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Plant defensins (PDFs) are small cysteine rich peptides with potential antifungal activity. In this work, a defensin-like cDNA of 237 bp was cloned from watermelon and named Cldef2.2 (GenBank accession No. KC481268). The amino acid sequence deduced from the coding region comprised 78 amino acids with a predicted molecular mass of 8.65 KD and isoelectric point of 8.325. Sequence alignment showed that Cldef2.2 had high amino acid homology to known PDF proteins from other plant species and contained the highly conserved eight cysteine motif. Phylogenetic analysis indicated that Cldef2.2 belongs to the Arabidopsis PDF2 cluster and is close to AtPDF2.5. Real-time PCR analysis revealed that Cldef2.2 was expressed in all the examined tissues, including leaves, roots, and stems. The highest expression level was observed in roots. We also investigated the differential expression profiles of Cldef2.2 gene in response to signalling molecules such as salicylic acid (SA), methyl jasmonate (MeJA) and ethylene or in response to Fusarium oxyporum f. sp. niveum (FON) challenge. For signalling molecules, Cldef2.2 was shown to be quickly induced by MeJA at 4 h after treatment (hat), while its response to ethylene and SA was relatively slow with the expression peaking at 48 hat. For FON challenge, Cldef2.2 showed highly responsive with mRNA accumulation peaking at 4 h after inoculation (hai).

To explore the physiological bases of the resistance of grafted watermelon to Fusarium wilt, watermelon ‘Sumi 1’ (SM1) was grafted onto bottle gourd rootstock ‘Chaofengkangshengwang’ (CFKSW) using the hole insertion grafting method. Grafted watermelon (SM1/CFKSW) and non-grafted plants were planted in soil infested with Fusarium oxysporum f. sp. niveum race 1 (FON 1). Resistance of grafted and non-grafted watermelon and rootstock against Fusarium wilt was evaluated, and the activities of plant defense enzymes, contents of malondialdehyde (MDA), along with activities of pathogenesis related proteases were determined. Results showed that grafted plants (2.3% disease incidence) were more resistant to FON 1 compared to non-grafted ones (95.7% disease incidence). Phenylalanine ammonia lyase (PAL) activity increased in all analyzed plants, but was higher in grafted plants and rootstock than in non-grafted plants up to 120 h after inoculation (HAI). The activities of the antioxidants peroxidase (POD) and polypheno1 oxidase (PPO) significantly increased in all plants within 12 HAI, and activities in grafted plants and rootstock remained at higher levels up to 120 HAI, whereas the level in SM1 gradually decreased. Content of malondialdehyde (MDA) in grafted plants and rootstock gradually decreased after 12 HAI and reached the lowest level at 120 HAI, while in SM1 the MDA content increased since FON 1 inoculation and was much higher than the other two samples at 120 HAI. By 24 HAI and 48 HAI, higher levels of I²-1,3-glucanase and chitinase accumulated in the grafted plants and rootstock, respectively. The grafted watermelon provided an intermediated resistant response to FON 1 under the experimental conditions of this study. These findings suggested that the resistance of grafted watermelon is associated with the high levels of defense enzymes and PR proteases in grafted plants.

Genomic DNA of watermelon was analyzed by optimizing the concentration of Mg 2+ , dNTPs, and primer in RAPD and ISSR analysis systems. The screened optimization system of RAPD for watermelon genomic DNA was 20 μl, which contained primer 0.4 μM L -1 , dNTPs 200 μM L -1 , Mg 2+ 2.0 mM L -1 , DNA 20 ng, 1×buffer, Taq E 1.0 U. The optimization system of ISSR was 16 μl, which contained primer 0.25 μM L -1 , dNTPs 160 μM L -1 , Mg 2+ 1.0 mM L -1 , DNA 20 ng, 1×buffer, Taq E 0.64 U. RAPD and ISSR were applied in polymorphism analyses of watermelon mapping parents, 64-1⊗-3 and 6⊗-2, and 23⊗-1 and 4-1⊗-2, which were inbred lines of district variety in China. The results of mapping parents 64-1⊗-3 and 6⊗-2 showed that 170 out of 181 RAPD primers generated 525 amplified bands, 55 produced 77 polymorphic bands, and the polymorphism rate was 14.67%; 45 out of 62 ISSR primers generated 188 amplified bands with an average of 4.18 bands per primer. And 15 polymorphic primers produced 23 polymorphic loci. The polymorphism rate of ISSR analyses was 12.23%. In mapping parents 23⊗-1 and 4-1⊗-2, 456 out of 499 RAPD primers amplified 1431 bands and 142 polymorphic primers produced 240 polymorphic loci, so the polymorphism rate was 16.77%. These showed that the genetic background of watermelon mapping parents was narrow.

The effects of changes of chlorophyll, carotenoid, anthocyanin, flavonoid, total phenols contents and pericarp color in the two late maturing longan cultivars (Lidongben and Jiuyuewu) during the time of fruit fresh keeping on tree were investigated. The results showed that the pericarp color was related with the contents of chlorophyll and carotenoid during fresh keeping period. When chlorophyll content declined, carotenoid content rose, pericarp color became dark in both cultivars. The pericarp total phenols content in Lidongben kept high and declined slowly, the circumstance of Jiuyuewu was opposite. We also found the total phenols content in Lidongben which led to browning which was higher than Jiuyuewu, but keeping stable relatively in the early days that would slow down the process of browning. The anthocyanin and flavonoid had little effect on the color change of fruit peel.