PLANT METABOLISM 



209 



A strong absorption of visible h'ght by organic compounds is asso- 

 ciated with extensive conjugation (alternating single and double 

 bonds) in the molecules. Ihe intensity and wave length of the absorp- 

 tion are determined by the extent of the conjugation and the types 

 of atoms involved, some producing a greater effect than others. As 

 indicated by the structures for chlorophyll a and /3-carotene, the photo- 

 synthetic pigments are highly conjugated, making them quite efficient 

 for the absorption of light. 



High efficiency is necessary to permit plant growth and even survival 

 in rainy climates, in shaded locations, or during protracted cloudy 

 weather. On the other hand, prolonged intense illumination may 

 provide so much energy that utilization lags behind the supply. Serious 

 injury could result unless some safety mechanism is provided. In 

 plants photosynthesis begins at low light intensities and increases with 

 the intensity to a limiting value (Figure 9-1). Absorption of light 

 beyond the break in the curve seems to have little effect at moderate 

 temperatures. Apparently the chlorophylls are protected somehow 

 from photochemical destruction. One theory attributes a protective 

 role to the carotenoids, but the problem is by no means solved. 

 Photosynthesizing surfaces are normally at temperatures above that 

 of the environment. When the external temperature rises much 

 above 40°C., leaf temperatures in bright light rise still more, photo- 

 synthesis slows down or stops, and leaf damage or death of the plant 

 may occur. However, these effects are attributed to excessive tempera- 

 tures rather than an excess of visible light, since prolonged exposure to 

 brilliant sunshine is not destructive at moderate environmental 

 temperatures. 



Light intensity- 



FIGURE 9-1. The eUcct of li,L;ht intciisily on the rate of photosynthesis. 



