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Yosef Burger and Arthur A. Schaffer

The relationship between sugar accumulation and sucrose metabolism enzyme activities was studied among seven genotypes of Cucumis melo L., covering the broad genetic range of sucrose accumulation found in the species. The primary determinant correlated with sucrose levels was the genetic variation for developmental loss of soluble acid invertase (AI) activity. Sucrose accumulation in the developing fruit began only when AI activity declined to less than an experimentally determined threshold value, and continued until removal of the fruit from the plant. In addition, the activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and neutral invertase (NI) were all positively correlated with sucrose accumulation among the genotypes. The low-sucrose-accumulating genotypes were characterized by low activities of each of the three enzymes, irrespective of their invertase activities. Final sucrose content was best predicted for each genotype by the number of days the fruit remained attached to the plant while characterized by “sucrose accumulation metabolism,” which was characterized primarily by AI activity less than threshold values, together with SPS, SuSy, and NI activities higher than threshold levels.

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Roni Cohen*, Yosef Burger and Menahem Edelstein

The use of grafted vegetables as one of the alternatives to soil disinfestation with methyl bromide is increasing in Israel. Watermelon (Citrullus lanatus) and melon (Cucumis melo) plants are grafted mainly onto Cucurbita rootstocks for lessening losses due to soil-borne pathogens. The contribution of the rootstock to the grafted plant's resistance depends on the nature of the disease. In general, damage caused by non-specific root-rot pathogens such as Rhizoctonia solani, Macrophomina phaseolina, Monosporascus cannonballus, and Pythium spp. are effectively reduced by using Cucurbita rootstocks. However, these rootstocks provide only partial protection from vascular diseases such as fusarium wilt, in which case better protection can be achieved by grafting susceptible melons onto monogenic fusarium-resistant melon rootstocks. The performance of the grafted plants depends not only on the rootstock but also on the scion response to pathogens and on the effect of the environment on disease development. The response of grafted and non-grafted melons of different cultivars to sudden wilt disease caused by the fungus Monosporascus cannonballus was evaluated in field trials conducted in the fall and spring growing seasons. Significant differences in disease incidence were found among cultivars, between grafted and non-grafted plants, and between seasons. Grafting reduced plant mortality in the spring and fall experiments but prevention of yield losses was more effective in the spring. More emphasis should be given to finding suitable rootstocks and adjusting agrotechniques for successful commercial cultivation of grafted melons in the fall.

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Menahem Edelstein*, Meni Ben-Hur, Ron Cohen, Yosef Burger and Israela Ravina

Excess of boron and salinity in soil and irrigation water can limit the production of melons (Cucumis melo). A greenhouse study was conducted in order to compare the responses of grafted and non-grafted melon plants to combinations of high levels of boron and salinity. Boron levels were 0.25, 0.8, 2.5, 5.0, 10.0 mg·L-1 and salinity levels were 1.8 and 4.6 dS·m-1. Foliar injury caused by boron was more severe in the non-grafted than in the grafted plants. Likewise, boron accumulation in leaf tissue from non-grafted plants was higher than in grafted plants. High salinity led to decreased boron accumulation in the leaves. Fruit yield was decreased only at a boron concentration of 10 mg·L-1, and the decrease in grafted plants was smaller than that in non-grafted plants. A negative correlation was found between boron accumulation in leaves and fruit yield. The results showed that melon plants grafted on Cucurbita rootstock are more tolerant than non-grafted ones to high boron concentrations, and this can probably be explained by the decrease in boron accumulation caused by the rootstock.

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Yosef Burger, Uzi Sa'ar, Asaph Distelfeld, Nurit Katzir, Yelena Yeselson, Shmuel Shen and Arthur A. Schaffer

The sweet cultivars of Cucumis melo are characterized by high sucrose levels, together with low acid levels in the mature fruit flesh. The trait of high sugar accumulation in C. melo fruit is determined by a single recessive gene, suc. High acid content, conferred by a single dominant gene, So, is found only in C. melo varieties that do not accumulate high levels of sugar and are used for nondessert purposes. We combined the genetic traits of high acid content (low pH) and high sugar levels by crossing the nonsweet, high acid C. melo var. flexuosus, `Faqqous' (So/So, Suc/Suc), with high sugar, low acid C. melo genotypes (so/so, suc/suc) and generating the recombinant genotype So/—, suc/suc. Segregating F2 populations derived from the cross between `Faqqous' and a standard high sugar, low acid line showed that the traits of high sugar and low pH were inherited independently of each other. The accumulation of acid and sugar in the developing fruit of a recombinant high acid, high sugar breeding line, A6, were also temporally independent, with acid accumulation preceding the rise in sucrose levels. The low pH of A6 was correlated with the developmental increase in titratable acidity and particularly of citric acid levels. The combination of increased acidity and high sugar provides the melons with a unique taste due to a sugar to acid ratio not present in sweet C. melo cultivars. These results are discussed in terms of the evolution under domestication of C. melo.

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Yosef Burger, Uzi Saar, Nurit Katzir, Harry S. Paris, Yelena Yeselson, Ilan Levin and Arthur A. Schaffer

Fruit sweetness is the major determinant of fruit quality in melons (Cucumis melo L.) and reflects the concentration of the three major soluble sugars, sucrose, glucose, and fructose, present in the fruit flesh. Of these three sugars, sucrose is the prime factor accounting for both the genetic and the environmental variability observed in sugar content of C. melo fruit. Faqqous (subsp. melo var. flexuosus), a cultivar having a low sucrose and total sugar content, was crossed with Noy Yizre'el (subsp. melo var. reticulatus), a cultivar having a high sucrose and total sugar content. F1 plants had a sucrose content averaging slightly higher than that of the low-sucrose parent, indicating that low sucrose content is nearly completely dominant. Segregation in the F2 and backcross progenies indicated that high sucrose accumulation in melon fruit flesh is conferred by a single recessive gene herein designated suc. When the high-sucrose parent was crossed with the moderate-sucrose landrace known as Persia 202 (subsp. melo var. reticulatus), the segregation in the filial and backcross progenies suggested that additional genetic factors affect the amount of sucrose accumulation.