Response of perennial ryegrass, plantain, and white clover to shade in agroforestry and agrivoltaics systems

As global demand for food and renewable energy drives the expansion of dual-purpose land-management systems, such as agroforestry and agrivoltaics, understanding understory crop performance is critical. The spectral composition of the radiation transmitted through photovoltaic (PV) panels differs significantly from that of a green canopy. This can affect plant photomorphogenesis and performance. This study investigated the morphological responses of three forage species to these distinct shade environments. Spectral measurements confirmed that PV panels act as neutral absorbers-reflectors, maintaining a Red/Far-Red ratio (R/FR) similar to full sun (i.e., R/FR > 1) and potentially increasing the blue (B) radiation fraction. In contrast, hazel ( Corylus avellana L.) canopy shade dramatically reduced the R/FR and B/FR ratios, providing classic spectral “shade signals” for plants. We hypothesized that the lack of such signals under PV panels might fail to induce at least some of the morphological changes induced by shade. However, all species exhibited the same pronounced morphological changes in PV shade, including significantly increased specific leaf area (SLA), leaf area ratio (LAR), and plant height, and decreased root/shoot ratio. This suggests that low absolute radiation intensities can override “sunny” spectral signals. Although all species reacted to shade similarly in terms of SLA, LAR, plant height, and root/shoot ratio, the partitioning of dry mass into the different plant organs differed due to different plant architecture among species. For instance, in perennial ryegrass ( Lolium perenne L.) and plantain ( Plantago lanceolata L.) there is no leaf petiole and the proportion of leaves in the harvestable part did not decrease in shade. In white clover ( Trifolium repens L.), instead, shade increased petiole length and thus increased the proportion of petiole dry mass in the harvestable part (i.e., leaves and petioles).