Effects of UV‐B on activities of enzymes of secondary phenolic metabolism in barley primary leaves

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Abstract
Barley (Hordeum vulgare L.) was grown in a glasshouse with 13.56 or 8.84 kJ m−2: biologically effective UV‐B (280–320 nm: UV‐BBE) simulating levels predicted to occur with 25 or 5% ozone depletion at 40°N latitude, with UV‐A (320–400 mm), or with no supplemental irradiation. Activities of L‐phenylalanine ammonia‐lyase (PAL, EC 4.3.1.5). chalcone‐flavanone isomerase (CFI, EC 5.5.1.6) and peroxidase (EC 1.11.1.7) were determined from the 5th through the 30th day after planting. PAL regulates diversion of L‐phenylalanine into precursors for secondary phenolics. CFI regulates an early step of flavonoid biosynthesis, and peroxidase activates phenolic precursors for cross‐linking and rigidifying cell walls. At all ages UV‐B decreased soluble protein leaf−1 but had little effect on fresh weight or CFI activity. Exposure to UV‐B decreased peroxidase activity only slightly in early growth stages but decreased it about 40% by day 30. PAL activity was highest 5 days after planting under all treatments, decreased thereafter, and was not detectable in control plants after day 10. UV‐B prolonged PAL activity through day 15 in plants given the highest level of UV‐B. This UV‐B prolongation of PAL activity is correlated with, and is a likely underlying mechanism to explain, the UV‐B‐ enhanced accumulation of flavonoids and ferulic acid in barley primary leaves. The results are discussed in terms of barley leaf adaptation to UV‐B as developmental response dependent on conditions of plant growth.