ALLOCATION, GROWTH RATES AND GAS EXCHANGE IN SEEDLINGS OF STEPHANOMERIA EXIGUA SSP. CORONARIA AND ITS RECENT DERIVATIVE S. MALHEURENSIS

The annual diploid plant species Stephanomeria malheurensis is known only from a single locality where it grows interspersed with its progenitor S. exigua ssp. coronaria. S. malheurensis is thought to have originated recently at its present site and, consequently, serves as a model to study how the phenotype of a species is assembled following its origin. The following were examined in the present study: dry matter allocation, growth parameters, net photosynthesis, and dark respiration of seedlings under three temperatures in order to identify specific features of the new species that might relate to its ecological properties. Under each test condition, S. malheurensis had leaves with a significantly smaller surface area, greater dry weight per unit area, heavier roots, and a higher root/shoot ratio. Both species had similar relative growth rates during the measured period of rosette growth, but in S. malheurensis this resulted from the combination of a lower leaf area ratio and a higher net assimilation rate, whereas in ssp. coronaria it resulted from the opposite combination. The mean rates of apparent net photosynthesis and dark respiration were similar in both species. The differences between the species may reflect the establishment of genes in S. malheurensis which still are present in ssp. coronaria. The implications of this and the possible ecological significance are discussed. THE ONLY KNOWN habitat of the diploid annual plant Stephanomeria malheurensis is a single locality in eastern Oregon where it grows side by side with its progenitor S. exigua ssp. coronaria (Gottlieb, 1973, 1978). Electrophoretic evidence suggested that the genome of the predominantly self-pollinating S. malheurensis was extracted from the richly polymorphic gene pool of the outcrossing ssp. coronaria, since the S. malheurensis alleles which have been identified are nearly all present in ssp. coronaria and comprise less than half of the number of parental alleles (Gottlieb, 1973, 1976). The electrophoretic results were supported by an analysis of 33 quantitative characters which revealed that S. malheurensis has narrower ranges of phenotypic variability (Gottlieb, 1977). Stephanomeria malheurensis probably descends from an inbreeding lineage which formed in the midst of the ssp. coronaria population following the survival of individuals which were able to self-pollinate after a mutation at the selfincompatibility locus. Reproductive isolation of S. malheurensis is apparently provided by the change in breeding system, which restricts pollen exchange between the species, by a chromosomal rearrangement, and by a partial block in cross-compatibility that reduces seed set following interspecific pollination. 1 Received for publication 3 April 1978; revision accepted 11 July 1978. In their shared natural habitat, the number of individuals of S. malheurensis is only about 2 to 3% of that of ssp. coronaria. The two species cannot be distinguished with certainty until flowering and seed set have begun since differences in their seed weights (those of S. malheurensis are two times heavier than those of ssp. coronaria) serve as the most reliable taxonomic character. However, when the species were grown under uniform conditions, they were found to differ markedly in growth patterns. The seedling leaf rosettes of S. malheurensis initially increased in total leaf length more rapidly than those of ssp. coronaria, probably aided by the greater reserves stored in its larger cotyledons (Gottlieb, 1977). By six weeks of age, however, the ssp. coronaria rosettes had achieved a faster rate of increase and went on to bolt first (Gottlieb, 1977). Although they bolted from smaller rosettes, the ssp. coronaria seedlings produced stems having the same height as those of S. malheurensis, yet they had significantly more and longer branches with twice as many florets (Gottlieb, 1977). Since the species are biotically sympatric, their numbers may be regulated by different ecological factors. With annual species these factors often have to do with requirements for seed germination and seedling establishment. From this standpoint, the developmental differences between S. malheurensis and ssp. coronaria may have been selected because they permit them to