Cadmium (Cd) is harmful to plant growth and can be easily transferred from soil to plants. Plant cell wall plays important role in preventing Cd from entering cells. Salicylic acid (SA) mediated defense response increases plant resistance to heavy metals. In this study, all tomato seedlings were pre-treated with 100 μM SA for 3 d, then seedlings were used to analyze the role of SA in regulating plant cell wall resistance to Cd stress. The results showed that exogenous SA significantly reduced Cd accumulation in tomato plants and changed Cd distribution. By analyzing the cell wall composition, it was found cellulose, hemicellulose, pectin, and lignin were induced by SA. Interestingly, the content of Cd in pectin decreased by SA pretreatment, however it was increased in cellulose. Gene expression analysis showed SA up-regulated the expression level of lignin and cellulose synthase genes, but down-regulated the expression of pectin methylesterase related genes. In addition, SA down-regulated the activity of pectin methylesterase. These results indicated that SA pretreatment up-regulated cell wall polysaccharide synthesis and related gene expression to thicken the cell wall and block Cd from passing through. Furthermore, SA decreased pectin methylesterase activity and content to reduce cell wall Cd accumulation and change the Cd partition ratio.

Abiotic stresses could have complex and diverse effects on the growth and development of forest trees. In this review, we summarized the responses of morphological, physiological and anatomical traits in woody plants to abiotic stresses, including drought, flood, extreme temperature, salinity, heavy metal, microplastics and combined stresses, especially from the xylem perspective. Under most abiotic stress, xylem hydraulic conductivity decreases, which is associated with leaf stomatal regulation and the inhibition of aquaporin (AQP) activity. Meanwhile, woody plants regulate the size and morphology of their roots and leaves to balance water absorption and transpiration. The anatomical traits are also altered, such as denser leaf stomata, narrower conduits and thicker cell walls. In addition, different stresses have unique effects, such as flood-induced adventitious roots and aeration tissues, forest fire-induced irreversible xylem damage, low temperature-induced tissue freezing, salt stress-induced hinderance of ion absorption and heavy metal-induced biological toxicity. Under stresses of drought, flooding and heavy metals, woody plants’ growth may occasionally be promoted. The effects of combined stress on the physiological, morphological and anatomical traits of woody plants are not simply additive, with the related mechanism to be further studied, especially in natural or near-natural conditions.