Differences in salt tolerance between diploid and autotetraploid apple seedlings exposed to salt stress

• We induced autotetraploid plants of the apple cultivar ‘Hanfu’ and ‘Gala’ using colchicine.
• Diploid and autotetraploid plants were treated with NaCl to assess their salt tolerance.
• Physiological measurements showed that the autotetraploid has enhanced salt tolerance.
• Increased expression of aquaporin genes may cause elevated salt tolerance in the autotetraploid.

Autotetraploid plants of apple (Malus domestica) cultivar ‘Hanfu’ and ‘Gala’ were induced from leaves by colchicine treatment. There were obvious morphological differences in the leaves of diploid and autotetraploid plants. We treated the diploid and autotetraploid apple seedlings with 200 mmol L−1 NaCl for 8 days in 0.5× Hoagland's nutrient solution and measured the physiological characters and the expression of aquaporin genes in leaves at different times during treatment. The results showed that the response of the autotetraploid to salt stress was better than that of the diploid; the relative water content (RWC) decreased in both over time, but the RWC was higher in the autotetraploid than in the diploid for the duration of the salt stress treatment. The proline content and the accumulation of malondialdehyde (MDA) were higher in the diploid than the autotetraploid, with both increasing over the course of the treatment. We also showed that the expression of aquaporin genes (MdPIP1;1, and MdTIP1;1) decreased initially before peaking at 24 h and then rapidly declining at 48 h and 96 h. The relative expression levels in the autotetraploid were higher than in the diploid at all sampling times, and were 4 to 10-fold higher at 24 h. Our results show that autotetraploid apple possesses better salt tolerance than diploid, which may be associated with the higher levels of expression of aquaporin genes in response to salt stress. The results of our study will make a practical and theoretical contribution to the breeding of polyploid apple cultivars.