ONION (ALLIUM-CEPA L) SEEDLING EMERGENCE PATTERNS CAN BE EXPLAINED BY THE INFLUENCE OF SOIL-TEMPERATURE AND WATER POTENTIAL ON SEED-GERMINATION

A theory is outlined that seedling emergence patterns in the field are largely generated by the influence of soil water potential on a critical event (i.e. the initiation of radicle growth) that determines the base water potential for germination. Above this base the mean rate and distribution of seedling emergence are largely determined by temperature. This proposal was expressed as a parsimonious stochastic model to test whether patterns of seedling emergence recorded in field experiments could be explained. The model applies current understanding of the influence of water potential and thermal time on germination under controlled conditions to the variable environment of horticultural seed beds. Where possible. parameter values were taken from the literature, others were inferred from laboratory experiments on seed germination and seedling emergence. The model accurately predicts the onset of seedling emergence and the occurrence and timing of subsequent seedling flushes in a wide range of seed bed conditions. The study indicates that a co-ordinated use of existing seed-bed preparation techniques to retain soil moisture for initial seed imbibition and limited irrigation, timed to coincide with the critical period when radicle growth is initiated in the seed population, may provide an approach to improving crop uniformity that is simple and practical.