Antagonistic Regulation of Solanum nigrum Hypocotyl Elongation by Temperature and Water Potential via ABA Biosynthesis and Signaling

Solanum nigrum L. is a problematic broadleaf weed in Xinjiang cotton fields. Its seedling hypocotyl elongation is highly sensitive to temperature and water potential, but the underlying molecular basis remains unclear. Here, we investigated hypocotyl elongation, hormone metabolism, and transcriptomic dynamics under five temperature (15 to 35 °C) and water potential (0 to −0.8 MPa) gradients. Germination and hypocotyl elongation were optimal at 30 °C and 0 MPa; the germination rate reached 95.56%. Both were severely inhibited when the water potential dropped below −0.4 MPa, particularly at 15 °C and 35 °C. At 30 °C, elevated indole-3-acetic acid (IAA)/abscisic acid (ABA) and gibberellin (GA)/ABA ratios were associated with promoted elongation, whereas under moderate stress (−0.4 MPa), these ratios decreased markedly, which was consistent with suppressed growth. Transcriptomic analysis revealed that temperature predominantly affected carbon metabolism and spliceosome pathways, while osmotic stress enriched photosynthetic carbon fixation and glyoxylate cycle pathways. Notably, at 30 °C, down-regulation of ABA biosynthesis genes (NCED3, ABA2, AAO3) and SnRK2.4, coupled with PP2CA up-regulation, was associated with attenuated ABA signaling, suggesting its involvement in facilitating elongation. Osmotic stress reversed this pattern—up-regulating ABA biosynthesis genes and SnRK2.4 while suppressing PP2CA—suggesting the involvement of activated ABA signaling in restraining elongation. Collectively, these results suggest that temperature and water potential antagonistically modulate ABA signaling to determine hypocotyl elongation. These findings suggest adaptive mechanisms of S. nigrum seedling establishment under combined stresses and provide potential molecular targets for early weed management in agricultural systems.