Photosynthetic acclimation to shade: probing the role of phytochromes using photomorphogenic mutants of tomato

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Abstract
Photomorphogenic mutants of tomato were used to probe the possible involvement of phytochrome A in photosynthetic acclimation to shade. Plants of wild‐typeLycopersicon esculentum, the near isogenic mutantsaurea(au, deficient in phytochrome A, and also possibly other phytochromes) andhigh pigment(hp, exhibiting exaggerated photomorphogenic responses), and theau/hpdouble mutant were grown in a greenhouse under either unfiltered daylight, or daylight attenuated by neutral density filters or green filters that simulated the spectral effects of vegetation shade. Growth and photosynthetic parameters were measured over a growing period of 15 d. Typical effects of simulated vegetation shade on extension growth were observed only in the wild‐type andhpstrains; the presence of theaulesion resulted in reduced capacity to respond to vegetation shade by increased extension. Photosynthetic light saturation curves obtained using infrared gas analysis at 15 d indicated that neither theaunor thehpmutations caused serious reductions in maximum net photosynthetic rate in plants acclimated to full daylight, although theau/hpdouble mutant showed a significantly reduced maximum rate. In the wild‐type andhpstrains grown under both neutral and simulated vegetation shade, the photosynthetic light saturation curves were similar. Plants with theaulesion showed a significantly lower capacity to acclimate to vegetation shade, as seen by their reduced maximum rate of net photosynthesis. However,in situmeasurements of photosynthetic rate in the greenhouse were indistinguishable between the four strains. Thylakoid stoichiometry measurements revealed a very large increase in PSII/PSI ratio under simulated vegetation shade as compared with either unfiltered daylight or neutral density shade. These increases were not diminished in the phytochrome‐deficientaustrains, and were even larger in thehpstrains. The results indicate that theaulesion has little effect on the capacity of tomato to exhibit photoadaptation to simulated vegetational shade when measured at the thylakoid level. On the other hand, net leaf photosynthesis under vegetation shade can be significantly affected by the phytochrome deficiency caused by theaumutation, implying a possible role for one or more of the phytochromes in photoadaptation at the leaf level.