knna
Member
so do people deny that this is the chlorophyll absorbtions spectra?
No, that is one graph of chlorophill's light absorbance, on a given solution. Graph varies a little depending on the solvent used, but give or take, shape is very similar.
What we say, is that chlorophill's absorbance on light on lab conditions cant be used to know what spectrum of light works better or what wavebands have more efficacy promoting photosynthesis.
Why we say that?:
1)Pure empirical checking. So its proof, not an opinion. Photosynthesis is a process revealed by CO2 uptake or O2 release by plants, that are mensurable. Just by exposing a plant to different wavelenghts and measuring CO2/O2 evolution, you can see that photosynthesis vs wavelenght response has little to do with chlorophill's absorbance in a solution (lab).
These measurements has been taken very carefully thousand of times, for several plant's species, by botanists. All have a similar profile, averaged on Mc Cree or Inada photosynthesis-wavelenght graphs. (McCree expresing photosynthesis of absorbed light, Inada showing photosynthesis for incident light). Those curves have been measured again for other species and particular strains on agronomic studies, all confirming it along the years.
2)Theoretical analysis. Just to understand empirical results, it is worth to know why chlorophills absorbance in lab cant be used as a guide for photosynthetic efficiency:
-One thing is light absortion, and other, light use. Of course, you need light get absorbed to be used. But once absorbed, it may be dissipated as heat (unuseless for photosynthesis), or used for photosynthesis at different efficacy depending of wavelenght.
-Absortion of light by chorophills in vivo plants is different of absortion of the pigment alone in a solution, that is how those chlorophill's absortion graphs are calculated. Light absortion is a quantum process, strongly affected by spatial orientation. Plants transport energy obtained by a photon absorbed on a chain of electron jumps, resulting on a estratification of chlorophill pigments into the chlorophill's molecule, with different optimal wavelenght absortion for each. End result is chlorophils in vivo being able to absorb along a way wider range than on lab conditions, and at different efficacies.
In fact, on most plant's leaves there is a estratification on clorophills layers into a single leaf, with upper layers absorbing better the red, under it absorbing better the blue and in the center of the leaf, absorbing better the green. Plants change leaf's morphology to use better the light they receive, adapting to both intensity and spectrum by changing the thickness and concentration of chlorophills on those layers. For example, a plant grown under HPS light usually have a ratio chlorophill a/b 4 times lower that those grown under sunlight, because they have a double concentration of chl b but half of a.
Its a process that can be observed by eye, as plants receiving for example, strong green light, develop thicker but smaller leaves than those receiving red light, and more if its a weak red light.
-Chlorophills are not the only light absorbing pigments of plants. There are others, less important, but that works too absorbing a decent percentage of the total light absorbed. Main ones are carotenoids.