A field‐experiment (2004/2005 and 2005/2006 seasons) was conducted in the coastal plain of south‐eastern Sicily (37°03′N, 15°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 Fv/Fm, internal CO2 concentration, diffusive leaf resistance and specific leaf area (up to 8 %, 34 %, 18 % and 68 %, respectively), and decreased Chl content, Tmax, 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 Fv/Fm increasing up until the onset of flowering, and thereafter declining. This implies that Fv/Fm 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.

Agron. Sustain. Dev. 30 (2010) Abstract -The environmental side effects of intensive agriculture have underlined the need to develop sustainable farming systems. In particular, the use of cover cropping in orchards is a means of improving cash crop yield and of reducing the quantity of applied fertilisers. In the Mediterranean environment, subterranean clover species could be the best choice for cover cropping, but they are only poorly adapted to the heavily shaded conditions characteristic of modern high-density orchards. The plant traits needed to improve adaptation of subterranean clover are not well understood. Therefore, in a two-year experiment we studied the effects of four shading levels, of 0%, 40%, 60% and 90% reduction of photosynthetic active radiation, on phenology, growth and development of two subterranean clover species: Trifolium brachycalycinum cv. 'Clare' and T. subterraneum ecotype 'Ragalna'. Our results show that shading progressively delayed seedling emergence by up to 21 days, the initiation of flowering by up to 27 days, and the end of flowering by up to 25 days. Shading also lengthened the life cycle from 237 to 267 days. Shading reduced both soil cover by up to 38.2% and cover crop density by up to 39.7%. Shading lowered both the quantity of above-ground dry biomass by up to 820 g m −2 and photosynthetically active surface area by up to 213 cm 2 plant −1 . Trifolium brachycalycinum 'Clare' was more productive in terms of above-ground dry biomass yield, but T. subterraneum 'Ragalna' was better adapted to shading in terms of rapid emergence, earliness and the time taken to achieve soil cover. These species differences suggest that the breeding targets for improving the adaptation of subterranean clover to heavy shading are the ability to maintain earliness and the capacity to quickly break down hard-seededness under conditions of partial shade. A rapid initial increase in photosynthetically active surface area is particularly needed for maximising light harvesting during the early growth period.Trifolium brachycalycinum / T. subterraneum / cover crop / orchard / low irradiance

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