Watermelon Plants Research Articles

Arbuscular mycorrhizal fungi for vegetable (VT) enhance resistance to Rhizoctonia solani in watermelon by alleviating oxidative stress

The beneficial effects of arbuscular mycorrhizal fungi (AMF) vary depending on the species of fungi and host plants. In this study, we tested 8 mycorrhizal consortiums to explore the best AMF conformation for watermelon plants. The results showed that colonization with mycorrhiza for vegetable (VT) resulted in the highest biomass accumulation, photosynthetic rate, and dependence in watermelon seedlings. VT increased the photosynthetic rate and dry weight by 68.6% and 63.4%, respectively, compared with non-AMF plants. Dose trial on inoculum density for VT revealed that 300 spores per plant caused the highest dry weight, photosynthetic rate, and dependence. Notably, different inoculum density differentially affected endogenous hormone levels, whereas 300 spores/plant caused the highest accumulation of abscisic acid, indole acetic acid, and gibberellic acid. When VT-inoculated plants were infected with soil-borne fungal pathogen Rhizoctonia solani, they showed only 18.4% disease incidence with a 59.7% control effect compared with the 45.7% disease incidence in control plants without VT inoculation. R. solani decreased the photosynthetic rate, chlorophyll a, chlorophyll b, and carotenoid content by 38.8%, 20.5%, 63.4%, and 33.6%, respectively, while VT colonization significantly improved those indexes. Moreover, VT significantly reduced the relative electrolyte leakage, H2O2 accumulation, and lipid peroxidation, and increased the root activity and antioxidant enzymes activities and their transcripts in roots, suggesting mitigation of oxidative stress caused by R. solani. Our results suggest that VT at optimal dose could enhance watermelon resistance to R. solani and thus can be considered as a biological control agent for R. solani management in cucurbits.

Transcriptomic Analysis of Short-Term Salt Stress Response in Watermelon Seedlings.

Watermelon (Citrullus lanatus L.) is a widely popular vegetable fruit crop for human consumption. Soil salinity is among the most critical problems for agricultural production, food security, and sustainability. The transcriptomic and the primary molecular mechanisms that underlie the salt-induced responses in watermelon plants remain uncertain. In this study, the photosynthetic efficiency of photosystem II, free amino acids, and transcriptome profiles of watermelon seedlings exposed to short-term salt stress (300 mM NaCl) were analyzed to identify the genes and pathways associated with response to salt stress. We observed that the maximal photochemical efficiency of photosystem II decreased in salt-stressed plants. Most free amino acids in the leaves of salt-stressed plants increased many folds, while the percent distribution of glutamate and glutamine relative to the amino acid pool decreased. Transcriptome analysis revealed 7622 differentially expressed genes (DEGs) under salt stress, of which 4055 were up-regulated. The GO analysis showed that the molecular function term “transcription factor (TF) activity” was enriched. The assembled transcriptome demonstrated up-regulation of 240 and down-regulation of 194 differentially expressed TFs, of which the members of ERF, WRKY, NAC bHLH, and MYB-related families were over-represented. The functional significance of DEGs associated with endocytosis, amino acid metabolism, nitrogen metabolism, photosynthesis, and hormonal pathways in response to salt stress are discussed. The findings from this study provide novel insights into the salt tolerance mechanism in watermelon.

Elucidation of resistance signaling and identification of powdery mildew resistant mapping loci (ClaPMR2) during watermelon-Podosphaera xanthii interaction using RNA-Seq and whole-genome resequencing approach

Watermelon is an important vegetable crop and is widely cultivated in USA with an approximate global production of > 100 million tons. Powdery mildew (PM) caused by Podosphaera xanthii is a major production-limiting factor on watermelon and other cucurbits. Numerous PM and multiple disease resistant (MDR) watermelon germplasm lines have been developed by the USDA in Charleston, SC. To gain a better understanding of the innate and activated molecular defense mechanisms involved during compatible and incompatible PM-watermelon interactions, we inoculated PM susceptible (USVL677-PMS) and resistant (USVL531-MDR) watermelon plants with 105 conidia ml−1 of P. xanthii. RNA-seq profiling was done on leaf samples collected at 0, 1, 3, and 8 days post inoculation (DPI). A total of 2,566 unique differentially expressed genes (DEGs) were identified between compatible and incompatible interactions with P. xanthii. The compatible interactions resulted in distinct plant gene activation (> twofold unique transcripts, 335:191:1762 :: 1:3:8 DPI) as compared to incompatible interaction (> twofold unique transcripts, 314:681:487 :: 1:3:8 DPI). Further, comparative whole-genome resequencing analysis of USVL531-PMR, USVL677-PMS and four introgressed PM resistant recombinant inbred lines (RIL, USVL531-PMR × USVL677-PMS) were performed to identify the region of PM resistance introgressed break points along with other traits inherent by USVL531-PMR by comparing the SNPs and InDels. Based on SNPs identification and CAPS markers, the resistance gene was identified as ClaPMR2, Citrullus lanatus PM Resistance gene 2 {Chr2 : 26750001 .. 26753327 (−)}, a NBS-LRR resistance protein (R) with homology to the Arabidopsis thaliana PM resistance protein, RPW8. The transcriptome data also revealed a complex regulatory network associated with the introgressed junctions mediated by PM resistance R proteins (R genes) that may involve multiple signal regulators and transducers, carbohydrate metabolism, cell wall modifications and the hormone-signaling pathway.

Mapping a Partial Andromonoecy Locus in Citrullus lanatus Using BSA-Seq and GWAS Approaches.

The sexual expression of watermelon plants is the result of the distribution and occurrence of male, female, bisexual and hermaphrodite flowers on the main and secondary stems. Plants can be monoecious (producing male and female flowers), andromonoecious (producing male and hermaphrodite flowers), or partially andromonoecious (producing male, female, bisexual, and hermaphrodite flowers) within the same plant. Sex determination of individual floral buds and the distribution of the different flower types on the plant, are both controlled by ethylene. A single missense mutation in the ethylene biosynthesis gene CitACS4, is able to promote the conversion of female into hermaphrodite flowers, and therefore of monoecy (genotype MM) into partial andromonoecy (genotype Mm) or andromonoecy (genotype mm). We phenotyped and genotyped, for the M/m locus, a panel of 207 C. lanatus accessions, including five inbreds and hybrids, and found several accessions that were repeatedly phenotyped as PA (partially andromonoecious) in several locations and different years, despite being MM. A cosegregation analysis between a SNV in CitACS4 and the PA phenotype, demonstrated that the occurrence of bisexual and hermaphrodite flowers in a PA line is not dependent on CitACS4, but conferred by an unlinked recessive gene which we called pa. Two different approaches were performed to map the pa gene in the genome of C. lanatus: bulk segregant analysis sequencing (BSA-seq) and genome wide association analysis studies (GWAS). The BSA-seq study was performed using two contrasting bulks, the monoecious M-bulk and the partially andromonoecious PA-bulk, each one generated by pooling DNA from 20 F2 plants. For GWAS, 122 accessions from USDA gene bank, already re-sequenced by genotyping by sequencing (GBS), were used. The combination of the two approaches indicates that pa maps onto a genomic region expanding across 32.24–36.44 Mb in chromosome 1 of watermelon. Fine mapping narrowed down the pa locus to a 867 Kb genomic region containing 101 genes. A number of candidate genes were selected, not only for their function in ethylene biosynthesis and signalling as well as their role in flower development and sex determination, but also by the impact of the SNPs and indels differentially detected in the two sequenced bulks.

Uso de bioestimulante no desenvolvimento inicial de melancieira em solo salino

O uso de bioestimulantes tem sido empregado como uma nova tecnologia que proporciona a atenuação do estresse salino nas plantas, como também, um maior incremento da produção vegetal. O objetivo dessa pesquisa foi avaliar a tolerância e o desenvolvimento inicial da melancia Crinson Sweet submetida a aplicações do bioestimulante VIUSID-AGRO, cultivada em solo salino. Foram testados cinco tratamentos: Salinidade do solo de 0,6 dS m-1 Sem Bioestimulante (SS0,6+SB); Salinidade do solo de 0,6 dS m-1 e Bioestimulante (SS0,6+B); Salinidade do solo de 1,6 dS m-1 e Bioestimulante (SS1,6+BA); Salinidade do solo de 2,6 dS m-1 e Bioestimulante Viusid-Agro (SS2,6+B) e Salinidade do solo de 3,6 dS m-1 e Bioestimulante (SS3,6+BVA), com 4 repetições. A aplicação do bioestimulante nas plantas submetidas a salinidade de 3,6 dSm-1 favoreceu, o número de folhas, massa fresca de haste, massa seca de haste, massa fresca total, porcentagem da massa seca, partição da massa seca de folha e partição da massa seca de haste. O tratamento SS2,6+B incrementou o número de folha, volume de raiz, massa seca de haste, massa seca de raiz e massa fresca total das plantas de melancieira, enquanto que o tratamento SS0,6+B aumentou o número de folhas, massa fresca total, massa seca de folha, massa seca de raiz e massa seca total. A presença do bioestimulante nas plantas submetidas a salinidade de 1,6 dS m-1 favoreceu o comprimento de planta, número de folhas, massa seca de haste e massa fresca total.

Case study for estimating the potential evaporation in tunnel farming in Central Japan

In this research, a method was proposed to estimate the potential evaporation in polytunnel fields. In order to calculate the potential evaporation by the Penman method, as part of an irrigation plan, the climatic conditions inside the tunnels needed to be accurately estimated by adjusting the climatic conditions outside the tunnels. Measurements taken in two polytunnels of watermelon plants from April to June of 2005 and 2006, respectively, revealed that the climatic conditions inside the tunnels differed from those in open fields outside the tunnels. The air temperature inside the tunnels was 2.0 to 4.5 °C higher than that in the open fields, the air saturation deficiency was equivalent to that in the open fields, and there was more or less no wind. The net radiation inside the tunnels was estimated by an equation that uses the transmission factor and shielding factor values. In this study, the crop coefficient of watermelon was assumed to be the ratio of the water consumption to the potential evaporation value estimated by the Penman method, which was almost identical to the design standard value used in irrigation planning in Japan. Furthermore, this study showed that rainfall outside the approximately 1.5-m-wide tunnels did not affect the soil moisture within the tunnels, because the soil moisture inside the tunnels did not increase in response to the 100 mm of rain that fell over two days. Thus, the potential evaporation inside a polytunnel can be estimated with the Penman method and then adjusted with respect to the climatic factor values from neighboring open fields. In irrigation planning, such tunnels should be reclassified as greenhouses, rather than open field sites, due to their lack of access to rainfall.

Effectiveness evaluation of Trichozone for Trichoderma harzianum and Fulzyme for Bacillus subtillus and Pseudomonas fluorescens in curbing causes of charcoal rot disease on the watermelon plant

The results of the study showed the inhibitory effect of the preparation of Trichozone and Fulzyme included the biocontrol agents Trichoderma harzianum and Fulzyme bacterial preparation in Bacillus subtillus and Pseudomonas fluorescens in the laboratory in the growth of colonies of the pathogenic fungus Macrophomina Phaseolina on the PDA agricultural medium, as the two preparations at three concentrations 2.5, 5 and 7.5% achieved in a case of singularity or their combination together a significant inhibition of the pathogenic fungi colony diameter compared to the control treatment, as the interaction between the two factors of Trichozone and Fulzyme achieved the lowest diameter of the growth of that colony as it reached 2.3 cm and a rate of inhibition of 74.44% at the third concentration 7.5%. The results of the study showed the effectiveness of Trichozone and Fulzyme in field to inhibit the charcoal disease on the watermelon plant caused by the pathogenic fungus M. phaseolina, where significant superiority was achieved in treating the combined of the trichozone and Fulzyme and by treating the seeds and watering the seedlings in reducing the severity of infection and reducing the loss in the studied growth characteristics of wet biological weight, dry weight of the root system, plant length and chlorophyll ratio, with an effect average reached 0.17, 1212.1 gm, 3.43 gm, 241.3 cm and 62.8 spad, respectively. Control of pathogen contaminated fungus and untreated achived averages of 0.63, 731.6 gm, 1.81 gm, 145.2 cm and 50.7 spad, respectively. Moreover, the results of the study showed a significant difference in the effect of the same treatment with the difference in the method of application that treatment, the method of using the seed treatment and watering the seedlings superiority on the seed treatment only and for all studied treatments.

Application strategies of saline water and nitrogen doses in mini watermelon cultivation

Salt stress is highlighted as one of the limiting factors for the establishment of agriculture in the semiarid region of Northeastern Brazil. In this context, it is essential to look for new strategies aiming at minimizing the effects of salt stress on the crops. The present work aimed to evaluate the photochemical efficiency, photosynthetic pigments, and growth of the watermelon cv. Sugar Baby under different use strategies with saline waters and nitrogen fertilization. The experiment was conducted in a protected environment at the Center of Technology and Natural Resources of the Federal University of Campina Grande, municipality of Campina Grande, Paraíba. An experimental design in randomized blocks was adopted, arranged in a 6 x 2 factorial scheme, with six management strategies of water salinity and two nitrogen doses (corresponding to 50 and 100% of the recommendation), with five replications. Two salinity levels of the irrigation water were studied, one with low and another with a high level of electrical conductivity of the water (ECw = 0.8 and 3.2 dS m-1). The watermelon cv. Sugar Baby expressed higher sensitivity to salt stress in the flowering phase, with a decrease in the synthesis of chlorophyll b, chlorophyll total, and carotenoids. The 50% dose of N provided an increment in the initial fluorescence, stem diameter, and the number of leaves of the watermelon cv. Sugar Baby. The length of the main branch of the watermelon plants decreased with the salt stress applied in the fructification phase; however, the fertilization with 100% of N stimulated a higher growth when the irrigation with saline waters was performed at the vegetative and flowering phases.

Fruit Maturity and Quality of Splice-grafted and One-cotyledon Grafted Watermelon

Splice grafting with both cotyledons removed from the rootstock may significantly increase watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] grafting efficiency, eliminate rootstock regrowth, and reduce costs of watermelon transplant production. We evaluated the efficacy of antitranspirant and sucrose treatments on the survival of splice-grafted transplants and assessed the effects of grafting method and rootstocks on fruit yield and quality. First, in a greenhouse experiment, four commercial antitranspirants, applied to rootstock seedlings before splice grafting, increased transplant survival 21 days after grafting (DAG) from 7% to 35% to 68% (P < 0.0001). In a second greenhouse experiment, survival of splice-grafted seedlings was 91% for plants that received 2% sucrose solution + antitranspirant, compared with 67% for plants receiving 2% sucrose alone and 25% for plants that received only water (P < 0.0001). Finally, in a field experiment we compared splice- vs. one-cotyledon grafting with two rootstocks (‘Shintosa Camelforce’ and ‘Tetsukabuto’) vs. nongrafted plants. At 54 days after transplanting (DAT), survival of all grafted transplants averaged 96% with a plant vigor rating of 7.7/10 (10 = most vigorous), compared with 84% survival (5.8/10 vigor rating) for nongrafted transplants. Flowering was delayed by an average of 2 days for splice-grafted watermelon (37 DAT) vs. one-cotyledon grafted and nongrafted plants (P < 0.0001), but harvest date was the same for all treatments (70 DAT). Fruit were harvested 0, 7, and 14 days after fruit reached physiological maturity, and there was no difference in total yield or fruit quality between grafted and nongrafted treatments, with two exceptions. Fruit with splice-grafted ‘Shintosa Camelforce’ rootstock had the firmest flesh (8.2 N) compared with nongrafted transplants (5.3 N), and lycopene increased from 16.7 µg·g−1 at physiological maturity to as high as 31.4 µg·g−1 when harvested 7 days after physiological maturity (P = 0.0002). These results indicate that application of sucrose with antitranspirant to rootstock seedlings before grafting can increase the survival of splice-grafted watermelon, and splice-grafted watermelon perform similarly to one-cotyledon grafted and nongrafted watermelon plants in field production.

Biochar and Trichoderma aureoviride applied to the sandy soil: effect on soil quality and watermelon growth

Trichoderma spp. are an alternative to increase plant growth and as biological control agents of diseases. Biochar added to soil and Trichoderma can result in the enhancement of crop development and can aid in preventing fertilizer run-off, improving plant health, retaining soil moisture, and helping plants through drought periods. However, a knowledge gap remains regarding the combined effect of biochar and Trichoderma on soil quality and crop growth. The objective of the present study was to evaluate this combined effect and show a new approach of biochar as a route of T. aureoviride (T) inoculation. We evaluated three sources of biochar, bean husks, coffee grounds, and coffee husks, with or without T and additional control that was devoid of biochar and T. The association of all biochar sources with T showed a significant increase in watermelon growth and soil quality. Biochar coffee grounds with T increased urease, β-glucosidase, and total organic carbon, showing the potential of this combination in these processes. Watermelon plants cultivated in soil with coffee grounds and T showed plant length and shoot dry biomass increases of 129% and 192%, respectively. Thus, this study found that biochar use plays an important role in the effectiveness of T. aureoviride applications and is a new instrument for sustainable agriculture.

Stachybotriaceae on Cucurbits Demystified: Genetic Diversity and Pathogenicity of Ink Spot Pathogens.

Recently, the incidence of Myrothecium leaf spot, a foliar disease of watermelon, has increased in South Carolina. However, the identity of the fungal species responsible for outbreaks of this disease has not been determined. Sequence data from four partial gene regions were used to conduct Bayesian inference in order to identify 95 isolates of Stachybotriaceae. Isolates were collected in South Carolina between July 2015 and May 2018. In total, six species of Stachybotriaceae were identified on watermelon and two other cucurbits: Albifimbria verrucaria, Gregatothecium humicola, Paramyrothecium foliicola, P. humicola, Xenomyrothecium tongaense, and Xepicula leucotricha. Two species, G. humicola and P. foliicola, were the predominant species found. Within these two species, genetic differences within small spatial scales were detected. Five species (all except Xenomyrothecium tongaense) were tested in experiments to determine their pathogenicity and relative virulence on three hosts grown in rotation in South Carolina. Southern pea plants were less susceptible than watermelon and tomato plants, which were equally susceptible. This constitutes the first reliable report of pathogenicity of any of the five tested species of Stachybotriaceae on these three vegetable crops. Another important finding was that none of the isolates were identified as P. roridum, the species considered to be the only causal agent of Myrothecium leaf spot on cucurbits. We propose the common name "ink spot" for the foliar phase of diseases caused by genera within the family Stachybotriaceae. This name is descriptive and likely to be accepted by growers. To prevent further loss incurred by ink spot, watermelon and tomato crops should be monitored for this disease.

Growth, root morphology and leaf physiology of watermelon as affected by various rates and forms of nitrogen in the hydroponic system

The aim of this study was to determine the effects of different rates and forms of nitrogen (N) on shoot growth and root morphological and leaf physiological responses of watermelon (cv. Crimson Tide F1) under hydroponic growth condition. The nutrient solution experiment was conducted between January - March in 2018 by using an aerated Deep Water Culture (DWC) technique in a fully automated climate room placed in the Plant Physiology Laboratory of Erciyes University, Faculty of Agriculture, Kayseri in Turkey. Plants were tested under two N-Rates (N1: 1000 and N2: 2000 µM N) and three different N-Forms (Am-N: NH4+, Nit-N: NO3-, 50% mixture of both N-Forms Mix-N: NH4+NO3) by growing in 8 L pots filled continuously aerated nutrient solution (modified Hoagland). The experiment was conducted with a completely randomized block design with four replications. From each pot two plants were harvested 42 days after treatment (DAT) by separating into stem, leaf and root fractions. The results indicated that shoot growth, root morphological and leaf physiological responses were significantly (p<0.001) affected by N-Rate, N-Form and N-Rate x N-Form interaction. The lowest performance under sole Am-N supply was achieved, since it severely reduced shoot and root growth and leaf area development as compared to sole Nit-N and Mix-N treatments. Irrespective of N rates, best growth performance in shoot growth was achieved under Mix-N supply, while root growth significantly improved under sole Nit-N supply. All these clearly indicate that the application of sole ammonium (1000 µM N) is detrimentally toxic for hydroponically grown watermelon plants. On the other hand, a 50% mixed of ammonium with nitrate even at a higher dose (N2: 2000 µM ammonium N) can be more advantageous for the growth and development of watermelon plants grown in the hydroponic system. Furthermore, our study showed that the effects of N-Form (Nit-N and Mix-N) on the improvement of shoot growth, root morphology and leaf physiological development and photosynthesis were significantly higher than the effects of N-Rate. Therefore, the application of nitrogen fertilizers in the form of Mix-N could be a useful N management strategy for growth and yield of watermelon plants under hydroponic conditions.

CRESCIMENTO E MARCHA DE ACÚMULO DE NUTRIENTES EM MELANCIA cv. TOP GUN EM VARZÉA IRRIGADA NO TOCANTINS

Um dos empecilhos ao crescimento da área cultivada com melancia tem sido a escassez de informações técnicas, sobretudo relativas à nutrição da cultura, o que pode afetar diretamente na produtividade e qualidade dos frutos. A marcha de absorção de nutrientes é uma ferramenta fundamental para o conhecimento da exigência nutricional da planta em diferentes estádios fenológicos, mostrando a época certa para aplicação dos nutrientes. Diante o exposto, este trabalho teve como objetivo determinar o crescimento e marcha de acúmulo de macronutrientes pela cultura da melancia cv. Top Gun em área de várzea no município da Lagoa da Confusão – TO. O experimento foi conduzido em propriedade rural localizada no município da Lagoa da Confusão – TO, no período de junho a agosto de 2016. O delineamento experimental utilizado foi em blocos casualizados com cinco épocas de coleta das plantas (15, 30, 45, 60 e 75 dias após a semeadura), em quatro repetições, sendo que em cada repetição foram coletadas duas plantas. Após cada coleta, as plantas foram fracionadas em parte aérea (ramas e folhas) e frutos, para determinação da matéria seca constante. Posteriormente realizou-se a determinação dos teores dos nutrientes N, P, K, Ca, Mg e S, sendo utilizado o método de extração por digestão nitro-perclórica. Os nutrientes mais acumulados pelas plantas de melancia durante o ciclo foram K, N, Ca, Mg, P e S, com 101,27, 75,54, 59,45, 14,91, 13,09 e 5,84 kg ha-1, respectivamente. Os nutrientes mais transportados pelos frutos foram K, N, P, Mg, Ca e S, correspondendo a 80,56, 50,96, 11,16, 6,45, 5,93 e 4,88 kg ha-1, respectivamente. O período de maior acúmulo de matéria seca em plantas de melancia cv. Top Gun foi de 30-45 DAS para a parte aérea (ramas e folhas) e 60-75 DAS para os frutos.
 Palavras-chave: Citrullus lanatus L.; desenvolvimento de plantas; macronutrientes; marcha de absorção.

Fungal Endophytes as Biocontrol Agents against the Main Soil-Borne Diseases of Melon and Watermelon in Spain

Watermelon and melon crops are affected by some important soil-borne fungal diseases like carbonaceous rot (Macrophomina phaseolina), collapse (Monosporascus cannonballus), or the most important pathology at an economic level, the Fusarium wilt (Fusarium oxysporum f. sp. niveum, F. oxysporum f. sp. melonis, F. solani f. sp. cucurbitae, Neocosmospora falciformis, and N. keratoplastica). The methods commonly used for their control are often ineffective, thus new approaches, as the use of biological control agents, are constantly being sought. This work aimed to isolate, identify, and test endophytic fungi for their antagonistic properties against the three mentioned diseases. For this, about 350 endophytic fungal strains were isolated from asymptomatic watermelon plants. Among these, 7 fungal species were selected to evaluate their antagonistic potential against 14 pathogens. Dual culture assays allowed to select two Trichoderma strains according to the high inhibition rates observed (up to 93%), that were further employed in melon and watermelon plants, showing that some of the pathogens were controlled in terms of disease incidence, exhibiting a decrease up to 67% for T. lentiforme. In addition, three concentrations of Epicoccum purpurascens extract was selected to evaluate the germicide effect, obtaining significant differences in the growth of the pathogens depending on fermentation parameters.

Salicylic Acid Stimulates Antioxidant Defense and Osmolyte Metabolism to Alleviate Oxidative Stress in Watermelons under Excess Boron.

Boron (B) is a microelement required in vascular plants at a high concentration that produces excess boron and toxicity in many crops. B stress occurs widely and limits plant growth and crop productivity worldwide. Salicylic acid (SA) is an essential hormone in plants and is a phenolic compound. The goal of this work is to explore the role of SA in the alleviation of excess B (10 mg L−1) in watermelon plants at a morphological and biochemical level. Excess boron altered the nutrient concentrations and caused a significant reduction in morphological criteria; chlorophyll a, b, and carotenoids; net photosynthetic rate; and the stomatal conductance and transpiration rate of watermelon seedlings, while intercellular carbon dioxide (CO2) was significantly increased compared to the control plants (0.5 mg L−1 B). Furthermore, excess boron accelerated the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and induced cellular oxidative injury. The application of exogenous SA significantly increased chlorophyll and carotenoid contents in plants exposed to excess B (10 mg L−1), in line with the role of SA in alleviating chlorosis caused by B stress. Exogenously applied SA promoted photosynthesis and, consequently, biomass production in watermelon seedlings treated with a high level of B (10 mg L−1) by reducing B accumulation, lipid peroxidation, and the generation of H2O2, while significantly increasing levels of the most reactive ROS, OH−. SA also activated antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) and protected the seedlings from an ROS induced cellular burst. In conclusion, SA can be used to alleviate the adverse effects of excess boron.

Pumpkin rootstock improves the growth and development of watermelon by enhancing uptake and transport of boron and regulating the gene expression

Boron (B) is an essential trace element that plays a vital role in metabolic and physiological functions of higher plants. The adequate supply of B is important for plant growth and development. Grafting is a technique used to improve the ion uptake and plant growth. In this study, a commercial watermelon cultivar “Zaojia 8424” [Citrullus lanatus (Thunb.) Matsum. and Nakai.] was grafted onto pumpkin (Cucurbita maxima × Cucurbita moschata) rootstock cv. “Qingyan Zhenmu No.1” with an aim to investigate the response of grafted plants to different levels of B supply (0.25 μM, 25 μM and 75 μM B) in the nutrient solution. Self-grafted watermelon plants were used as control. Pumpkin rootstock improved the plant growth, chlorophyll and carotenoid contents, photosynthetic assimilation, stomatal conductance, transpiration rate, B accumulation and up-regulated the expression of NIP5;1, NIP6;1 and B transporter (BOR2, BOR4) genes in the roots and leaves at 25 μM B compared with self-grafted watermelon plants. Moreover, pumpkin rootstock reduced the oxidative stress and cell damage by reducing H2O2 and MDA contents, and down-regulating the expression of PDCD2-1, PDCD2-2 genes. Moreover, it enhanced the antioxidant activity of watermelon by up-regulating the expression of SOD1, SOD2, CAT2-1, and CAT2-2 genes. Based on these observations, we concluded that pumpkin rootstock has ability to improve the plant growth of watermelon by enhancing the B uptake. This study may help adjust the B concentration in the nutrient medium for watermelon production where pumpkin grafted plants are utilized.

A rapid fluorescence-based real-time isothermal assay for the detection of Cucurbit yellow stunting disorder virus in squash and watermelon plants

Cucurbit yellow stunting disorder virus (CYSDV) is a single-stranded positive-sense RNA virus that produces devastating disease in watermelon and squash. Foliar symptoms of CYSDV consist of interveinal yellowing, brittleness, and thickening of older leaves leading to reduced plant vigor. A rapid diagnostic method for CYSDV would facilitate early detection and implementation of best viral-based management practices. We developed a rapid isothermal reverse transcription-recombination polymerase amplification (exo RT-RPA) assay for the detection of CYSDV. The primers and a 6-fluorescein amidite (6-FAM) probe were developed to target the nucleocapsid gene. The real-time assay detected CYSDV at 2.5 pg purified total RNA extracted from CYSDV-infected leaf tissue and corresponded to 10 copies of the target molecule. The assay was specific and did not cross-react with other common cucurbit viruses found in Florida and Georgia. The performance of the exo RT-RPA was evaluated using crude extract from 21 cucurbit field samples and demonstrated that the exo RT-RPA is a rapid procedure, thus providing a promising novel alternative approach for the detection of CYSDV.

Genome Analysis of Melon Necrotic Spot Virus Incursions and Seed Interceptions in Australia

Melon necrotic spot virus (MNSV) was detected in field-grown Cucumis melo (rockmelon) and Citrullus lanatus (watermelon) plants in the Sunraysia district of New South Wales and Victoria, Australia, in 2012, 2013, and 2016, and in two watermelon seed lots tested at the Australian border in 2016. High-throughput sequencing was used to generate near full-length genomes of six isolates detected during the incursions and seed testing. Phylogenetic analysis of the genomes suggests that there have been at least two incursions of MNSV into Australia and none of the field isolates were the same as the isolates detected in seeds. The analysis indicated that one watermelon field sample (L10), the Victorian rockmelon field sample, and two seed interception samples may have European origins. The results showed that two isolates (L8 and L9) from watermelon were divergent from the type MNSV strain (MNSV-GA, D12536.2) and had 99% nucleotide identity to two MNSV isolates from human stool collected in the United States (KY124135.1, KY124136.1). These isolates also had high nucleotide pairwise identity (96%) to a partial sequence from a Spanish MNSV isolate (KT962848.1). The analysis supported the identification of three previously described MNSV genotype groups: EU-LA, Japan melon, and Japan watermelon. To account for the greater diversity of hosts and geographic regions of the MNSV isolates used in this study, it is suggested that the genotype groups EU-LA, Japan melon, and Japan watermelon be renamed to groups I, II, and III, respectively. The divergent isolates L8 and L9 from this study and the stool isolates from the United States formed a fourth genotype group, group IV. Soil collected from the site of the Victorian rockmelon MNSV outbreak was found to contain viable MNSV and the virus vector, a chytrid fungus, Olpidium bornovanus (Sahtiyanci) Karling, 18 months after the initial MNSV detection. This is a first report of O. bornovanus from soil sampled from an MNSV-contaminated site in Australia.

Characterization of Tomato leaf curl New Delhi virus associated with leaf curl and yellowing disease of Watermelon and development of LAMP assay for its detection.

Diseases caused by begomoviruses are becoming the major limiting factors for the production of watermelon in India. Survey for the incidence of plants showing symptoms typical to begomovirus infection was conducted in watermelon fields. The study revealed that 40% of the watermelon plants were showing the yellowing and downward curling symptoms. Twenty infected samples were collected from the different farmer's fields to know the association of begomoviruses. The PCR amplification using begomovirus-specific primers resulted in an expected 1.2kb PCR product indicating the begomovirus association with the watermelon samples. The sequence comparison results of 1.2kb representing partial genome revealed that all sequences obtained from watermelon samples have a nucleotide (nt) identity of more than 98% among them and are maximum homology with Tomato leaf curl New Delhi virus (ToLCNDV). One watermelon sample (WM1) was selected for complete genome amplification using RCA method (rolling-circle amplification). Amplification of DNA B and no amplification of betasatellites and alphasatellite indicated this virus as bipartite. Sequence Demarcation Tool (SDT) analysis of the DNA A component of the WM1 isolate showed the maximum nt identity of 94.6-97.9% and 85.2-95.8% with ToLCNDV infecting cucurbits. The recombinant analysis showed that the genome was likely to be derived from the recombination of already reported begomoviruses (ToLCNDV, ToLCPalV, and MYMIV) infecting diverse crops. The whitefly cryptic species predominant in the begomovirus-infected watermelon fields were identified as Asia-II-5 group. The LAMP assay developed based on coat protein gene sequence was able to detect the ToLCNDV in the infected samples. Visual detection of the LAMP-amplified products was observed with the hydroxy naphthol blue. LAMP assay was also validated with ToLCNDV infected sponge gourd, spine gourd, ivy gourd, ridge gourd, and cucumber. This is the first report of ToLCNDV association with leaf curl and yellowing disease of watermelon from India and World based on complete genome sequencing.

Metabolomics-based biomarkers of Fusarium wilt disease in watermelon plants

Herein, we used a metabolomics approach to identify biomarkers of Fusarium wilt infection in watermelon. The observed metabolic profiles of infected plants were genotype specific and distinct in leaf and stem tissues compared with roots. The leaf tissue of Fusarium wilt-symptomatic plants had detectable or elevated levels of jasmonic acid–isoleucine, abscisic acid, indole-3-acetic acid, and melatonin compared to their asymptomatic and healthy plants of other varieties. In addition, leaf tissue of symptomatic plants showed increased contents of arginine, aspartic acid, citrulline, histidine, valine, lysine, and phthalic acid. These differentiating metabolites can be used to monitor or diagnose Fusarium wilt in watermelon plants.