Effects of Soybean Seed Coat Cracks on Seed Exudation and Seedling Quality in Soil Infested with Pythium ultimum

Abstract

Nonblemished soybean [Glycine max] seeds with intact seed coats exuded 5.3 .mu.g glucose equivalents/h per seed when buried in glass beads and leached by dripping distilled water at 3 ml/h for 24 h. Anthrone-positive leachates of 17.5 .mu.g glucose equivalents/h per seed were collected after soaking nonblemished seeds in 1 ml water for 3 h. Exudation from soaking seeds was not influenced by cracks in the hypodermal seed coat layer, but scarifying through the remaining layers increased exudation to 217 .mu.g/h. Exudation of ninhydrin-positive substances from leached intact seeds was 1.7 .mu.g glycine equivalents/h per seed. This was increased 2-fold by soaking and an additional 5-fold by scarification. Germination of washed sporangia of P. ultimum was low in distilled water or in a sucrose-asparagine solution containing 1 .mu.g of C/ml but increased to 48% or more with 5 .mu.g or more C/ml. Spermosphere effects could be detected up to 5 mm from intact and 7.5 mm from scarified seeds after 5 h in soil at 24 .+-. 1.degree. C and -0.3 bar matric water potential. Lower quality seedlings resulted from scarified seeds than from seeds with intact or cracked seed coats when planted in saturated soil or soil infested with sporangia of P. ultimum (130 propagules/g dry soil) at 15.degree. C, but addition of sucrose (12.7 mg) to each seed had no added effect. A high population of P. ultimum at 24.degree. C reduced quality of seedlings produced from both intact and scarified seeds to the same level. Apparently adequate sugar is exuded from nondamaged soybean seed for optimum development of P. ultimum and scarification effects on seedling quality are not the result of additional stimulation of P. ultimum by exudates.