Physiological Adaptation Mechanisms Of Different Ploidy Kiwifruit Along Elevational Gradients

Polyploidization is an important mechanism for speciation and evolutionary adaptation in plants.Physiological and ecological function will change and may increase stress tolerance,thus polyploids can occupy more extreme environments relative to diploids,which contributes to polyploid establishment.Actinidia is a large perennial woody vine.The natural populations of Actinidia chinensis complex of different ploidy exhibit an alternative distribution pattern along the altitude gradient.Diploid is distributed in low-altitude hilly areas,while high ploidy is distributed in cold high-altitude mountains.The factors responsible for this distribution pattern are still unclear.In order to explore the key environmental factors that cause the differentiation of the distribution of different ploidy kiwifruit,clarify the adaptability of different ploidy kiwifruit to different altitude habitats and their physiological adaptation mechanisms.In this study,we used the niche model to predict the potential suitable areas for different ploidy kiwifruit,the niche differences between ploidy and diploid,and the environmental factors that cause niche difference.Then,the field reciprocal transplant experiment was used to compare the fitness of different ploidy kiwifruit along the altitude gradient.Finally,the related physiological characteristics of different kiwifruit ploidy were determined under the natural altitude gradient,including the ice nucleation temperatures of leaves（INT）,the leaf/stem lethal temperature(LT₅₀),and leaf dehydration resistance ability.The main results were as follows:1 Niche divergence and potential distribution of different ploidy kiwifruit（1）The results of the niche model show that there were a clear geographic pattern in the potential distribution of cytotypes,with diploids occurred in lower altitude Hunan foothills,and tetraploids nearly overlapped with diploids but tended to northern Guizhou and eastern Chongqing,while hexaploids were centred in Guizhou plateau,northwestern Hunan,southwestern Hubei and southern Shanxi under the current climate conditions.With the global warming,the suitable areas of different ploidy kiwifruits would being gradually decreasing,but hexaploid kiwifruit had a smaller decreasing trend.Compared with diploid,polyploid kiwifruit was obviously shifted to high altitude and high latitude area,and had a wider area of highly suitable area.（2）Niche identity test demonstrated overlapped niches of diploid and tetraploid kiwifruits,but the niches between diploid/tetraploid and hexaploid kiwifruits were obviously different.Min temperature of the coldest month（Bio6）and precipitation of the driest month（Bio14）were key environmental factors that driving the niche shift between different cytotypes.Regions occupied with polyploid kiwifruits were characterized by lower Bio6 and Bio14,which indicated more extreme niche in cold and arid mountains for polyploids than diploids.2 Differences in fitness of different ploidy kiwifruits along the altitude gradientThe results of the field reciprocal transplant experiment in 2019 showed that the seed germination rate and seedling survival rate were significantly different at ploidy level and altitude level.The polyploid initially showed higher fitness with the trend of that as the ploidy increased,the germination rate and survival rate increased significantly;as the altitude increased,the germination rate and survival rate were significantly reduced.The results of unary linear regression analysis of environmental factors and fitness indicators showed that ploidy,altitude and temperature did not have a significant impact on the germination stage（germination rate）of kiwifruit seeds,but altitude and temperature had an important impact on the seedling establishment stage（survival rate）.Low temperature and high altitude limited the growth of kiwifruit seedlings.3 Differences in low temperature tolerance of different ploidy kiwifruitThe test results of low temperature tolerance index of different ploidy kiwifruit under natural altitude gradient showed that hexaploid and tetraploid grown in high altitude areas had lower LT₅₀and INT,and LT₅₀was much lower than INT,which was a tolerant plant in terms of strategies to cope with freezing damage,while diploid LT₅₀occurred at the same time as INT,which was an avoidant plant,indicating that polyploid kiwifruit was more resistant to low temperature.In addition,the test results of leaf water physiological indicators showed that hexaploid and tetraploid leaf water physiological indicators were more negative in water potential at turgor loss point(Ψ_TLP),cell elastic modulus（ε）was larger,and leaf mass per area（LMA）was higher,indicating that the polyploid might be more tolerant to physiological drought caused by low temperature.In conclusion,our results revealed that there was a clear niche divergence among different ploidy kiwifruits.Bio6 and Bio14 were key environmental factors that driving the niche shift between different cytotypes.Polyploid kiwifruit occupied extreme niche of high altitude,low temperature and drought.With the global warming in the future,the suitable areas for different ploidy kiwifruits will decrease,but the hexaploid’s are wider and shift to high altitude and latitude zones.At the field reciprocal transplant experiment,polyploid kiwifruit had a higher fitness advantage in different altitude environments.The more,compared with diploid kiwifruit,polyploid kiwifruit had lower LT₅₀and INT,and the cells were more resistant to dehydration.These physiological characteristics enable polyploid kiwifruit to grow normally in high-altitude,low-temperature and dry habitats,and thus can be distributed in higher altitude areas.