Foliar carbon dots improve drought tolerance in soybean through genotype-dependent hydraulic and metabolic pathways

Although foliar application of carbon dots (CDs) has emerged as a promising strategy to alleviate drought, whether CDs act through hydraulic or biochemical pathways, and whether their effects are genotype-dependent, remains unclear. Here, we investigated the effects of foliar CDs application (50 mg L^-1 CDs) at the podding and seed-filling stages on photosynthetic performance, hydraulic property, abscisic acid (ABA) and indole-3-acetic acid contents, oxidative stress, antioxidant defense, and osmotic adjustment substances in wild (Y15) and cultivated (Zhonghuang 13) soybeans under full (75%-95% field capacity, FC) and deficit (55%-65% FC) irrigation. The results showed that leaf net photosynthetic rate (A_n) and stomatal conductance (g_s) were reduced by 10.6%-54.1% and 34.5%-75.9%, while intrinsic water use efficiency was increased by 31.5%-124.7% during the podding and seed-filling stages under deficit irrigation. Leaf hydraulic conductance (K_leaf) was decreased by 21.8% at the podding stage, and bulk modulus of elasticity (ε) was increased by 31.0% at the seed-filling stage under deficit irrigation. Wild soybean exhibited greater K_leaf and catalase activity, whereas cultivated soybean showed superior photosynthetic capacity and growth potential. Foliar CDs application primarily enhanced predawn leaf water potential by 27.9%, and ɛ by 49.6% at the seed-filling stage, and K_leaf by 35.4% at the podding stage, while concurrently down-regulating the stress-induced over-accumulation of CAT, proline, and soluble sugars (SS). Crucially, in wild soybean, CDs improved A_n by increasing K_leaf, which promoted stomatal opening and carbon assimilation. ABA played only a secondary role by negatively regulating K_leaf. In cultivated soybean, CDs promoted SS accumulation and then osmotic adjustment substances thereby enhancing g_s and A_n, while ABA and hydraulic pathways were not significant. Collectively, these findings demonstrate that foliar CDs alleviate drought in soybean through genotype dependent mechanisms of hydraulic facilitation in wild soybean versus metabolic reprogramming in cultivated soybean. This study provides a physiological basis for the genotype-sensitive application of CDs as a drought-resilience strategy in soybean production.