FarRed :Photosynthesis .

stardustsailor said:

Yes ....The two chlorophylls A & B do favor blue and red wls ,
regarding their absorptance ....

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churchhaze said:

I don't remember the source, but I've read that unshaded leaves tend to have more chlorophyll A and that shaded leaves tend to generate a higher percentage of chlorophyll B.

From this, I can speculate that chlorophyll B is meant for being under chlorophyll A and that a lower R:FR ratio signals the lower leaves to generate pigments that can effectively absorb the "lower quality" wavelengths that the top canopy missed.

From a practical perspective, does it even matter if FR can contribute to photosynthesis? If you tried to power your plant with FR, wouldn't it just stretch to death?.

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Plants experience continually changing light quality and quantity. Light varies, not only over a wide range of intensities and spectral qualities (full sun, early morning, late evening, cloud and canopy shade), but also over vastly different time scales from short sunflecks to long-lasting canopy gaps, and seasonal variations (Anderson and Osmond 1987; Anderson et al. 198. Moreover, a continuum of light intensity and quality is experienced from stromal to granal thylakoids within chloroplas- ts (Terashima 1989), to different chloroplasts within leaves, to leaves within and between species.

Com- pared with the exposed canopy, light received in shade is a blend of very weak diffuse irradiance greatly enriched in far-red (PS I light), deficient in red and to a lesser extent in blue light, interspersed with gap sunlight or sunflecks. Non-limiting to saturating light strategies: Shade or low light plants have more Chl b and more of the light-harvesting chlorophyll proteins of PS II (LHC II) and PS I (LHC I) for maximal light capture.

This increase in light-harvesting components occurs at the expense of electron transport, photophosphorylation and carbon fixation components, particularly Rubis- co, resulting in lower photosynthetic rates which satu- rate at lower irradiance.

Conversely, under sun and high light, plants are limited in electron transport rather than light capture and conversion: they have greater amounts of cytochrome b/fcomplex, ATP syn- thase, plastoquinone, plastocyanin, ferredoxin, and more carbon fixation enzymes, to support high max- imal photosynthetic rates which saturate at high irra- diance (Anderson and Osmond 1987). These modula- tions in the composition, organization and function of the photosynthetic apparatus are so well regulated, that even the Chl a/b ratios of leaves are a simple index of light intensity acclimation: they are linearly related to the content of cytochrome b/fcomplex, ATP synthase and Rubisco, and inversely correlated with amounts of LHC II and LHC I (Anderson et al. 198 and stacked membranes (Anderson and Aro 1994). Only P680 and P700 are not proportional to Chl a/b ratios.

Light-limiting strategy: Nevertheless both pho- tosystems undergo acclimation: two strategies are involved. First, the amounts of LHC II and LHC I serving each reaction centre decrease with increasing irradiance. Second, the amount of PS II reaction centres relative to PSI reaction centres, i.e. the photosystem stoichiometry, is altered with varying light quantity or quality. Shade and low light plants have lower PS II/PS I ratios of 1.0-1.3, due to fewer PS II units each with larger light-harvesting antennae, while sun and high light plants with PS II/PS I ratios of 1.8-2.4, have more PS II units each with smaller light-harvesting units, relative to PSI (Anderson et al. 198. This adaptation of the photosystem stoichiometry which is influenced more by alterations of spectral quality than intensity, serves to regulate the distribution of excita- tion energy between the photosystems and correct any imbalances: eg plants grown in PSI light have high- er PS II/PS I ratios due to increased amounts of PS II, and vice versa (Chow et al. 1990b; Melis 1991). Acclimation of plants to only light intensity or light quality demonstrates the opposing nature of regulation by quality and quantity: lower light intensity causes a decrease in PS II content on a chlorophyll basis, while the concomitant increase in PSI light in extreme shade enhances PS II content. Acclimation also ensures that plants operate effi- ciently under limiting light. It is remarkable that the quantum yields of all C3 plants are high and con- stant (Demmig and Bj6rkman 1987). Measurements of quantum yields of plants grown in PS II or PSI light demonstrate that high quantum yields were obtained only when the quality of the measuring light was the same as that of the growth light (Chow et al. 1990b; Waiters and Horton 1995b). This strongly suggests that modulation of the photosystem stoichiometry is a com- pensatory factor, along with modulations of effective light-harvesting antennae size, that ensures all plants have constant, high quantum yields at limiting light. Regulation of photosystem stoichiometry is important since most chloroplasts function for most of the time in non-saturating light, due to pronounced attenuation of light within chloroplasts, cells, leaves and canopy.

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Photosystems II and I act as sdf-regulatory light sensors


To simulate extreme shade conditions, peas were grown in white light supplemented with high levels of far-red light (FR/R ratio of 0.04) (long-term FR), comparable to extreme shade conditions (Smith 1982). Compared to control peas, long-term FR also produced phytochrome-regulated morphological changes simi- lar to those described above for brief FR. However, in contrast to brief FR, there was a marked increase in photosynthetic capacity on a Chl basis (Table 1), due to increased activities of Rubisco and ATP synthase and levels ofCyt blfcomplex, and to a lesser extent PS II content, as well as a significant decrease in PSI on a chlorophyll basis. We hypothesized that the observed enhanced Pmax and chloroplast components in long- term FR was induced by far-red light acting through PS I, to increase photosynthesis by extra cyclic electron transport and/or enhanced non-cyclic electron trans- port due to state transitions (Chow et al. 1990a). The extra supply of ATP would then be available to regu- late gene expression to increase protein synthesis, pro- tein translocation across membranes and assembly of complexes in a post-translational manner. Thus, long- term FR induces an increase in chloroplast compo- nents (Table 1) by a feedback mechanism regulated by photosynthesis.

Modulation of these energy storage pools of ATP and NADPH in the chloroplast stroma will regulate gene expression by enhanced transcrip- tion/translation and biosynthesis/assembly activities being directed towards those components of chloro- plast function that are rate-limiting, as well as degra- dation of other components (e.g. decrease of PSI inPS 1-growth light)(Melis et al. 1985, 1991; Chow et al. 1990a; Anderson and Chow 1992).

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