Bioactive compounds of legume based swards are affected by light intensity in a Mediterranean agroforestry system

In Mediterranean grazed woodlands, microclimate changes induced by trees influence the growth and development of the understory but very little is known about its phenolic composition in relation to the contrasting exposure to full sunlight or shade. Phenolic acids, flavonoids and tannins are the most important compounds for biological activities and especially antioxidant properties and related implications in animal nutrition and welfare. The research was carried out in a cork oak agrosilvopastoral system in Sardinia, where we investigated the bioactive compounds and antioxidant capacity of different legume-based swards subjected to variations in light intensity. Open areas with full sunlight exposition (FS) and areas under tree canopy with partial shade conditions (PS), under a cork oak (Quercus suber L.) density of 450 trees ha-1 were carefully identified. Light levels of photosynthetically active radiation were measured using a SunScan canopy analysis system (Delta-T Devices, Cambridge, UK). For both FS and PS, the following legume based swards were compared: Fertiprado commercial legume mixture, with 100% annual legume composition, CNR ISPAAM mixture, with 80% legume composition, Bladder clover, Trifolium spumosum L., elite Sardinian accession pure sward and semi natural pasture with 60% of legume composition. Total phenolics, total flavonoid and antioxidant capacity were determined and single phenolic compounds were identified. Light interception by cork trees was 85, 77 and 70% in January, April and May, respectively due to the different solar azimuth angle of the seasons. The antioxidant capacity,total phenolics and total flavonoid of the different legume-based swards were significantly affected by the contrasting conditions of light intensity, as well as by the type of legume-based sward. Compared to full sunlight, partial shade reduced antioxidant capacity values by 29 and 42%, and total phenolics content by 23 and 53% in Fertiprado and semi natural pasture. Twelve phenolic compounds were detected, verbascoside in bladder clover mixture was the most abundant in full sunlight and neochlorogenic acid and isorientin were abundant in partial shade in CNR ISPAAM mixture and in unsown semi-natural pasture respectively. As antioxidant capacity and the content of plant secondary metabolites ascertained in the legume-based swards could potentially affect the nutritional properties of forage, their variations caused by contrasting light intensities thus represent a particular benefit, which must be exploited as an additional service from agroforestry. Our results provide new insights into the effects of light intensity on plant secondary metabolites from legume based swards, underlining the important functions provided by agroforestry systems.