A study on the phenotypical, physiological and biochemical characteristics of the cold resistant Sanming wild banana (Musa itinerans)

Abstract: 【Objective】Banana plants are generally low in cold resistance, and thus often suffer great yield losses in some cultivation regions in south China, such as Guangdong, Guangxi and Fujian provinces due to the chilling or freezing injuries.Fujian has rich wild banana resources, among which the Sanming wild banana (Musa itinerans) has been identified as the most cold-resistant one.The semi-lethal temperature for tissue cultured Sanming wild banana seedlings is -1.776 ℃, which is about 1 ℃ lower than most of the cultivated bananas.The clarification cold resistance mechanism of Sanming wild banana will undoubtedly provide a solid basis for the improvement of banana cold resistance, which is of great significance for reducing the loss of banana farmers and sustainable development of local banana industry.However, up to now, the mechanism is still unclear.In the present study, experiments were performed to reveal the cold resistance mechanism of Sanming wild banana.【Methods】To explore the phenotypical, physiological and biochemical characteristics that might contribute to the cold resistance of Sanming wild banana, 2-month-old plants were used, and the main banana variety of Fujian province'Tianbaojiao' (Musa acuminata) , a cold susceptible banana variety, was used as the control.We first compared the leaf tissue structures of Sanming wild banana and'Tianbaojiao'using conventional paraffin sections.Then, they were subjected to low temperature treatment (4 ℃, the critical temperature that causes chilling injury on most cultivated bananas) .The phenotypic differences between Sanming wild banana and'Tianbaojiao'banana before and after low temperature treatment were measured using the LemnaTec (Germany) plant phenotypic imaging system.Moreover, by using M-series modulated chlorophyll fluorescence imaging system IMAGING-PAM (Germany, Walz) , the chlorophyll fluorescence parameters on the second leaf of the seedlings prior to and 24 h after low temperature treatment were measured.The measured indexes included the maximal photochemistry efficiency of PS Ⅱ (F_v/F_m) , non-photochemical and photochemical quenching coefficient (qP and qN) , efficient quantum yield [Y (Ⅱ) ], yields of non-regulated and regulated energy dissipation of PSⅡ [Y (NPQ) and Y (NO) ]and electron transport of PSⅡ (ETR) .Additionally, cold resistance related physiological and biochemical indexes, such as antioxidant enzyme activities [including superoxide dismutase (SOD) , peroxidase (POD) , catalase (CAT) and polyphenol oxidase (PPO) ], and the contents of membrane lipid peroxidation products (malondialdehyde, MDA) and osmotants [proline (Pro) and soluble sugars (SS) ] were assayed.【Results】Compared with the cold susceptible'Tianbaojiao'banana, the palisade and spongy tissues of Sanming wild banana leaf were thinner and the cell tense ratio (CTR) was higher, while the spongy ratio (SR) was lower.After low temperature treatment, the Sanming wild banana plants were still upright without injury symptoms in their leaves, while leaf wilting and drooping was found in'Tianbaojiao'banana and typical chilling injuries such as leaf softening and leaf margin browning appeared.After low temperature treatment, the Fv/Fm, qP, qN, Y (Ⅱ) , Y (NPQ) and ETR parameters of Sanming wild banana leaf still remained at a relatively high level.Its Y (NO) value was stable, indicating that low temperature did not cause significant damage on its photosynthesis system.The chlorophyll fluorescence parameters of'Tianbaojiao'banana, however, revealed a decreasing trend with reduced photosynthetic capacity after low temperature treatment.The Y (NPQ) and qN values of'Tianbaojiao'banana decreased slightly, and its Fv/Fm, qP, ETR, Y (Ⅱ) and Y (NO) values were significantly decreased (p < 0.01) , indicating that low temperature treatment damaged its PSⅡ reaction center, induced the photoinhibitory effect, significantly decreased the chloroplast dissipated energy, and affected the carbon assimilation of photosynthesis.Before and after low-temperature treatment, the activities of SOD, POD and PPO, and the content of Pro in Sanming wild banana were all significantly higher than in'Tianbaojiao'banana, suggesting that these physiological and biochemical indexes might contribute to the cold resistance of Sanming wild banana.Low temperature significantly reduced the PPO activity and MDA contents in Sanming wild banana, while those in'Tianbaojiao'banana showed opposite change patterns.Before low temperature treatment, no significant difference in CAT activity was found between the two banana species.However, after low temperature treatment, the CAT activity in Sanming wild banana became significantly higher than in'Tianbaojiao'banana.【Conclusion】The results suggest that the Sanming wild banana is adaptable to temperature due to its complex genetic background, and its ROS scavenging capacity is strong enough to eliminate excessive accumulation of free radicals, and thus reduce the cell membrane leakage.It is deduced that the cold-resistance characteristics of Sanming wild banana is, at least partially, dependent on its higher CTR, antioxidant capacity and proline content.