Christopher J. Beggs

UV-B Radiation and Adaptive Mechanisms in Plants

Further examples of UV-B dependent formation of UV-B screening pigments have been investigated. This active protective mechanism also has been found to be present in the leaf epidermis of various lightgrown vegetables (greenhouse conditions excluding UV-B radiation). Typical action spectra for this kind of positive UV-B effect are interpreted in terms of effectiveness under natural growth conditions’ and also with respect to the role of interaction with longer wavelengths. The importance of photorepair of UV-B damage is demonstrated for two systems, bean leaves and mustard cotyledons. Photorepair phenomena are further characterized and their importance for adaptation to an increased UV-B environment are pointed out. UV-B effects on growth and the hypersensitivity reaction (isoflavonoids, phytoalexins) in bean are discussed both from the viewpoint of damaging and beneficial consequences for the plant.

ANALYSIS OF LIGHT‐CONTROLLED ANTHOCYANIN FORMATION IN COLEOPTILES OF Zea mays L.: THE ROLE OF UV‐B, BLUE, RED AND FAR‐RED LIGHT

Abstract— Light‐induced anthocyanin formation in Zea mays L. coleoptiles was investigated in seven different varieties of this species. Under the test conditions, four varieties showed practically no response to any waveband used (UV, continuous red and continuous far‐red), two responded strongly to both UV and far‐red, and one showed a strong response only to far‐red. The radiation‐sensitive varieties showed, however, only a very weak response to continuous red light. In those varieties sensitive to far‐red light, a pretreatment with continuous red light led to a greatly enhanced response to UV or in one case the manifestation of a response to UV that was previously lacking. Further investigations in one radiation‐sensitive variety (INRA) showed that the UV response was to UV‐B radiation below 350 nm. The UV response, as well as the far‐red and blue responses in this variety, showed fluence‐rate dependency. Red light was almost ineffective and showed only a very weak fluence‐rate dependency.

Phytochrome-induced flavonoid biosynthesis in mustard (Sinapis alba L.) cotyledons. Enzymic control and differential regulation of anthocyanin and quercetin formation.

Phytochrome-induced increases in enzyme activities for phenylalanine ammonia-lyase (EC 4.3.1.5) and chalcone isomerase (EC 5.5.1.6), and in amounts of the related end products, anthocyanin and the flavonol, quercetin, were measured in cotyledons of mustard (Sinapis alba L.). There was no correlation between the activities of these enzymes and the rate of anthocyanin accumulation; however, some correlation was found with the quercetin accumulation rate. Since anthocyanin and flavonol accumulation is spatially separated in mustard (flavonols in the upper epidermis, anthocyanin in the lower epidermis), it was possible to measure anthocyanin-associated phenylalanine ammonia-lyase independently. This activity correlated well with the accumulation rate for anthocyanin during the first few hours after induction. The phytochrome effect on anthocyanin formation differed from that on quercetin formation: anthocyanin was strongly induced by continuous far-red light and by both continuous red light and red light pulses, whereas quercetin was only effectively induced by continuous far-red light.

DEPENDENCE OF Pfr/Ptot‐RATIOS ON LIGHT QUALITY and LIGHT QUANTITY

Abstract— Not only the spectral distribution of the light source determines the relative proportion of phytochrome in the Pfr(Pr) form, the Pfr/Ptot‐ratio also depends strongly on the fluence rate of the irradiation. This dependence has been observed in the cotyledons of etiolated mustard seedlings for blue light of fluence rates below 20 Wm‐2. It has also been observed for white light and seems to be a characteristic of the phytochrome system resulting from the involvement of phytochrome thermal reactions as well as Pr Pfr photoconversions. The fluence rate dependence of Pfr/Ptot‐ratios can be used to analyze the characteristic transformations of the phytochrome system. Phototransformations together with a fast thermal transformation (τ½⋍ 3min) are consistent with the results obtained for blue and white light.

High irradiance response promotion of a subsequent light induction response in Sinapis alba L.

Relative quantum responsivity curves for inhibition of hypocotyl elongation in Sinapis alba L. seedlings previously grown in white light confirm that a marked “end of day” inhibition response can be induced by a monochromatic light treatment (30 min) at the end of the light period. In dark grown seedlings, however, no growth inhibition can be induced by a 30 min monochromatic light treatment. A prerequisite for an induction response appears to be a pretreatment with continuous light. Far red light is most effective with blue and red light showing a lesser effectiveness. The light pretreatment also shows a marked fluence rate dependency with respect to its ability to allow an induction response to manifest itself. The pretreatment required shows all the characteristics of a classical “HIR” response. The appearance of the effect in plants treated with the herbicide SAN 9789 seems to exclude chlorophyll as being the photoreceptor.

A comparative study of the responsivity of Sinapis alba L. seedlings to pulsed and continuous irradiation

Anthocyanin formation in 36h dark grown Sinapis alba L. seedlings and inhibition of hypocotyl elongation in 36h and 54h dark grown and 54h and 7 day light grown seedlings in response to continuous red light could be substituted for by hourly 5 min light pulses where the total fluence over the irradiation period is the same. These pulses are partially (36h) or almost totally (54h and 7 day) reversible by subsequent far-red (RG 9) light pulses. In contrast to 654 nm light, hourly light pulses with 552 nm, 449 nm and 715 nm can at best only partially substitute for continuous irradiation. These data are discussed with respect to the commonly used models for the phytochrome high irradiance response.