Agrivoltaic systems can be expected to integrate photovoltaic power generation with agricultural cultivation. The emerging agricultural production mode can better balance the clean energy output and agricultural productivity under the dual-carbon goals; However, the shading effects can be induced by photovoltaic (PV) modules, leading to an uneven distribution of the internal light environment. The synergistic benefits of agrivoltaic systems have also constrained the photosynthetic efficiency and crop yield. Thereby, this study aims to simulate the light environment under different photovoltaic module layouts in an open-field agrivoltaic system. The PV module layout configurations were systematically regulated under the internal light environment. A PV agricultural installation was also supported by a 3.8 m-high steel structure in Guanyun County, Lianyungang City, Jiangsu Province, China. The spatiotemporal distribution of solar radiation was captured over an entire year. A parametric framework was established using Rhino and Grasshopper. Annual light environment simulations were conducted with the Ladybug plugin. While field measurements of solar radiation were used to validate the accuracy of the simulation model. A systematic investigation was implemented to quantitatively analyze the effects of PV module arrangement patterns and vertical projection ratio on the distributions of photosynthetically active radiation (PAR), light homogeneity index (XLHI), and monthly average daily light integral (DLI). The results indicated that the vertical projection ratio was a key parameter governing both the total available radiation and the uniformity of light distribution in agrivoltaic systems. The vertical projection ratio was reduced from 43.7% to 29.2%, while the internal PAR increased by 18.3%–38.5%, indicating the significantly uniform light distribution during the summer season. In addition, the double-row arrangements also exhibited superior light penetration and homogeneity under the same projection ratio, compared with the single ones. Specifically, the summer XLHI increased by up to 10.3 percentage points when the longitudinal spacing between the two rows in a double-row structure increased from 0 to 0.4 m. Seasonal variation further intensified the spatial heterogeneity of the light distribution. Among them, the irradiance levels in the inter-row areas were markedly higher than those directly beneath the PV modules during summer and autumn, indicating the more suitable for the spatially differentiated planting. By contrast, the relatively uniform light distribution during winter and spring greatly contributed to the unified cultivation of the shade-tolerant or moderately light-demanding crops. Overall, the PV module layout can be expected to integrate the seasonal light distribution with the crop light requirements. The finding can provide scientific support for the structural layout optimization and planting mode in agrivoltaic systems. The light use efficiency can also coordinate the photovoltaic power generation and agricultural production.

Root regeneration is an important factor influencing the healing rate of graft union and the survival of double-root-cutting grafting. To date, little information is available on how to enhance root regeneration of rootstock in grafted watermelon (Citrullus lanatus) seedlings. In this study, the effects of different light treatments on root regeneration were determined. This revealed that addition of far-red light (Fr) could significantly expedite root formation in the rootstock. Moreover, the results of transcriptome analysis revealed that plant hormone pathway and auxin-related genes were greatly induced by Fr, especially for auxin-response proteins (including CmIAA11, CmIAA17, and CmAUX28), Small auxin-up RNA genes (including CmSAUR20 and CmSAUR50) and the auxin efflux transporter (CmPIN3). In addition, the expression of Phytochrome Interacting Factor (PIFs), such as CmPIF1, CmPIF3 and CmPIF7, was remarkably increased by Fr. These genes may act together to activate auxin-related pathways under Fr treatment. Based on the results of HPLC-MS/MS analysis, the concentrations of different auxin-types in adventitious root were significantly influenced by Fr. Furthermore, the better growth of rootstock root displayed superior vasculature transport activity of the graft union with Fr treatment, which was determined by the acid magenta dyeing experiment. Therefore, all the results suggested that Fr could induce AR formation in rootstocks, which may be associated with the auxin accumulation by regulating the transcriptional level of auxin-related and PIF genes. The findings of this study demonstrated a practicable way to shorten the healing period of graftings and improve the quality of grafted watermelon seedlings, which will provide a theoretical basis for the speeding development of industrialized seedlings production.