Effects of Shading on Chlorophyll Content, Chlorophyll Fluorescence and Photosynthesis of Subterranean Clover

A field-experiment (2004/2005 and 2005/2006 seasons) was conducted in the coastal plain of south-eastern Sicily (37 degrees 03'N, 15 degrees 18'E, 15 m a.s.l.), on a Calcixerollic Xerochrepts soil, aimed at quantifying the effect of shading on chlorophyll (Chl) content, Chl fluorescence, photosynthesis and growth of subterranean clover. Four levels of photosynthetically active radiation reduction (from 0 % to 90 %) were tested on Trifolium brachycalycinum cv. 'Clare' and Trifolium subterraneum ecotype 'Ragalna'. In both species shading progressively increased F-v/F-m, internal CO2 concentration, diffusive leaf resistance and specific leaf area (up to 8 %, 34 %, 18 % and 68 %, respectively), and decreased Chl content, T-max, photosynthetic rate and plant dry weight (up to 9 %, 24 %, 79 % and 39 %, respectively). As plants aged, characteristic bell-shaped trends were evident for photosynthetic parameters, with F-v/F-m increasing up until the onset of flowering, and thereafter declining. This implies that F-v/F-m may be a useful indicator of earliness in subterranean clover genotypes. The aboveground dry biomass response to shading was both genotype- and season-dependent, but was predictable from the measurement of relative leaf Chl content. Moreover, our results suggest that an improvement in the interaction between host-rhizobium may represent a major potential breeding target for enhancing subterranean clover tolerance to shading.