Nutrient Supply and Mycorrhiza Development in Caribbean Pine

Changes are reported in mycorrhiza development of Pinus caribaea Mor. seedlings, following the addition of nutrient salts; the plants were raised in the green-house in topsoil collected from a young pine plantation. Problems of assessing mycorrhiza frequency are discussed, and samplingprocedures described. Treatments were compared on the basis of (a) the number of infected seedlings expressed as a percentage of the total number of seedlings per pot, and/or (b) "mycorrhiza percent," defined as the number of mycorrhizae expressed as a percentage of the total number of shortroots per seedling. The presence of dichotomous branching or forking of the short roots was taken as the criterion of mycorrhizal infection. Mycorrhiza development was reduced by increasing the level of soil nitrate, either by adding ammonium nitrate or by liming, but was unaffected by changein soil reaction per se. An inverse, nonlinear relationship was found to exist between the total nitrogen content of seedling roots and mycorrhiza percent (multiple correlation coefficient R = 0.81). The addition of sodium dihydrogen phosphate lowered the percentage of nitrogen in the roots,and increased mycorrhiza development. A test was made of Björkman's hypothesis that the formation of mycorrhiza depends on soluble carbohydrate being present in the roots. Mycorrhiza percent was correlated with the ratio of soluble carbohydrate (percentage reducing sugars) to total nitrogen(r = 0.72) but not with the level of soluble carbohydrate alone (r = 0.13).