Absorption Of Radiant Energy By Leaves

Leaves of common plants absorb 80—95% of the blue (400—500 mμ, 60—80% of the green (500—600 mμ, and 80—90% of the red (600—700 mμ. Absorption in the infrared is about 5% at 800 to 1,200 mμ and increases to near 100% beyond, 3,000 m(μ) or 3 μ. Absorption at 550 mμ by equal quantities of leaf pigments was two to six times greater for intact leaves than for methanol solutions. Shifts of the absorption bands toward shorter wavelengths, which are characteristic of chlorophyll solutions, were not observed in isolated whole or sonically disintegrated chloroplasts. Leaves in sunlight are heated from a few degrees above air temperature for thin leaves to 30 degrees C or more for very thick leaves before reaching a steady temperature. The heating and cooling curves shown by wilted leaves were not significantly different from those of transpiring leaves. Dried leaves heated less and cooled faster than normal, transpiring leaves. Leaves in sunshine were cooled quickly toward air temperature by wind at 5 mph. It is concluded that leaves tend to assume the temperature of the surrounding air; that they are heated rapidly by radiant energy, and cooled primarily by conduction of energy to the air.