The Physiological Responses of Tea to pH and Cd Conditions and the Effect of the CsHMA2 on Cd Transport

Soil acidification in tea (<i>Camellia sinensis</i> L.) gardens leads to nutrient depletion, inhibits the growth of tea plants, reduces tea quality, and activates heavy metals such as cadmium (Cd) in the soil. To clarify the impact of soil pH under acidified conditions on tea plant growth physiology and the key genes involved in Cd²⁺ transport in tea plants, this study planted 'Longjing 43' under different pH levels (4.0, 4.5, and 5.5) and Cd concentrations (T1 = 0 mg L^-1, T2 = 0.01 mg L^-1, T3 = 0.05 mg L^-1, and T4 = 0.2 mg L^-1). The results showed that the concentration of Cd in tea plants from highest to lowest was root > stem > mature leaves > young leaves. Under T4, with decreasing pH, the total chlorophyll significantly decreased, the <i>Fv</i>/<i>Fm</i> significantly decreased, stomatal aperture reduced, and net photosynthetic rate and transpiration rate significantly decreased. In the T4 treatment at pH = 4.0, the contents of free proline and malondialdehyde were both the highest, while superoxide dismutase (SOD), peroxidase (POD), and catalase from micrococcus lysodeiktic (CAT) showed a significant negative correlation with pH. By screening the tea genome data, a total of nine CsHMAs involved in metal ion transport were identified. The qRT-PCR results indicated that the expression level of <i>CsHMA2</i> was the highest in young leaves of tea, and <i>CsHMA2</i> was localized on the cell membrane. Under T4 and pH = 4.0, transient overexpression of <i>CsHMA2</i> enhanced the ability of tea to transport Cd²⁺, whereas transient silencing of <i>CsHMA2</i> weakened this ability. These findings not only help understand how tea adapts and regulates its physiological processes in acidic environments but also provide an important theoretical basis and technical guidance for soil improvement in tea gardens, the control of heavy metal pollution, and the optimization of tea quality.