Comparative Analysis Of Seedling Growth And Photosynthetic Physiological Features Of Hippophae Goniocarpa And Its Parents

Natural hybrid is a common phenomenon in plant kingdom,which plays an important role in plant evolution and is an important way of speciation.Natural hybrid maintains and enriches species diversity by enhancing reproductive isolation or forming stable hybrid zones.The hybrid zone produced by the natural hybrid after the differentiated species is contacted again,contains various genotypes,and is a natural laboratory for studying species evolution.According to the hybrid zone theory,stable hybrid zone are maintained by the balance between selection and diffusion,namely,the combined effect of endogenous selection and exogenous selection.Comparative studies on the fitness of parents and hybrids in the hybrid zone at different stages of life cycle can provide evidence for the maintenance mechanism of the hybrid zone.Hybrids can exhibit a unique combination of their parents'physiological characteristics that can affect the fitness of the hybrid offspring and the evolutionary trajectory of the hybrid zone.Natural hybrid in the area where Hippophae rhamnoides ssp.sinensis and Hippophae neurocarpa ssp.neurocarpa intersect to produce the natural hybrid zone where the parent and hybrid Hippophae goniocarpa intermingle.The following three aspects were studied in this paper through laboratory control experiments and field experiments:Firstly,the seed germination and seedling growth characteristics of H.goniocarpa and its parents were analyzed in order to explore the fitness of three species of Hippophae in the early stage of life history.Secondly,through the analysis of photosynthetic and physiological characteristics of the three species of Hippophae seedlings,the adaptability differences of the three species of Hippophae under different temperature conditions were compared and analyzed.Finally,in order to provide a theoretical basis for revealing the maintenance mechanism of the hybrid zone from physiological point of view,the diurnal variation of photosynthesis of three adult seabuckthorns?male and female?plants was studied in natural hybrid zone.The main results are as follows:1.H.goniocarpa and its parent species,H.rhamnoides ssp.Sinensis and H.neurocarpa ssp.neurocarpa,were germinated at 15°C,20°C,25°C,30°C.The germination rate of H.rhamnoides ssp.Sinensis was the highest at 92±7.89%-100±3.03%.At higher temperature,the germination initiation time was earlier,the germination process was shorter and the germination rate was faster,but the germination time was longer at lower temperature.The germination initiation time of H.neurocarpa ssp.neurocarpa was earlier at four temperature treatments,and the germination process was the shortest,and the germination rate was the fastest,but the high temperature restricted the germination of H.neurocarpa ssp.neurocarpa seeds.At a higher temperature of 30°C,its germination rate is the lowest.The germination rate of H.goniocarpa at higher temperature?25°C,30°C?is higher,its germination rate is slow,the germination start time and germination history and the parent species?H.rhamnoides ssp.Sinensis?show similar characteristics.Three three species of Hippophae adopt different germination strategies to adapt to different altitudes.H.neurocarpa ssp.neurocarpa is a“adventurous”type,H.rhamnoides ssp.Sinensis and H.goniocarpa are“slow-eating”and“transitional”.2.Seedlings of three species of Hippophae were cultured at different temperatures for two months.Lower and higher temperatures have a certain effect on the growth of three species of Hippophae seedlings.There was no significant difference in the survival rate and plant height of the three species of Hippophae at15°C,but the difference was significant at 30°C.At this time,the survival rate,plant height and specific leaf area of H.neurocarpa ssp.neurocarpa were significantly lower than the other two seabuckthorns.High temperature showed significant inhibition on the survival and growth of H.neurocarpa ssp.neurocarpa.The plant height,leaf number and leaf area of H.rhamnoides ssp.sinensisn under four temperature gradients are larger.However,low temperature has a certain influence on the growth of its seedlings.Its specific leaf area at 15°C is significantly smaller than that of H.neurocarpa ssp.neurocarpa,and the plant height,leaf number,leaf area and specific leaf area at this temperature are lower than other temperatures.H.goniocarpa has maintained the intervening level at the other three temperatures except for the lowest survival rate at 15°C.The plant height and specific leaf area were higher at25°C,and the growth traits at lower and higher temperatures did not exceed the parents.3.Biomass distribution and root to shoot ratio of three species of Hippophae seedlings cultured at four temperatures for two months were studied.Lower temperature and higher temperature have a certain degree of inhibition on the growth of seabuckthorns'seedlings.At 15°C,H.neurocarpa ssp.neurocarpa gives priority to the allocation of nutrients to the growth of stem organs,The leaf biomass of H.rhamnoides ssp.sinensis and H.goniocarpa is relatively large.The higher temperature has a significant inhibitory effect on H.neurocarpa ssp.neurocarpa,the total biomass of H.neurocarpa ssp.neurocarpa was significantly lower than the other two species of Hippophae and the root to shoot ratio was relatively large At 25°C and 30°C.H.rhamnoides ssp.Sinensis adapts to the temperature range of survival.The total biomass under four temperature gradients was higher than that of H.neurocarpa ssp.neurocarpa,with higher leaf distribution and lower root to shoot ratio.4.There are some differences in photosynthetic physiological indexes of three species of Hippophae seedlings at different temperatures.At 15?,the chlorophyll content of H.neurocarpa ssp.neurocarpa was the highest,and the photosynthetic rate was higher than the other two species of Hippophae seedlings.At the same time,it had lower stomatal conductance,transpiration rate,non-photochemical quenching coefficient and maximum photochemical efficiency under light adaptation.However,at 30?,the photosynthesis rate and water use efficiency of H.neurocarpa ssp.neurocarpa were significantly reduced,the stomatal closure degree was the largest,which result in a decrease of intercellular CO₂ concentration and the higher sensitivity to high temperature.Stomatal restriction is one of the main reasons for the decrease of photosynthetic capacity of H.neurocarpa ssp.neurocarpa at high temperature.H.rhamnoides ssp.sinensis had the strongest water use capacity and showed the highest photosynthetic rate,water use efficiency,actual photochemical efficiency and low non-photochemical quenching coefficient at higher temperatures.The hybrid showed an intermediate state all the time,and the photosynthetic physiology of the hybrid was not more than two parents in the seedling stage.5.The diurnal variation of photosynthetic characteristics of three male and female species of Hippophae on natural hybrid zones was studied.There is no significant difference in the diurnal variation of photosynthesis between different genders of seabuckthorn.The diurnal variation curves of net photosynthetic rate were bimodal or near bimodal,which showed a certain photosynthetic noon break,It is limited by the factors of the pores and is also affected by other factors.H.rhamnoides ssp.sinensis showed high net photosynthetic rate and water use efficiency at 10:00and 14:00 of higher photosynthetic effective radiation and temperature.The highest values are 19.53±5.35?mol·m^-2·s^-11 and 6.72±0.95?mol·mmol^-1,respectively,and have lower stomatal limit values.The photosynthetic capacity and water use efficiency of H.neurocarpa ssp.neurocarpa and H.goniocarpa were limited by photosynthetic effective radiation and air temperature to some extent.At 8:00,which is relatively low in photosynthetically active radiation,there is a higher net photosynthetic rate value,with a maximum of 15.27±2.43?mol·m^-2·s^-11 and13.43±3.43?mol·m^-2·s^-16.The parental H.rhamnoides ssp.sinensis has strong photosynthetic physiological adaptability at the mid-low altitude and high temperature in the seedling stage and adult stage.In the photosynthetic physiology,H.neurocarpa ssp.neurocarpa is resistant to high altitude and low temperature environment.The hybrid species H.goniocarpa does not exhibit higher photosynthetic physiological adaptability than the parents to compete to occupy new habitats.