Photosynthesis in Climatic Races of Mimulus. II. Effect of Time and CO2 Concentration on Rate

Abstract

Compara-tive measurements were made on 6 climatic races of Mimulus cardinalis whose native habitats range from 31[degree] to 38[degree] N and from 45 to 2220 m elevation. During 12 hours'' continuous photosynthesis at 30[degree] C, saturating light intensity, and 0.04% CO2 the rate fell 10-25% below its maximum. The fall in rate was least for races from low elevations, greatest for those from high altitudes. The photosynthetic rate increased linearly with CO2 concentration to about 0.05%, and more slowly at higher CO2. No increase in rate was observed above 0.11 to 0.15% CO2. Races differed in the response of their rates to CO2 concentration. During continuous photosynthesis with 0.15% CO2 the rate fell much faster than with 0.04%. The total CO2 uptake in 12 hours with 0.15% was only 1.15-1.66 times that with 0.04% CO2. The stimulating effect of high CO2 on the CO2 uptake became less with increase in elevation of the native habitats of the races.