Ecophysiological studies on Sonoran Desert plants,VI. Seasonal photosynthesis and production of Machaeranthera gracilis,a winter ephemeral*

Abstract. A comparative study of two chromosomal races of the winter-active ephemeral Machaeranthera gracilis showed that the seasonal magnitudes of photosynthesis were only slightly greater for a progeny desert race than for an ancestral foothills race. Maximum observed photosynthetic capacity and the seasonal reduction in foliar photosynthesis occurred earlier in the year for the desert race. The relative growth rate was higher in this race up until the time of its peak seasonal biomass. A ratio of harvested net production to estimated gross primary production decreased until anthesis. Photosynthesis contributing to net growth continued into periods with moderate environmental stress. The continuation of growth by the desert race was enhanced by maintenance of a higher root-shoot ratio, as well as greater relative stem growth. During reproduction, foliar CO₂ assimilation could not solely provide the measured dry matter accumulation, suggesting the importance of assimilate contribution by photosynthetic stems. Seasonal increases in the enthalpy content of whole plants and plant organs occurred for both races, indicating the absence of significant translocation during reproduction and the potential for stem photosynthesis.