Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green

Abstract

The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosynthetic capacity profile. In strong white light, therefore, the quantum yield of photosynthesis would be lower in the upper chloroplasts, located near the illuminated surface, than that in the lower chloroplasts. Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light. Based on the assessment of effects of the additional monochromatic light on leaf photosynthesis, we developed the differential quantum yield method that quantifies efficiency of any monochromatic light in white light. Application of this method to sunflower leaves clearly showed that, in moderate to strong white light, green light drove photosynthesis more effectively than red light. The green leaf should have a considerable volume of chloroplasts to accommodate the inefficient carboxylation enzyme, Rubisco, and deliver appropriate light to all the chloroplasts. By using chlorophylls that absorb green light weakly, modifying mesophyll structure and adjusting the Rubisco/chlorophyll ratio, the leaf appears to satisfy two somewhat conflicting requirements: to increase the absorptance of photosynthetically active radiation, and to drive photosynthesis efficiently in all the chloroplasts. We also discuss some serious problems that are caused by neglecting these intra-leaf profiles when estimating whole leaf electron transport rates and assessing photoinhibition by fluorescence techniques.

Optical Property Analyses of Plant Cells for Adaptive Optics Microscopy

“…[19,20] However, strong refraction also impairs the fine imaging of live plant cells and tissues, which has impeded research in plants. Here, we analyzed the optical properties of Physcomitrella leaf cells as a model for plant cells, using bright field and phase contrast microscopes, and used fluorescent beads to examine the image degradations caused by plant cellular structures.…”

“…Plant cells show stronger light refraction and scattering than animal cells, [19,20] and, therefore, live images of plant tissues tend to be degraded, even with a few layers of the cells. [21,22] Many epidermal cells with a planoconvex shape function as lens to focus light on chloroplasts in the subadjacent mesophyll cells, most likely to increase the efficiency of photosynthesis, and cylindrical rhizoid cells of bryophytes also refract light as lenses.…”

“…[21,22] Many epidermal cells with a planoconvex shape function as lens to focus light on chloroplasts in the subadjacent mesophyll cells, most likely to increase the efficiency of photosynthesis, and cylindrical rhizoid cells of bryophytes also refract light as lenses. [19,23,24] These lens effects appear to be caused by the plant cell wall; the cell wall has a refractive index of 1.41 À 1.52 in flowering plants, [20,[25][26][27] a value higher than the refractive indices of medium (n ¼ 1.33) and cytosol (n ¼ 1.35 À 1.39, estimated from that in animal cells). [28,29] These results indicate that the cell wall is expected to cause degradation in the images of living plant cells.…”

Certain Biominerals in Leaves Function as Light Scatterers

“…This is similar to the well known 'sieve effect' caused by chloroplasts at the cellular level. [28] Because the loss under very dim light is 5% of very slow photosynthetic activity, whereas the gain under stronger light regimes is 5% of fully active photosynthesis, changing illumination conditions as well as changes in leaf orientation are not important as long as the leaf, on average, is illuminated by a reasonable flux. It is impossible to quantify from our data the net photosynthetic gain for the tissue, but unless extremely dim light conditions persist throughout long periods, the overall contribution is positive.…”

The Operation of Two Decarboxylases, Transamination, and Partitioning of C4 Metabolic Processes between Mesophyll and Bundle Sheath Cells Allows Light Capture To Be Balanced for the Maize C4 Pathway

“…Leaf pigments preferentially absorb the blue and red region of the spectra, and some wavelengths penetrate deeper into leaves. It was shown in C 3 leaves that exposure to different wavelengths results in characteristic light penetration profiles, which, translated into different gradients in PSII yield, rates of ATP production, and assimilation (A) within the leaf (Terashima et al, 2009). In C 4 leaves, because of the concentric anatomy, light reaches M cells before the deeper BS (Evans et al, 2007) and could alter the balance between light harvesting and energetic partitioning between BS and M.…”