| The effect of polyploidy on the genetic variation of AFLP molecular markers, morphological characters of the pollen grain, leaf epidermal anatomy, photosynthetic characteristics of watermelon (Citrullus lanatus) was firstly investigated of different ploidy watermelon (diploid, triploid crossed with 4x×2x and autotetraploid induced by colchicine), and salt-tolerances, chilling-tolerances, waterlogging-tolerances mechanisms among different ploidy were studied. According to the analysis of molecular biology, cytology, physiological and biochemical, the relationship among chromosome, gene dosage and trait expression was discussed. The researches provided some useful information of polyploid breeding and adversity resistance breeding of watermelon. The main results were as follows:1. Amplified fragment length polymorphism (AFLP) was applied to identify the genetic variation of watermelon. We employed 48 AFLP primer combinations on different ploidy, A total of 4800-5000 AFLP bands were observed: AFLP bands of different ploidy watermelon amplified by most AFLP primer pairs were not apparently different, it indicated low genetic variation among 4x, 3x, 2x. 3 bands of Mimei's and 7 bands of JM's were polymorphic, There were 1 specific bands in M2x, 1 specific band in M3x and 1 specific band in M4x when compared with its other ploidy; There were 2 specific bands in JM2x, 2 specific bands in JM3x and 3 specific bands in JM4x when compared with its other ploidy. There were present or absent of specific bands of autotetraploid compared with diploid, this reflected that DNA molecular genetic structure was changed by colchicines and allelic variation of autotetraploid. There were present or absent of specific bands of triploid FI compared both parents, this reflected that gene mutation occurred in the fertilization process after crossing the tetraploid and diploid.2. Morphological characters of the pollen grain of diploid, triploid and tetraploid watermelons were observed with the aid of scanning electron microscope. Population pollen of the diploid watermelon were in good order; their pollen's shape with 3 apertures was subprolate and average P/E ratio was 1.283; The pollen grains of the tetraploid was spheroidal with 3, 4 and more apertures, average pollen's P/E ratios was 1.111; Heteromophic, adhesive and shrivelled pollen were observed on tetraploid watermelon. All of the pollen grains of the triploid were shrivelled. The reasons of lower germination of autotetraploid pollen and abortive pollen of triploid were discussed from palynological observations.3. There were significant variations in leaf anatomy among different ploidy watermelon observed with the aid of scanning electron microscope, the number of stomata and epidermal cells per unit leaf area decreased with increasing ploidy, reflecting larger cells in the tetraploids than that of diploids and triploids. Observing fluorescence of guard cell chloroplasts with a microscope and UV light, leaves from diploid, triploidand tetraploid had 19.9±3.4, 15.9±2.2, 11.5±1.5 chloroplasts per guard cell pair, respectively, the ratio was 4 : 3 : 2. The method can be used to identify the ploidy of watermelon in early stage.4. Under field cultivation conditions, the area per leaf, specific leaf mass, Photosynthetic pigment (chlorophll-a, chlorophll-b and carrotene) contents per unit area leaf of watermelon increased with increasing ploidy, Photosynthetic pigment per unit fresh weight decreased, adversely. There was significant difference among three ploidy watermelons. Net photosynthetic rate (Pn) of different ploidy watermelon showed 'midday depression' phenomenon at anthesis, The Pn diurnal changes varied as a two-peaked curve, its values reached at 10:00 am. and 13:00 pm.; The Pn of first peak was higher than the second. The Pn was M3x>M2x>M4x, because there were high intercellular CO2 concentration (Ci) and lower stomatal resistance (Rs) of M3x and M2x than M4x. M4x had showed serious 'midday depression' phenomenon.5. To explore the salt-toleran...
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