treatments were soil liming at a rate of 0.67 g Ca(OH)2 kg-1, gypsum application at 0.49 g CaSO4 2H2O kg-1 soil placed around the seed, and an untreated control. Beans were grown under three temperature regimes with constant night and one constant day vs. two sinusoidal day temperatures. To examine patterns of seed and seedling exudation at regular intervals leachates of germinating seeds were collected on filter paper soaked with equilibrium solutions from soils of the three treatments. The severity of root rot in the control treatment was highest when plants were stressed by temperature extremes. At a sinusoidal day temperature peaking at 40-degrees-C soil liming and gypsum application to the seed increased the number of healthy seedlings similarly by over 60%. However, only liming which effectively eliminated growth constraints by low pH and high aluminum concentrations led to an increase in hypocotyl elongation by 22% and in total root length by 8%. Both calcium amendments increased the calcium and potassium contents in the hypocotyl tissue. From seeds exposed to the equilibrium solution of unlimed soil with pH 3.7, 1 mM Ca, and 0.6 mM Al considerable amounts of amino acids and carbohydrates were leached. In contrast, exposure to the equilibrium solution from limed soil with pH 4.3, 3 mM Ca, and negligible concentrations of Al led to a net uptake of amino acids and decreased leaching of carbohydrates. Exposure to the equilibrium solution of the gypsum treatment with pH 3.6, 20 mM Ca, and 1.2 mM Al resulted in a somewhat smaller net uptake of amino acids compared to liming. During germination pH around the seeds steeply increased in the untreated control but significantly less with both amendments. The results indicate that pH and the Ca/Al ratio in the soil solution around bean seeds determine their pattern of exudation and solute uptake. For bean germination and early growth on acid soils locally placed application of small amounts of gypsum as seed pelleting seems as effective as soil liming in reducing the incidence of root rot. The results indicate that this may be accomplished by decreasing the amount of leachates available for fungal development.