Seedling growth characteristics of Stylosanthes

Abstract

Twenty-four lines of annual (S. humilis, S. subsericea, S. sundaica and S. hamata) and perennial (S. viscosa, S. scabra and S. fruticosa) species of Stylosanthes were grown for a 6 week period after sowing, and sampled weekly to determine whether differences in seedling dry weight, earlier exhibited by single members of each species in the field, were (1) representative of each species, and (2) attributable to differences in seed size, relative growth rate (RGR) or distribution of dry matter between tops and roots. Seedlings of all lines of the perennial species had much lower dry weights than the annual species. This was due largely to the low RGR values of the perennials, and to low initial photosynthetic areas of the extremely small-seeded (< 1.3 mg) lines of S. viscosa and S. scabra. Seedling weight at the first harvest was highly correlated with seed weight, embryo weight and cotyledon area regardless of species. With each successive harvest, this correlation was reduced owing to differences in RGR which were independent of seed weight. The poor top growth of the perennial species could not be attributed to increased root growth. In fact, between harvests 1 and 3, the small plants of the perennial species allocated a greater proportion of dry matter to the tops than did the annuals. The perennial species did, however, develop deeper and less-branched root systems than the annuals when both had similar root weights. The results suggest that the perennial species of Stylosanthes may present more problems than the annual species both in establishment and in competition with the vigorous tropical grasses.