Changes in UV-B radiation screening effectiveness with leaf age in Rhododendron maximum

We examined how ultraviolet-B radiation (UV-B; 300 nm) screening effectiveness changes with leaf age in Rhododendron maximum growing in a shaded understory by measuring depth of penetration and epidermal transmittance with a fibre-optic microprobe. Depth of penetration (and epidermal transmittance) of UV-B decreased with leaf age in 1- to 4-year-old leaves, averaging 62 (32), 52 (22), 45 (16) and 48 μm (13%), respectively. Epidermal thickness increased with age in 1- to 4-year-old leaves due to a thickening of the cuticle from an average of 20 to 29 μm. Ultraviolet-B-absorbing compound concentrations increased with age from 1.3 to 1.5 A₃₀₀ cm^-2 leaf area. Concentrations of UV-B-absorbing compounds (area basis) were a strong predictor of depth of penetration (r² = 0.82) and epidermal transmittance (r² = 0.95) of UV-B in mature (1-4 year-old) foliage. Chlorophyll concentrations (area basis) increased in leaves up to 3 years of age. Current-year leaves (30 d old) were exceptional in that while they were particularly effective at screening UV-B (depth of penetration and epidermal transmittance averaged 39 μm and 5%, respectively) they had relatively low concentrations of UV-B-absorbing compounds (1.3 A₃₀₀ cm^-2). Our findings show that UV-B-screening effectiveness is not necessarily related to absorbing compound concentrations on a whole-leaf basis, possibly due to anatomical changes within the epidermis that occur with leaf age.