A Comparison of Photosynthesis in Individual Pine Needles and Entire Seedlings at Various Light Intensities

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Abstract
The rates of apparent photosynthesis of individual, fully exposed loblolly pine {Pinus taeda) needles were measured at various light intensities and a constant temp. of 25[degree]C. Photosynthesis was detd. by measuring the difference in CO2 conc. of the air before and after passing over the needles. CO2 conc. of the air was detd. by measuring the change in conductivity of alkali in absorption towers. Fully exposed pine needles reached their max. rate of photosynthesis at about 1/3 of full sunlight and showed no further increase in rate up to the highest light intensity to which they were exposed, 9200 f.c. They behaved in essentially the same manner as the hardwood seedlings investigated by Kramer and Decker, but differed from entire pine seedlings which attained their max. rate of photosynthesis at max. light intensity (9300 f.c.) and reached only 60% of the max. photosynthesis at a light intensity equal to 1/3 of full sun. It is believed that the relatively lower rate of photosynthesis of entire pine seedlings in the shade results principally from mutual shading of the needles by one another. Because of the arrangement of the needles, there is much more mutual shading in pine than in hardwood seedlings.