Salix viminalis is a typical dioecious shrub willow, which has been widely used in the heavy metal polluted soil phytoremediation due to its fast growth rate, deep root system, and metal-resistance trait. It has been reported that dioecious plants present sex-specific characteristics when exposed to biotic and abiotic stress. However, different responses to heavy metal cadmium stress between male and female plants of S. viminalis have not been clarified. In this study, male and female S. viminalis clones were treated by 100 μM cadmium stress for a period of 21 days, and the root morphology, hydrogen peroxide content, malondialdehyde, and free proline content, antioxidant enzyme activities, and glutathione content were investigated after every 7-day interval. The results showed that female plants of S. viminalis developed thicker root diameter and volume and also accumulated more hydrogen peroxide than males at the first 7-day interval. Furthermore, sexual dichotomous indexes showed the most significant on the second 7-day interval. These findings suggest that male plants of S. viminalis have stronger cadmium tolerance than female plants and sexual differences in adaptive responses to cadmium are duration dependent, which adds to present evidence of sex-related responses to environmental stress in dioecious plants.

Premise: Sexual dimorphism in dioecious plant species is often not obvious or is absent. Dioecious species populations also often exhibit deviations from expected sex ratios. Previous studies on members of the Salicaceae family have shown strong, partial, and no sexual dimorphism. Some studies have shown sex-biased ratios in several Salix spp., however, S. purpurea has never been examined for evidence of sexual dimorphism or for the presence of sex-ratio bias, and therefore a comprehensive phenotypic study is needed to fill this knowledge gap. Methods: This study examined a suite of morphological, phenological, physiological and wood composition traits from multi-environment and multi-year replicated field trials in a diversity panel of unrelated S. purpurea accessions and in full-sib F ₁ and F ₂ families produced through controlled cross pollinations to test for sexual dimorphism and sex ratio bias. Key Results: Significant evidence of sexual dimorphism was found in vegetative traits with greater means for many traits in male genotypes compared to females across three populations of S. purpurea , measured across multiple years that were highly predictive of biomass yield. Male plants exhibited greater nitrogen accumulation under fertilizer amendment as measured by SPAD in the diversity panel, and males showed greater susceptibility to fungal infection by Melampsora spp in the F ₂ family. There were also consistent female-biased sex ratios in both the F ₁ and F ₂ families. Conclusions: These results provide the first evidence of sexual dimorphism in S. purpurea and also confirm the prevalence of female-biased sex ratios previously found in other Salix species.

Phosphorus (P) plays a crucial role in the translocation and accumulation of cadmium (Cd) in plants; however, its effects on Cd transport via the apoplastic pathway remain unclear. In this study, Salix caprea was used to systematically investigate the regulatory roles of P on apoplastic barrier deposition (casparian strips and suberin lamellae), apoplastic Cd transport, and Cd accumulation through an integrated approach combining physiological, biochemical, anatomical, and transcriptomic analyses. The results showed that under Cd stress, P-deficient conditions accelerated the development of apoplastic barriers, with the initiation of casparian strips and suberin lamellae occurring 0.5% and 5% closer to the root tip, respectively, compared with P-sufficient conditions. Transmission electron microscopy (TEM) further revealed that P deficiency significantly increased the thickness of endodermal cell walls by 37.2% relative to P sufficiency when exposed to Cd stress. Moreover, root lignin content and the activities of lignin- and suberin-related enzymes (POD and PAL) were significantly higher under P deficiency. Transcriptome analysis indicated that under Cd stress, P deficiency markedly upregulated genes involved in lignin and suberin monomer biosynthesis (PAL, POD, KCS20, LACS), as well as casparian strip polymerization (CASP, MYB36). In addition, under P-deficient conditions, the net Cd²⁺ flux at the root tip was reduced by 21.3%, and the 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (PTS, a tracer for the apoplastic pathway) concentration in leaves decreased by 36.3%, further confirming that P deficiency limits Cd transport via the apoplastic route. This may explain why, under P-deficient conditions, Cd concentrations in leaves and shoots were significantly reduced by 48.7% and 63%, respectively, compared with P-sufficient conditions. This study provides new insights into improving phytoremediation efficiency for extreme heavy metal pollution by P application.