Barley (Hordeum vulgare L.) was grown with UV‐B (280–320 nm) at levels simulating 25 nr 5% ozone depletion on the date of the summer solstice al 40°N latitude, with UV‐A (320–400 nm), or with no supplemental irradiation. In plant growth chambers providing 300 μmol m−2 s−1 photosynthetically active radiation (PAR). UV‐B‐grown leaves elongated more slowly than controls but reached the same final length 1 day later. Leal specific fresh weight (mass leaf area−1) was significantly increased by UV‐B after the 7th day of growth. IV‐B did not significantly affect leaf area, fresh weight, dry weight, total chlorophylls, total carotenoids or photosynthetic quantum efficiency. CO2 assimilation was decreased by UV‐B only at internal CO2 levels above 250 μl l−1. By the 8th day of growth, UV‐B increased flavonoid (saponarin and lutonarin) accumulation in both the lower epidermis and the mesophyll: about 40% of the saponarin and 20% of the lutonarin were in the lower epidermis under all experimental conditions. Glasshouse conditions proved too variable for reproducible determination of growth and photosynthesis but were reliable for determining developmental changes in flavonoid (saponarin and lutonarin) accumulation and provided up to 800 μmol m−2 s−1 PAR. In the glasshouse UV‐B‐grown leaves had more flavonoids than controls al all stages from 5 to 30 days after planting: ca 509 more saponarin and 100% more lutonarin. Levels of soluble (vacuolar) ferulic acid esters were similar under all conditions on day 5. and on day 20 or later, but were significantly higher in UV‐B‐grown plants on days 10 and 15. UV‐B decreased insoluble (cell‐wall‐bound) ferulic acid esters on a whole leaf basis but significantly increased this fraction in the lower epidermis. UV‐A had no significant effects on growth, photosynthesis or ferulic acid, but it slightly increased flavonoid accumulation. The results are discussed in terms of secondary phenolics as a tissue‐specific, developmentally regulated adaptive response to UV‐B.

SummaryStem tissues from different internodes of 4-6 weekold Zinnia elegans cv. Envy plants were sectioned and stained with chromogenic substrates previously used in studies of laccases (p-diphenohO2 oxidoreductasss) isolated from tree tissues. The pattern of color development found when stem sections were stained in the presence and absence of H202 suggested that p-diphenohO= oxidoreductase activity was tightly correlated spatially and temporally with the lignification of secondary cell walls in developing primary xylem. The correlation between this laccasslike phenoloxidass activity and Iignification appeared tighter than that between lignification and peroxidases stained using the same substrates. Zymogram analysis of the phenoloxidaee activities catalyzed by enzymes that were not boiled prior to separation by SDS-PAGE suggested that a single enzyme was predominantly responsible for the laccase-like phenoloxidase activity in Zinnia stems. Some of this enzyme was released from cell wall residue by washing with high ionic strength buffer; however, substantial amounts of the enzyme could only be recovered after treatment of the residue with cell walldegrading enzymes. This phenoloxidaee appears to share significant characteristics with the coniferyl alcohol oxidass isolated from developing secondary xylem in pines, which suggests that such enzymes may be widespread in vascular plants.

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