A study of seed vigour in kale (Brassica oleracea L. var. acephala DC.)

Abstract

The present study investigated variables for the accelerated ageing (AA) test for kale, the effect of seed development stage and post-cutting environment, sowing date, pod position and seed size on kale seed vigour, and the relationship between laboratory test results and field performance of kale seed. These parameters were divided into four different experiments. In the first experiment, six seed lots of kale with germinations above 90% were aged at three different temperatures (40, 41, 42°C ± 0.3 °C) and two ageing times (48, 72 h ± 15 minutes). In experiment 2 the effects of seed development stage, sowing date, pod position and the environment pre and post-cutting on kale seed vigour were determined. Also, the effect of storage on seed quality was determined after 12 months storage at room temperature. In experiment 3, six seed lots were size graded into four categories, large (retained on a 2.36 mm screen), medium (retained on a 2.0 mm screen), small (retained on a 1.5 mm screen) and very small (passed through a 1.5 mm screen) and quality was evaluated. Finally, in experiment 4, the relationships between laboratory tests for seventeen seed lots of cultivars Kestrel and Gruner and field emergence/cold room emergence were evaluated. In experiment 1, both temperature and time had a significant influence on kale seed germination after AA. Increasing the ageing time from 48 h to 72 h decreased mean germination by 28, 34 and 76% at 40, 41 and 42°C respectively, while an increase in temperature from 41 °C (the recommended temperature for Brassica spp) to 42°C reduced mean germination results by 50% and 80% in the 48 hand 72 h treatments respectively. Testing kale seed at 40 °C/48 h and 41 °C/48 h allowed a reasonable ranking of vigour results, but for a practical reason, AA testing at 41 °C/48 h is recommended. In experiment 2, the crop reached physiological maturity (PM) at 38% SMC, while maximum seed vigour was attained 2-3 weeks later. Continual exposure for a longer time (i.e. more than 20 d after cutting) to inclement field conditions in the swath reduced vigour because of seed ageing. The study suggests that the kale crop should be cut at 38% SMC and combined within two weeks after cutting, when SMC is between 12-15%, then seed dried to 8% for safe storage. A difference in sowing time of two weeks produced only a small reduction in mean germination, but had no significant effect on vigour. Seeds from middle pods had superior vigour than top pods, but there was no significant difference in germination. Also, storing kale seed for one year at room temperature did not affect vigour of high quality kale seed. In experiment 3, the medium sized seeds generally had better vigour than other seed sizes, although medium and small seeds did not always differ. However as medium and small sized seeds made up around 90% of the seed lots, this suggests that it may be difficult to improve seed lot quality through seed grading. In experiment 4, when seed quality data for all seed lots were correlated with field and controlled temperature emergence, germination was usually more strongly related to emergence than vigour test results. However while removing two low germinating seed lots from the analysis changed this result, neither germination nor vigour was correlated with field emergence. When data for the cultivars were analysed separately, vigour results for cv. Gruner had a stronger relationship with emergence than did germination, but neither vigour nor germination results were correlated with emergence for cv. Kestrel.