Evaluation in Contrasting Climates of Dormancy Characteristics of Seed of Digitaria milanjiana

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Abstract
Seed of 4 ecotypes of the grass D. milanjiana (Rendle) Stapf, previously shown to differ in dormancy characteristics related to climatic origin, was placed on the soil surface in nylon mesh envelopes at three sites with contrasting climates in northern Australia on June 4, 1981. The sites were Katherine (14.degree.28''S; warm dry winter with no rainfall), Narayen (25.degree.41''S; cool winter, 25% of rainfall occurring in winter) and Lansdown (19.degree.40''S; winters intermediate in temperature and expectancy of rainfall). Envelopes were retrieved after 4, 8, 12, 16, 20, 24, 32 and 41 wk and tested for germinability, dormancy and viability. At Narayen no accession had adaptive germination characteristics; the high-rainfall equatorial ecotype germinated in winter when survival would have been improbable; the low rainfall equatorial and low rainfall tropical ecotypes failed to break dormancy by the following wet season. At Lansdown only the low rainfall tropical ecotypes retained dormancy through the dry season and were fully germinable by the start of the wet season. At Katherine the total lack of rain and high tempertures during the dry season resulted in all accessions being live and non-dormant at the start of the wet season. There was some evidence of induced or secondary dormancy in accessions from low rainfall provenances. A laboratory study showed that all accessions had an optimum temperature for germination of about 25.degree. C. Temperatures above 32.5.degree. C resulted in death of continuously moist seed. The results of the field experiment were in accordance with an earlier laboratory experiment in which accessions from low rainfall tropical regions were shown to require extended periods of high temperature to break dormancy. The genetic variation evident in this characteristic in D. milanjiana is considered to have significance in the evolution of climatic adaptation and is also of potential importance in the selection of cultivars for sown pastures in the tropics.

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