Studies On Cross Breeding And Flood Tolerance Of Poplar

Poplar, a widely cultivated tree species in the world, was of significance for meeting the inconsistency between increase of woody products requirement and scarce of provision. In china, poplar was one of the most important tree species for afforestation in woody production forests and ecology protection forests.In the present study, we adopted Aigerious section poplar clones as our research materials for their maximum economic values. The study was focused on cross breeding and flood tolerance of poplar to breed some new clones with characteris of fast-growing and strong tolerance.The major results are as follows:1. Cultured in vitro, time courses of viabilities of poplar pollens stored in different temperatures were studied, as well as the feasibilities of the stored pollens used for cross breeding. Results showed that the optimum concentrations of sucrose and boric acid for germination of I-63 and Liaoning poplar pollens were, respectively, 20%, 100 mg/L and 15%, 100 mg/L when medium containing 0.7% agar+300 mg/L CaCl₂+200 mg/L MgSO₄+100 mg/L KNO₃ The viabilities of I-63 and Liaoning poplar pollens after one-year storage in -20℃were 23.77% and 34.19%, respectively, decreasing 48.51% and 62.23% compared with fresh pollens. The length of pollen tube decreased 48.90% and 48.45%, respectively. It was feasible to obtain vigorous seeds using the pollens for pollination in cross breeding, which were stored in 4℃for one year or in - 20℃for two years. The optimum storage temperature was - 20℃.2. Storage capacities of poplar seeds were studied using germination method indoor. Results showed that drying method, moisture content, storage temperature and time all significantly affected viabilities of I-72 and Liaoning poplar seeds, including germination rate, seedling rate, seedling height and radicle length. In terms of storage, poplar seeds were suitable to be slowly dehydrated indoor naturally, but not be rapidly dehydrated by silica dehydration, and the optimum duration was one day. In this way, 10.16% of moisture content could be obtained for Liaoning poplar seeds. Storage capacities of Sect. Aigeiros poplar seeds could be enhanced in low temperature. Seeds of Liaoning poplar preferred cryopereservation below 0℃and -70℃was the best. However, I-72 could get better effect if it was stored at 4℃. After stored for 110 days, viability index would respectively decrease 19.73% and 65.02% compared to fresh seeds for Liaoning poplar seeds in -70℃and I-72 seeds in 4℃. With storage time prolonging, all items related to seed viabilities were declined significantly. For storage of poplar seed, storage temperature was more important than drying method and moisture content of seed.3. Cross breeding was performed using superior poplar clones of Aigeiros Section (Liaoning poplar, I-63,I-69 and I-72) widely cultivated in Hubei Province to breed new poplar clones. The progenies showed abundant genetic variation and obvious heterosis. Seedling growth of height and root-collar diameter both were characters with high heritabilities. Heritabilities in the broad sense of one-year old seedlings with one-year old roots in height and root-collar diameter were respectively 76.74 % and 5.22 %. The general combining ability variable and specific combining ability variable of height were 21.56 % and 78.44 %, respectively, as well as 61.39 % and 38.61 % in root-collar diameter. All of them were consistent with the one-year old seedlings with two-year old roots. Resistance of poplar to rust disease, canker and longicom beetles were controlled by multiple genes. The resistance of progenies to rust disease was obviously relative with their parents, but this kind relativity have not occurred in resistance of canker and longicom beetles. According to height growth of widely cultivated clones, obvious heterosis of higher-parent and genetic gain could be obtained after primary selection. Some progenies had much higher growth than the widely cultivated clones. Compared with Nanlin 895 poplar, the highest progenies increased 14.98% and 18.86%, respectively, in growth of height and root-collar diameter of one-year old seedlings with one-year old roots, as well as 13.87 % and 41.38 % in growth of height and root-collar diameter of one-year old seedlings with two-year old roots, and possessed characters of superior resistance to disease and insects.4. Flood tolerance of 20 clones, including progenies and their parents, were studied to select flood-tolerant clones primarily, as well as to estimate genetic variances in flood tolerance of poplar. Results showed that all the flooded cuttings showed significant reduction in growth of height, root-collar diameter, leaf, and root, as well as total biomass yield. All 20 clones formed hypertrophied lenticels and adventitious roots by Day 6 to 14 of flooding. And for flooded cuttings, net photosynthesis, stomatal conductance, transpiration, and chlorophyll fluorescence decreased significantly compared with the control. After flooding ended, all plants recovered rapidly. Heterosis existed in F1 generations, regardless of flooding condition. Whereas heterosis in flooding condition was lower than that of in watered condition. Under flooding, the maximum heterosis of higher-parent in height and root-collar diameter was 68.63% and 20.83%, respectively. Besides, variance of flood tolerance among progenies was obvious in growth of height and root-collar diameter. Selection criterions of parents in cross breeding were different between watered and flooding. Relative effect values of the specific combining ability (SCA) and relative effect values of the general combining ability (GCA) of parents were more important than their flood tolerance. Progenies with higher level of flood tolerance also could be obtained, although their parents were intolerant. In terms of breeding, height growth would be most important in flooding condition, and root-collar diameter growth was also very useful. Based on data of all measured values, the tested 20 clones were classified into three groups using hierarchical cluster analysis. Clones Lu, E4, E9, E29, A2, A8, A9, B1, B3, B4, and D8, were flood-tolerant. Clones Lf, Ha, Lm, D1, D7, F9, and F21 were moderately flood-tolerant. Clones Sm and F13 were flood-susceptible.5. Responses to soil flooding of two poplar clones with significantly different flood tolerance were studied to illuminate mechanism of poplar flood-tolerance. Results showed that P. deltoides cv.Lux ex. I-69/55 (Lu) was flood-tolerant, whereas P. simonii (Si) was flood-susceptible. They differed in morphological, ecophysiological, and anatomical characteristics when subjected to flooding. The difference between Lu and Si visualized in latter flooding period (8～22 d), but it was closely related to their responses to flooding in former flooding period (1～8 d). In terms of ecophysiology, in former flooding period, Lu could keep a high level of photosynthesis, as well as high free water content and water use efficiency, through inducing pore closure and reducing water lost due to decrease water potential of leaves rapidly. Meantime, chlorophyll of Lu was decomposed quickly to reduce sunlight absorbation and to avoid destruction to photosynthesis system by photooxidation due to light energy overplus. Protein of Lu was also rapidly decomposed to synthesize other substances to enforce its flood tolerance. Reversely, for Si, much water was evaported due to slow water potential decrease of leaves and pore closure in former flooding period, leading disorder of water system directly, as well as much free water transformed into bound water and lowered water use efficiency. Meanwhile, slow decompose of chlorophyll of Si lead light energy overplus and photosynthesis systemâ…¡(PSâ…¡) destruction occurred seriously. Therefore, photosynthesis rate of Si seriously decreased under flooding. As for morphology, all hypertrophied lenticels of Lu were fine during flooding, whereas many hypertrophied lenticels of Si were easy to be infected by bacteria and rot. Furthermore, Lu could form and maintain large root porosities during flooding period. However, Si could form large root porosities, but its roots easily rot to decrease the aerenchyma in latter flooding period. Under submergency, Lu could keep intact leaves structure. Nevertheless, leaf epidermis and structure of Si were destroyed seriously. In terms of anatomy, ultrastructure of leaves of Lu were still intact at the end of flooding, whereas ultrastructure of leaves of Si were destroyed seriously, and many organelles were decomposed. Therefore, the authors thought that flood-tolerant poplar clones were both with mechanisms of avoidng oxygen scarcity and tolerating oxygen scarcity. Whereas, flood-susceptible poplar clones lacked these mechanisms. Under soil flooding, responses of rapidly forming and maintaining hypertrophied lenticels, quickly decreasing water potential and closing pores, keeping high level of photosynthesis and water use efficiency, rapidly decomposing chlorophyll and protein of leaves, enlarging and maintaining root porosities, maintaining intact ultrastructure of leaves etc. were very important to survive and growth of poplar, which could be adopted as references and indexes for selecting flood-tolerant poplar chones.