Seed Germination Performance and Seed Coat Mucilage Production of Sweet Basil (Ocimum basilicum L.)

Abstract

Sweet basil (Ocimum basilicum L.) is a warm season herb usually propagated from seeds. Establishment of basil is difficult as seed germination may be limited, particularly during field seeding at cold soil temperatures. The germination of six cultivars (\'Italian Large Leaf\', \'Italian Large Leaf\' 35X, \'Nufar\', \'Genovese\', \'Genovese Compact Improved\' and \'Aroma 2\') of sweet basil seeds were tested on a one dimensional thermo-gradient table over temperatures ranging from 0 to 50"C. At temperatures below 20"C, germination among cultivars was more variable and the mean time to germination (MTG) increased to greater than 25 days for some cultivars. Germination declined sharply and had a sudden termination at high temperatures above 40"C for all six cultivars. There were statistical differences among the cultivar base temperatures, which ranged between 10.1 and 13.3"C. The optimal and ceiling temperatures for germination were similar and did not differ statistically among the cultivars compared in this study. The average optimal temperature for all cultivars was 35 ± 0"C, while the average ceiling temperature was 43 ± 1.3"C. Stored seeds (> 5 years) had lower seed vigor and lower germination percentage, also lower ceiling temperature compared with the fresh seeds of the same cultivar (\'Italian Large Leaf\'), but the base temperatures were the same for both new and old seeds.

Sweet basil (Ocimum basilicum L.) seeds produce a thick layer of mucilage around the pericarp within minutes after hydration. Mucilage is most prevalent among plant species adapted to surviving in arid sandy soils, though its significance in determining ecological fitness is unclear. The mucilage produced by seeds is reported to be composed of cell-wall polysaccharides that are deposited in testa pericarp cells during development. In this study, sweet basil seeds were examined using light and environmental scanning electron microscopy. The mucilage of basil seeds is held together by columnar structures that unfolded from the pericarp and helped hold and stabilize the mucilage to the outer surface. The mucilage was removed using diluted hydrochloric acid to compare performance of seeds with and without mucilage. Mucilage removal did not inhibit seed germination under ideal laboratory conditions but decreased the water content of seeds significantly. The water content of intact seeds was almost 4 times greater than seeds without mucilage. Mucilage enabled seeds cling to an incline board set to a steeper angle than seeds without mucilage. The fully hydrated seeds approached zero water potential, so the mucilage did not prevent seeds from fully hydrating. Soil (media) germination testing showed the seeds with mucilage had higher germination percentage than the seed without mucilage on several different types of media. Seeds with mucilage also had higher survival percentages after 10 days on different types of media. Basil seeds mucilage acts as a reservoir to hold loosely bound water at high water potential so it is available for seed germination and early seedling development.