Frost avoidance and frost resistance in Centrosema virginianum.
Accessions of C. virginianum were tested for response to frosting in the field and in controlled environments. Accessions differed in their ability to survive mild field frosts; these differences were correlated with latitude of origin. Differences in leaf frost resistance in the field, in the early winter, were not closely correlated with plant survival at the end of the winter. Marked differences were observed between accessions in ability to survive controlled frost of -1 to -7 deg C (at 5 cm height). These differences were closely correlated with winter survival in the field, and were associated with variation in the height of the lowest potential growing points (the axillary buds at the cotyledon node). There were only small differences between accessions in leaf frost resistance and in ability to survive freezing at the cotyledon node. It was concluded that differences in ability to survive frost were primarily due to frost avoidance rather than frost tolerance. The height of the cotyledon node was negatively correlated with both latitude of origin and winter survival in the field. It is suggested that cotyledon node height (or the underlying causal character, hypocotyl length) has adaptive significance in C. virginianum. Environmental factors (e.g. temp., sowing depth) affected the height of the cotyledon node, but the relative ranking of accessions remained constant. Cotyledon node height is under strong genetic control; a heritability of 0.79 was recorded in a diallel cross of 6 contrasting accessions. A hardening regime, similar to that experienced by plants in the field, did not improve leaf resistance to frost or improve plant survival in controlled environments. Exposure to the hardening regime after frosting led, in some cases, to increased plant mortality.