Comparative Growth, Photosynthetic Pigments, and Osmolytes Analysis of Hemp (Cannabis sativa L.) Seedlings under an Aeroponics System with Different LED Light Sources

The performance of hemp seedlings was evaluated through morphological traits, photosynthetic pigments, and osmolytes under 11 light treatments (10 LED light compositions + natural light) in an aeroponics system. The seedlings were brought under treatment at 25 days of age, where the light intensity was 300 µmol m−2s−1 and duration was 20 days. A higher leaf number and node number were observed in L10 (R4:B2:W2:FR1:UV1) and L11 (R2:B2:G2:W2:FR1:UV1), and a higher leaf length and leaf width were recorded in the L2 (white), L3 (R8:B2), and L5 (R7:B2:FR1) treatments. Furthermore, a higher shoot length was recorded in L3 (R8:B2), L6 (R6:B2:G1:FR1), and L9 (R6:B2:FR1:UV1) while roots developed more in the L1 (natural light), L5 (R7:B2:FR1), and L9 (R6:B2:FR1:UV1) treatments. On the other hand, the L3 (R8:B2) treatment manifested higher chlorophyll a, chlorophyll b, and photosynthetic quantum yield (Fv/Fm). The hierarchical clustering and heatmap analysis revealed that higher leaf numbers and node numbers resulted in bushy plants with shorter shoots and longer roots. A negative correlation was also observed in photosynthetic traits (pigments and fluorescence) with osmolytes and root length. Importantly, the treatments L4 (R7:B2:G1), L6 (R6:B2:G1:FR1), L8 (R5:B2:G1:FR1:UV1), and L11 (R2:B2:G2:W2:FR1:UV1) manifested higher nodes with a higher osmolyte content, such as proline, ascorbic acid, total soluble carbohydrate, and sucrose, which may be a helpful indicator for higher branches and inflorescences, and ultimately higher cannabinoids accumulation in the plants. The approach and findings of this study could provide future research with the baseline information on optimizing the light composition to produce hemp plants with ideal phenotypes.