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.