Induction of Polyploidy and Physiological Evaluation of Salt Tolerance in Apple

Polyploid induction technology is one of the most important means used to breed highly resistant varieties. Therefore, this study regenerated isolated leaves of ‘Qin Cui’ using 4.43 g · L -1 of MS + 30 g · L -1 sucrose + 8 g · L -1 agar + 6.0 mg · L -1 thidiazuron (TDZ) + 0.3 mg · L -1 1-naphthaleneacetic acid (NAA), yielding the highest germination rate of 64.3% and regeneration rate of 147.71%. Further, the study used different concentrations of colchicine and dimethyl sulfoxide (DMSO) to screen for the best concentration combination to induce chromosome doubling in ‘Qin Cui’ plants, with a mixture of 0.08% colchicine and 2% DMSO yielding the highest mutagenicity rate of 19.64%. The chromosome doubling effectively improved the salt tolerance of ‘Qin Cui’ plants, as manifested by the more compact leaf tissues, shorter but denser and thicker root systems, stronger root vigour, higher photosynthetic efficiency, and more increased antioxidant enzyme activities in tetraploids than diploids. Polyploidy also maintained Na + /K + homeostasis and regulated amino acid metabolism in ‘Qin Cui’ plants, which are also important pathways in enhancing salt tolerance in tetraploids. Therefore, chromosome doubling enhanced the salt tolerance in ‘Qin Cui’ plants by regulating phenotypic changes and stomatal opening, alleviating chlorophyll degradation to prevent photosynthetic impairment, increasing antioxidant enzyme activities, mitigating ion toxicity and regulating amino acid metabolism. The findings of this study could also provide new germplasm resources for apple quality improvement, as well as a theoretical basis for the regulation of apple polyploid resistance.