Plant root excretions in relation to the rhizosphere effect

Summary

The root exudates from seedlings of ten plant species grown under conditions of controlled environment and nutrition were biassayed for six vitamins of the B-group. Biotin was consistently present in the exudates in amounts sufficient to influence the growth of rhizosphere micro-organisms. Pantothenate and niacin were generally present, but usually at low levels unlikely to influence the microflora; riboflavin and thiamine were occasionally found in traces; pyridoxine was not detected in any root exudate.

The vitamin content of the exudate varied with plant species. Field pea released large quantities of biotin, pantothenate, and niacin, but other plants including legumes, produced exudates medium to low in vitamin content and varying in relative amounts of each. Subterranean clover produced moderate amounts of vitamins, and from seed samples of graded size exuded vitamins in quantities unrelated to seed size. A comparison of five species of clover showed distinct differences in patterns of exudation in closely related plant species.

Raising temperature and reducing light intensity by shading, produced only small effects upon vitamin exudation. Improved nutrient status produced marked increases in plant growth, but only small increases in amount of vitamin exuded, with pantothenate an exception tending to be released in greater amounts under unfavourable growing conditions. The presence of a root microflora caused sharp reduction in vitamin concentration of the culture solution.