Volume 27, 276

Tropical Grasslands (1993) Volume 27, 276–290

Tropical pasture establishment.
2. Seed characteristics and field establishment

J.M. HOPKINSON

QDPI, Walkamin Research Station, Walkamin, Queensland, Australia

Abstract

The relationships between measured properties of tropical pasture seeds and their ability to produce emerged seedling populations in different seedbed environments are reviewed. Experimental records of the behaviour of the grasses, green and Gatton panic (Panicum maximum), and the legume, Seca stylo (Stylosanthes scabra), are then examined in attempts to elucidate further the principal interrelationships among seed properties and seedbed factors that influence seedling emergence.
Critical seed characteristics were perceived to be vital quality, measured satisfactorily as viability, and the impediments to germination imposed primarily by dormancy in tight-husked panicoid grass seeds and hardseededness in legumes. Experimentally, survival of seeds and seedlings in soil was closely related to vital quality. Dormancy in grass seeds, however, led to complications that hampered prediction of field emergence. Inconsistencies arose because: dormancy diminished with age; threshing damage of variable severity had opposing effects of breaking dormancy but shortening life expectancy; and a level of dormancy existed that failed to prevent germination but prejudiced seedling survival in conditions of stress in the field. A further variable was imposed by differences in the course of germination of seed populations, patterns of which tended to be species-specific and led to widely differing risks of seedling death from desiccation during a field germination event.
Seedling emergence from Seca seed-in-hull had a close, positive, linear relationship with laboratory germination in conditions that precluded further seed softening. Scarified, naked seed had high absolute emergence levels but relatively high seedling mortality in soil, apparently from pathogen invasion of tissue damaged by scarification. The course of germination of populations of soft seeds depended on the route of water penetration, either immediate through fractures, or slow and staggered through the strophiole. By inference, scarified and strophiole-softened seed would have markedly different survival patterns under different sequences of available soil moisture.