© 1993 Heron Publishing—Victoria, Canada
The effects of ultraviolet-B radiation on photosynthesis of different aged needles in field-grown loblolly pine
Shawna L. Naidu (1), Joe H. Sullivan (2), Alan H. Teramura (2) and Evan H. DeLucia (1, 3)
1. Department of Plant Biology, University of Illinois, Urbana, IL 61801, USA / 2. Department of Botany, University of Maryland, College Park, MD 20742, USA / 3. Author to whom correspondence should be addressed / Received May 13, 1992
Summary
We examined the effect of supplemental UV-B radiation (290–320 nm) on photosynthetic characteristics of different aged needles
of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: I, most recently fully expanded, year 3; II, first flush, year 3; III, final
flush, year 2; and IV, oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease
(6%) in the ratio of variable to maximum fluorescence (Fv/Fm) following dark adaptation only in needles from the youngest age class, suggesting transient damage to photosynthesis. However,
no effects of enhanced UV-B radiation on other instantaneous measures of photosynthesis, including maximum photosynthesis,
apparent quantum yield and dark respiration, were seen for needles of any age. Foliar nitrogen concentration was unaffected
by UV-B treatment. However, the 13C/12C carbon isotope ratios (δ13C—a time integrated measure of photosynthetic function) of needles in age classes II and IV were 3% (P < 0.01) and 2% (P < 0.05) more negative, respectively, in treated plants than in control plants. Exposure to enhanced UV-B radiation caused
a 20% decrease in total biomass and a 4% (P < 0.05), 25% (P < 0.01), and 9% (P < 0.01) decrease in needle length of needles in age classes I, II, and IV, respectively. The observed decreases in δ13C, and Fv/Fm of the needles in the youngest needle age class suggest subtle damage to photosynthesis, although overall growth reductions
were probably a result of decreased total leaf surface rather than decreased photosynthetic capacity. Needles of age class
IV had lower light- and CO2-saturated maximum photosynthetic rates (39%), lower dark respiration (34%), lower light saturation points (37%), lower foliar
nitrogen concentration (28%), and lower δ13C (14%) values than needles of age class I. Apparent quantum yield and Fv/Fm did not change with needle age. The observed changes in photosynthesis and foliage chemical composition with needle age are
consistent with previous studies of coniferous trees and may represent adaptations of older needles to shaded conditions within
the canopy.
Keywords:
carbon isotope ratio, leaf age, leaf nitrogen, ozone, photosynthesis, Pinus taeda, UV-B radiation.
