Assessing Viability of Prevariety Germplasm of Native Coreopsis Species

Abstract

Dormancy issues and lack of approved protocols have led to increasing use of the pregermination tetrazolium test to assess whether seeds of premium-priced, prevariety germplasm of native wildflower species are living and have the potential to germinate and develop normally under field conditions, that is, whether seeds are viable. A major concern is the limited amount of industrywide training on how to conduct and interpret results of tetrazolium (TZ) tests for the multitude of native species. Precise methods that yield uniform results are especially critical for testing of prevariety germplasm of native species given their high value and economic consequences of inaccurate test results. In preliminary work, we observed that embryos extracted from intact seeds of native Coreopsis species subjected to TZ testing were viable if they were turgid and appeared normal and were stained pink, red, or were pure white with or without the radicle tip stained pink to red. In this study, we conducted an in-depth analysis of TZ testing of intact seeds of prevariety germplasm of four native Coreopsis species to: 1) verify our preliminary conclusions; and 2) to determine if staining could be improved by preconditioning or gibberellic acid (GA3) because relatively low percentages of embryos of intact seeds stained pink or red. In addition, we evaluated two other methods to assess viability: the excised embryo test and emergence tests in soil and a soilless medium under ambient conditions. We confirmed that embryos extracted from intact seeds subjected to TZ testing were viable provided that embryos were turgid and appeared normal and were stained pink, red, or were pure white ± a pink to red radicle tip. Embryo staining was not consistently improved by moist preconditioning or GA3. Results of pregermination TZ tests of intact seeds and/or germination plus postgermination TZ tests of intact, nongerminated seeds were not consistently accurate and/or uniform across all species and seed lots, issues usually encountered in seed lots that were relatively dormant or of poor quality, and in postgermination TZ testing of seed lots with relatively low dormancy. Moreover, these issues are likely to be encountered even with standard TZ testing protocols given the widely accepted challenges of accurately interpreting TZ tests of native species in general. The most precise, uniform method of assessing viability was the excised embryo germination test because all viable seeds germinated, results were easy to interpret so the likelihood of false-positives or -negatives was nil, and results were very uniform across replications. Emergence tests under ambient conditions substantially underestimated viability. In summary, the high cost of prevariety germplasm seeds of native Coreopsis species, inherent genetic and phenotypic variability, and unknown dormancy characteristics warrant the use of the embryo excision test for determining viability and for increased sample sizes, especially for any tests involving TZ.