Pasture seed dynamics in a dry monsoonal climate,II The effect of water availability,light and temperature on germination speed and seedling survival of Stylosanthes humilis and Digitaria ciliaris

Environmental factors affecting speed of germination and survival of naturally occurring seed of the legume Stylosanthes humilis and the grass Digitaria ciliaris were studied. Previous studies on the seedbed environment had shown that germination speed, defined as the proportion of seeds capable of germinating in the first 12 h period, was likely to be the most relevant laboratory measurement. Ten studies were conducted to compare species performance and to devise a standard technique for the measurement of germination speed under laboratory conditions. For S. humilis, the level and duration of submergence in water in the germination test had a large effect on germination speed (from 0–70% in 12 h). The absence of light delayed germination (7% in 12 h compared to 52% under light), but did not prevent complete germination over 7 days. Optimum temperature was 25°C with a decrease to 0% at 30°C. A standard technique was developed for measuring potential germination speed which gave reproducible results allowing differences in seed lots to be described. D. ciliaris seeds sampled just prior to field germination in the early storm period were less sensitive to environmental control. Temperature optimum for germination decreased from 40 to 30°C with the duration of the germination test. Seeds sampled at the time of seed dispersal (early dry season) had a high fraction (80%) requiring light for germination but this requirement had been lost by the early storm period. Investigations on the role of leaching (water movement) showed that long periods of water movement (12–18 h) increased the speed of germination of both species and in the case of S. humilis overcame environmental blocks to rapid germination (absence of light or 30°C temperature). Root elongation rates in S. humilis were not greatly affected by temperature over the range of seedbed temperatures when moisture is available (25–40°C) in the field. However, D. ciliaris was delayed in development at 25°C, the most likely overnight surface soil temperature. D. ciliaris was more susceptible to death by desiccation once germination had occurred compared to S. humilis. In both species seedling death due to desiccation increased with stage of development. The better relative performance of S. humilis in the germination-establishment phase of the life cycle, which had been observed in a previous study, could be explained by a qualitative comparison of the species' attributes. This suggested that D. ciliaris was more likely to germinate at rainfall events following which there was a greater chance of seedling death