Field changes in germination requirements: effect of natural rainfall on potential germination speed and light requirement of Stylosanthes humilis, Stylosanthes hamata and Digitaria ciliaris

Abstract

The effect of isolated rainfall events on germination speed and light requirement was studied for different seed populations of Stylosanthes humilis, Stylosanthes hamata and Digitaria ciliaris. Initial germination, measured as the proportion of seeds germinating within 12 or 18 h under optimal laboratory conditions, was an important determinant of field germination where surface soil moisture is available for only short durations (12-18 h). Seeds were placed on the soil surface prior to rainfall, usually in the late afternoon, were resampled the following day when dry, and then stored in the laboratory before standard germination tests. The seed populations were exposed to 20 different rainfall events. Compared with unexposed seeds the seeds of all species germinated faster after exposure to rainfall, even a few mm, provided the rainfall was insufficient to cause field germination, e.g. germination of S. humilis increased from 46 to 70% in 12 h. Field germination was common in S. humilis, but little field germination of S. hamata and D. ciliaris occurred because of their slow germination. Initial germination of resampled seed was correlated with duration of surface moisture, whether this was received over one or two rainfall cycles. Seeds of S. hamata and S. humilis with the greatest increases in initial germination had a shorter imbibition time. The light requirement for rapid germination of S. humilis was not changed by rainfall, but the light-requiring fraction of D. ciliaris was reduced by exposure to rainfall (the reduction was positively correlated with duration of surface moisture) and high temperature (>50¦C).