Enhanced Stem Immobilization Mitigates Leaf Cadmium Accumulation and Modifies PSII Photochemistry in a Tobacco Line with Low Cadmium Accumulation

Tobacco (<i>Nicotiana tabacum</i> L.) has a propensity to accumulate cadmium (Cd), especially in its leaves, which can have a detrimental impact on yield, quality, and product safety. The development of low-accumulation cultivars is a vital mitigation approach; however, the underlying mechanisms remain inadequately understood. In this study, through pot experiments, the physiological mechanisms responsible for the differential Cd accumulation between the low-accumulating tobacco line CF986 and the high-accumulating Yuyan5 were explored. A comprehensive analysis was conducted on the organ-specific Cd distribution, chemical speciation, subcellular compartmentalization, and photosynthetic responses across a gradient of Cd exposure. In comparison with Yuyan5, CF986 accumulated significantly higher amounts of Cd in the roots and stems, but substantially lower amounts in the leaves. Specifically, the Cd content in the leaves of CF986 was only 64.32-68.74% of that in Yuyan5 across different Cd exposure levels. The organ-specific Cd distribution pattern in CF986 followed the order: leaf > stem > root. Moreover, the proportion of Cd partitioned to the leaves was lower in CF986 compared to Yuyan5, while the roots and stems exhibited enhanced Cd retention, with Cd levels in stems reaching up to 2.04 times higher than those in Yuyan5. Analysis of the chemical forms and subcellular distribution of Cd indicated that the mobile Cd fractions in the stems of CF986 were significantly reduced compared to Yuyan5. A larger proportion of Cd was immobilized in the stem cell-wall fraction, which enhanced Cd retention and restricted xylem-mediated transport to the leaves. Cd exposure did not significantly affect the concentration of foliar photosynthetic pigments in CF986; however, it notably inhibited the activity of the photosystem II (PSII) reaction center. At higher Cd levels, the photoprotective thermal dissipation gradually failed, with a decrease of up to 41.36% in Φ_NO for CF986 compared to CK under Cd4.0 treatment. This research unveiled a stem barrier mechanism, whereby Cd translocation to the leaves is restricted through chemical and subcellular sequestration in the stem. This mechanism provides a novel perspective on both plant heavy metal allocation and the assurance of crop safety.