عنوان مقاله [English]
Introduction: Watermelon, originated from warm regions of Africa, is one of the most important vegetable crops used as human food, forage and for producing alcohol. The area under its cultivation in our province was 1031 ha as irrigated and 3829 ha as rainfed. The average fruit yields under above-mentioned conditions were 23784 and 14888kg ha-1, respectively. Watermelon fruits suffer from a nutritional disorder that is attributed to shortage of calcium in developing fruits. The adsorption and uptake process of calcium by plants is comprised of three main stages; the first stage is passing through root cortex into xylem, the second one is its transport within xylem, and the last one is its distribution into leaves and fruits. Plants obtain calcium preferably from soil solution. Calcium moves upward in apoplastic pathways (with transpiration flow), and reaches different organs of the plants through xylem. Blossom-end rot (BER) is a nutritional disorder of watermelon, tomato and pepper that is caused by a shortage of calcium in enlarging fruits. Blossom-end rot of watermelon commonly occurs in our province when fruits are growing rapidly. BER is frequently seen in watermelon genotypes with long fruits, Charleston gray type in particular. Calcium requirement increases at rapidly developing stage of fruit. BER might happen as a result of increased demand of blossom-end rot of fruit (rapidly growing section of the fruits) for calcium. It can occur even when there is abundant calcium in the soil, because demand exceeds supply. Since fruits do not transpire as much as leaves, less calcium is deposited when calcium containing water is transpired, resulting in a localized calcium deficiency in the fruit.
Materials and methods: In order to investigate the effects of foliar application of calcium on BER of watermelon, an experiment was carried out at the agricultural research station of Gonbad, Gonbad-e Qabus, Iran, in 2008. The soil of the experiment site was calcizerol, thermic. The soil was river alluvial plain type and the slope was moderate .The soil was not saline and PH was slightly alkaline. The amount of phosphorus, potassium, iron, boron, and calcium were sufficient but the amount of available zinc was low. The experiment was carried out in a factorial based on randomized complete block design (RCBD) with two factors and three replications. The first factor was two watermelon cultivars (Charleston gray and Charlie), and the second factor was four levels of calcium (0, 3, 5 and 8 gl-1 of calcium chloride). Each plot consisted of four ten-meter long rows. Row and within-row distances were 3 and 0.75 meters, respectively. Three seeds were planted in each hill, and they were thinned to one seedling at first hoeing when they were completely established. Irrigation was applied six times in furrows to avoid moisture stress. Foliar application of calcium was initiated before fruit set and applied with fifteen-day intervals afterwards. Number of healthy fruits, mean fruit weight, and number of fruits affected by blossom-end rot were counted from the two inner rows discarding the first and last plants of the rows. Brix was determined using a hand refractometer (OSK 7926). The data obtained was analyzed by using SAS.
Results and discussion:The effect of foliar application of calcium and genotypes were not significant for fruit length. The interaction of genotype and calcium did not significantly affect fruit length. Calcium application did not have any effect on the length of blossom-end rot affected fruits. There were not any differences between the genotypes studied. These findings were in contrast with those of some other researchers, who reported enhanced fruit length by calcium application. Brix (soluble solid content) is an important trait in watermelon, so that an increase in it raises the marketability of the product. Brix was not significantly affected by calcium, genotypes, and their interaction. The results of analysis of variance showed that fruit rind width of the genotypes was not affected by calcium, whereas calcium and genotype interaction significantly affected fruit rind width at 0.05 probability level. These findings were consistent with those of some other researchers. Fruit yield was not significantly affected by calcium. The effect of calcium on the number of fruits affected by blossom-end rot disorder was significant at 0.01 probability level, while the effect of genotype and calcium interaction on this trait was not significant. The average number of fruits affected by BER were 1667, 1144 and 588 per hectare at 0, 3, 5 and 8 gl-1calcium chloride, respectively. These findings were consistent with those reported by some other researchers.
Conclusion We concluded that under our experiment's conditions, calcium foliar application was ineffective in improving watermelon fruit yield, but it was promising in decreasing blossom-end rot, which reduces marketability of the fruit.