Variation in temperature requirements for germination and early seedling root development in Chamaecrista rotundifolia and three allied species

Two experiments are discussed. In the first, rates of root elongation durin g the first four days of growth at 25 degrees C were studied in Chamaecrist a rotundifolia (34 accessions), C. nictitans (2 accessions), C. pilosa (1 a ccession) and C. serpens (3 accessions). In the second, a comparison was ma de of germination characteristics of 14 accessions of C. rotundifolia and o ne of C. pilosa at nine constant temperatures (11-27 degrees C). Entries in cluded accessions from a wide latitudinal range. There were statistically s ignificant differences in root length on all four days of measurement. No a ccession had significantly longer roots than the commercial cultivar cv. Wy nn on day 1, but over the following three days, 4, 10 and 13 accessions had significantly longer roots, respectively. Root length on day 4 was positiv ely correlated with latitude of origin. In the second experiment, maximum g ermination occurred at 25.5-27 degrees C, with progressively lower percenta ges at lower temperatures. There were highly significant differences betwee n accessions at each temperature (P < 0.001), and a highly significant acce ssion x temperature interaction. No accession had a higher germination perc entage than cv. Wynn at any temperature. Percentage mortality differed sign ificantly (P < 0.05) at all temperatures except 27 degrees C. Mortality at low temperature was negatively correlated with latitude of provenance. Incr eased temperatures up to 22.5 degrees C reduced the time taken to reach 50% of maximum germination (T-50), and thereafter it was reasonably constant. With one exception (22.5 degrees C), no accession germinated significantly more quickly than cv. Wynn, and at 13 degrees C cv. Wynn germinated signifi cantly more rapidly than all other entries. Over all temperatures, there wa s a significant negative correlation between days to 50% germination and la titude of provenance. Estimated base temperature for germination (T-b) rang ed from 10.6 degrees C to 13.6 degrees C. T-b was positively correlated wit h latitude of provenance. It is argued that accessions from higher latitude s have developed mechanisms for survival of soft seed at low temperatures, low base germination temperature, germination rates and root elongation rat es that adapt them to an environment with summer-dominant rainfall and wint ers which are too cool to support growth.