February 2013 Newsletter (vol.15)   >>   Watermelon - One of the Summer Pleasures

Chick Magic

 

Watermelon - One of the Summer Pleasures

 

By GEORGE S. AYERS

Department of Entomology; Michigan State University, East Lansing, MI 48824-1115

 

Taken from the September 2012 edition of the American Bee Journal

 

Watermelon

 

Scientific name: Citrullus lanatus Synonyms: Citrullus vulgaris, Citrullus vulgaris var. citroides, Citrullus citrullus, Citrullus colocynthis var. lanatus, Colocynthis citrullus, Cucubertia citrullus Origin: The species seems most likely to be native to tropical and southern Africa.[6] Plant description: Watermelon is a slender, sprawling, slightly hairy, vining, generally monoecious1 annual made up of individual stems (runners) that generally grow to lengths of 3.5 to15 ft (~1.1 to 4.6 m). The species requires a fairly long growing season that provides high temperatures.
Depending on the cultivar, the deeply lobed leaves vary from 1 to 6 inches (~2.5 to 15.2 cm) in width and from 2 to 10 inches (~5 to 25.4 cm) in length. Watermelon fruits, again depending on the cultivar, vary in size and generally weigh between 10 and 50 lbs (~4.6 kg to 22.7 kg), but there are growers, especially in the southern US, who specialize in producing large melons that might weigh in excess of 100 lbs (45.5 kg). Different cultivars also vary in shape from roundish to oblong and the skin color varies from solid dark green to light green, to striped in a light and dark green pattern. The flesh color of the melon varies from reddish to yellowish and more rarely from light green to white. The seed color varies through the spectrum of white, yellow, brown, black and reddish black. Some seedless cultivars have been developed (See ‘Additional information’ below). The time to maturity also varies with the cultivar.
 

The relatively rare citron melon (variety citroides), sometimes called preserving melon, is a small fruited variety with hard white flesh and is occasionally grown for watermelon rind preserves. It should not to be confused with the true citron, which is a species of Citrus. Visually from the outside this variety is essentially indistinguishable from other watermelons, but can be opened only with great difficulty, and is inedible when raw. The flesh color of this variety varies from light greenish to white. Nearly all cultivars of watermelon are monoecious2, but a relatively few varieties bear hermaphrodite flowers (with both male and female parts in the same flower i.e. perfect) along with staminate flowers, but no pistillate flowers3. Typically the number of pistillate flowers (female or hermaphrodite) is low and as you proceed out a stem there will be 4 to 15 male flowers (seven a common number) followed by a pistillate flower (either female or hermaphroditic). The flowers are pale yellow to greenish and about 3 to 4 cm (1.2 to 1.6 inch) wide and are generally less conspicuous than in other members of the family, squash for example. The flowers are borne singly in the axils (upper angle between the leaf stem and the main stem). The corolla is united into a small tube with five deeply cut lobes. There are three stamens in the male flowers. In the female flowers, the ovary is inferior5, and the style is short and blunt, terminating in a three-lobed stigma.

 

In the hermaphroditic flowers, the stamens and pistil are tightly crowded into the corolla tube. Nectar is secreted at the base of the corolla. There does not seem to be a definite cycle to fruit setting, the fruit setting occurs more or less irregularly throughout the season or at least while the plants are still growing vigorously. Distribution: The species thrives in southern, southwestern and central states, but fast maturing cultivars are needed for northern states. In 2010 the major watermelon production areas, based on area harvested, were in descending order: TX, FL, GA, and CA. Blooming period: Watermelon is a cold intolerant annual and the blooming period will depend to some extent on the variety planted and planting date, which will in turn depend on the local climate. As an example, the general planting dates, and harvesting dates for Texas, Virginia and Arizona are: March 1 and July 31, May 1 and September 15, July1 and October 31, respectively. Importance as a honey plant: Pellett states that the bees visit the blossoms eagerly for nectar and in commercial watermelon areas they make some honey from watermelon, but that it is important in only a few areas. Honey potential: Wolf et al. found the following variation in nectar sugar concentrations to vary across varieties: Sucrose:11.7-20.5%, Glucose: 4.4-6.0%, Fructose: 4.5-5.6%, total sugar: 21.5-32.1%, and that there was a positive relationship between nectar sugar concentration and attractiveness to bees. The bees came earlier in the morning to the high sugar concentration varieties than to the low sugar concentration varieties. Honey: I doubt that any pure watermelon honey has ever been collected. Pollen: Pellett states that the bees eagerly collect watermelon pollen. Additional information:

Pollination considerations

 

Except for the relatively few hermaphroditic varieties, for fruit set, pollen must be transferred from male flowers to female flowers. Although the hermaphrodite flowers are self-fertile, none set seed when bagged to prevent insect pollination unless they are hand pollinated, indicating that some transfer agent is necessary to transfer pollen. The results of hand and open pollination are similar. Stanghellini came to the same conclusion using ‘Royal Jubilee’, presumably not a hermaphrodite variety. Pollination is apparently almost entirely done by insects, and honey bees seem to be the main pollinator, but may not improve melon weight or seed yield. Bees visit the blooms for both nectar and pollen, but because of the small number of flowers, rarely store surpluses of either of these products. Because of this, bees are easily attracted to surrounding, competitive flora. McGregor states that the flowers open 1 to 2 hours after sunrise and the female flower and the male flower just below it open on the same day, The anthers have dehisced when the corolla expands, but the sticky pollen remains with the anthers. Apparently, there is no self-sterility, as pollen from a flower on the same vine is as effective at setting fruit as pollen from another plant. In 1943 Mann showed that the pollen needed to be deposited on all three stigmatic lobes or the melon will be misshapen. His work showed that despite the fact that some pollen tubes will move laterally into adjacent carpels6, the mean difference in cross-sectional areas of the mature melons corresponding to the pollinated and unpollinated carpels was significant at beyond the 1 per cent level.
 

Adlerz, by allowing varying numbers of bee visits, studied the effect of number of bee visits on fruit set and yield. No fruit set occurred if the flowers remained covered and only two of 64 flowers that received one bee visit, and one of 72 receiving two bee visits, set fruit, and in both instances the fruits were small and misshapen. He found that as the number of bee visits increased, both fruit set and yield increased until about eight visits at which point they were equal to what was produced through open pollination (allowing the bees full availability to the plants) and hand pollination. Bees spent less time on male flowers than on female flowers. Maximum times for male and female flowers were 60 and 27 seconds, respectively, the average times being 5.7 and 8.0 seconds, respectively. Upon landing, bees seldom changed positions unless the time on the flower exceeded about 10 sec. This behavior suggests that the bees receive considerable reward from a flower. Because of the sticky nature of the pollen, Adlerz concluded that movement by ‘long staying’ bees was not likely to redistribute previously deposited pollen on the stigma. Taken together, his bee observations and the sticky nature of the pollen helped confirm his final conclusion that total pollination of the stigma was essentially dependent on the number of visits rather than upon movement while on the flowers.

 

This conclusion was important because of the previous study by Mann. Adlerz, using a cutoff point of a minimum of 6 bee visits, found that the effectiveness of bee pollination increased from 6AM to 7AM and then leveled off from 7 to 9 AM in 1959, but there was a general increase from 6 to 9 AM in 1960. In both years, the effectiveness of hand pollination increased from 6 to 10AM (the last time reported). These two studies tend to indicate that bee activity in the morning is of great concern to the melon producer. In his 1959 study, Adlerz found that length of ovary at the time of pollination affected fruit set, and over the of range of 20 mm to 28 mm fruit set increased from 19 to 100 %. The same trend occurred during 1960, but was not pronounced as in 1959. Overall, the factors that appear to improve fruit set include the number of bee visits, possibly the time of the visits, the size of the ovary, plant vigor, and number of melons already set on the vine[1, 9 & 10].

Pollination recommendations McGregor[10] reviewed pollination recommendations that included: One to five hives per acre in relatively small fields, One colony per five acres with the colonies placed in small groups, One colony per acre with the bees placed on opposite sides of a 40 acre field, One colony per every two acres, One bee for each 100 flowers in all parts of the field. McGregor seems to prefer the last recommendation because it automatically incorporates certain important factors such as attraction to surrounding bee forage, climatic conditions etc. Adlerz[1] made his studies in fields with one colony/acre (2.47 colonies per ha) and concluded this exceeded the number necessary to give the 8 bee visits per flower discussed above. In addition to the recommendations reported by McGregor, Delaplane and Mayer supply the following from their search of the literature:
2 to 3 hives per acre (5-7.4 colonies/ha). 1 to 2 colonies per acre (2.5-5 colonies / ha). 0.2-2 colonies per acre (0.5 – 5 colonies per ha). ≥8 honeybee visits per flower (this from Adlerz’s work). They also provide a literature average of 1.8 colonies per acre (4.5 colonies per ha). Free[4], at the end of his review of watermelon pollination, stated, “….there is little actual evidence that the presence of honeybee colonies in a watermelon field increases yield.” He then goes on to cite Goff’s 1931 report which indicated that watermelon set was better on the periphery of an essentially 1000 acre (405 ha) field made up of adjoining fields than in its center. Goff felt this was because there were fewer pollinators (largely honey bees, though he also lists seven halictid species7) in the center of the field than on the periphery.

 

Goff also mentioned a field that had only a tenth the number of bees as another; the one with a lower number of bees had a lower fruit set than the other. In both cases, no actual data was provided. The use of bee attractants to raise the population of bees in watermelon:
The use of bee attractants to increase honey bee populations in watermelons is not very promising.  Schultheis found the use of Bee-Scent or Beeline provided “no significant improvement in cucumber or watermelon yield or monetary returns”.  Ambrose apparently reporting the same experiment in another journal, recommended “….growers shoud invest financial resources in renting additional colonies of bees for pollination of cucumbers and watermelons rather than investing in commercial honey bee attractants.” Loper and Roselle found that while Bee-scent did increase bee visitation for up to two days in watermelon fields, watermelon yields were not increased.

 

Pollination efficiency of honey bees versus bumble bees:
Honey bees are experiencing population decline for a variety of reasons and Stanghellini set out to study the pollination efficiency of the honey bee versus the commercially available bumble bee Bombus impatiens, on both watermelon and cucumber by assessing the abortion rate that occurred after different numbers of visits by the two bees.  For cucumber, at the different visitation levels, the abortion rates of fruit visited by B. impatiens was consistently lower or equal to that of the honey bee.  For watermelon, however, statistical significance was essentially not detected except at the single visit level where the abortion rate for the bumble bee was noticeably lower than for the honey bee.  Stanghekkini found that seed production in watermelon at the different visit levels (1, 6, 12 and 18 visits) was always higher for th bumble bee than for the honey bee.  To obtain the same seed production level as the open-pollinated plants with unlimited exposure to both species of bees as well as other pollinators, required 12 visits by the bumble bee and 18 visits by the honey bee.

Seedless watermelon production:
In my opinion, raising something that will naturally enhance a honey display, at for example a farmers market, is one of the better ways that small beekeepers can increase their income from their beekeeping operation.  While seedless watermelons won’t noticeably increase honey production, they might increase the number of perspective clientele that visit the beekeeper’s booth and thereby increase honey sales.  It is in the spirit that I offer the following.  Because of the high seed cost, it also provides critical, though demanding, pollination opportunities.  I provide the advance warning, however, from my personal experience, the production of seedless watermelons is a very exacting process.  If you decide to try it, start small.  Beyond that, however, it’s an interesting story, but one with daunting complexity. 
 

The production of seedless watermelons sounds a little like an oxymoron.  If they don’t produce seeds, from where do the seeds come?  Normally the cells of a watermelon plant have 11 sets of 2 chromosomes, 11 chromosomes from the male and 11 chromosomes from the female for a total of 22 chromosomes consists of two identical sister chromatids held together at a constricted area known as the centromere.  The cell is now ready to divide.  Two sets of fibers, known collectively as the spindle, appear on opposite sides of the cell and the nucleus wall breaks down.  The spindle fibers ‘seem’ to align the chromosomes up on a plane within the cell and then ‘pull’ the two chromatids derived from each of the chromosomes to opposite sides of the cell.  New nuclear membranes and an intervening cell wall are then laid down between what are now two cells, each with 11 sets of two chromosomes (the starting number). 
 

Seedless watermelons start with the production of a tetraploid watermelon that has 11 sets of four chromosomes each, i.e. double the normal number.  While several methods have been used to develop these tetraploids, the most common method is by applying the chemical colchicine to the shoot apex of young seedlings.  Cochicine inhibits spindle formation, so that the chromatid pairs are not pulled apart, and the cell ends up with 11 sets of four chromosomes each (a tetraploid) instead of the normal two.

 

Next a selected tetraploid female is crossed with a selected diploid (2n, with 2 sets of chromosomes) male, which results in an essentially sterile triploid (with 3 chromosomes).  Because the triploid requires the pollen from a diploid plant to set a satisfactory seedless melon, the triploid seeds are planted along with a diploid variety.  It is the pollination of the triplod plants with pollen from the diploid plants that provides the seedless watermelon we buy in the grocery store.  Apparently, because the tiploids that the bee is carrying is diluted, and for maximum production, this last crossing apparently can require greater than 16 visits from pollinating bees.  The seedless melons, of course, need to be distinguishable from the seeded melons produced by the normal diploid plants.  This can best be done by selecting a diploid pollen donor whose fruit is distinguishable from the seedless melons (darker green vs. light green, solid color vs. striped, etc.).  And all of this so Johnny won’t spot seeds at his sister.
 

For the average person, the starting point for producing seedless watermelons is the triploid seed, which is commercially available.  Frequently for the home gardener a mix of triploid and diploid seeds are provided, though you may not be told which is which,  If you decide to try raising seedless watermelons, deal with a  company that is willing to tell you what you are purchasing and to provide suggestions concerning the intricacies of raising what they are selling.  Here are a couple of things to ask about: The triploid seed often provides some problems during germination and is usually best germinated under controlled conditions including warm soil temperatures, 82°F with the radicle end of the seed oriented upward of 45 or 90 degrees .  It is imperative that the plants from the triploid and diploid seed bloom together.  You might also search the web for additional useful information.

References:
1.    Adlerz, W. C. 1966. Honey bee visit numbers and watermelon pollination. Journal of Economic Entomology 59: 28-30.
2.    Ambrose, J. T., J. R. Schultheis, S. B. Bambara, and W. Mangum. 1995. An evaluation of selected commercial bee attractants in the pollination of cucumbers and watermelons.  American Bee Journal 135:267-272.
3.    Delaplane, K. S. and D. F. Mayer. 2000. Crop Pollination by Bees. CABI Publishing.  New York.
4.    Free, J. B. 1993. Insect Pollination of Crops (2nd Edition). Academic Press. London.
5.    Goff, C. C. 1937. Importance of bees in the production of watermelons. Florida Entomologist 20:30-31.
6.    Liberty Hyde Bailey Hortorium Staff. 1976. Hortus Third. A concise Dictionary of Plants Cultivated in the United States and Canada.  Macmillan Publishing Co. Inc. New York.
7.    Loper, G. M. and R. M. Roselle. 1991. Experimental use of BeeScent to influence honey bee visitation and yield of watermelon. American Bee Journal 131:777.
8.    Maynard, D. N. and G. W. Elmstrom. 1992. Triploid watermelon production practices and varieties. Acta Horticulturae 318:169-178.
9.    Mann, L. K. 1943. Fruit shape of watermelon as affected by placement of pollen on stigma.  Botanical Gazette 105:257-262.
10.    McGregor, S. E. 1976. Insect Pollination of Cultivated crop Plants. Agricultural Handbook No. 496. Agriculture Research Service United States Department of Agriculture. Washington D.C.
11.    Pellett, F. C. 1978. American Honey Plants. Dadant and Sons, Hamilton, IL.
12.    Schlising, R. L. 1993. Cucurbitaceae—Gourd Family. In: The Jepson Manual Higher Plants of California (Hickman, J.C. Editor). University of California Press. Berkeley, CA.
13.    Schultheis, J. R., J. T. Ambrose, S. B. Bambara, and W. A. Mangum. 1994. Selective bee attractants did not improve cucumber and watermelon yield. HortScience 29:155-158.
14.    Spangler, H. G. and J. O. Moffett. 1979. Pollination of melons n greenhouses, Gleanings in Bee Culture 107:17-18.
15.    Stanghellini, M. S., J. T. Ambrose, J. R. Schulthesis. 1997. The effects of honey bee and bumble bee pollination on fruit set and abortion of cucumber and watermelon.  American Bee Journal 137:386-391.
16.    Stanghellini, M. S., J. T. Ambrose, J. R. Schulthesis. 1998. Seed production in watermelon: a comparison between two commercially available pollinators. Hortscience 33:28-30.
17.    Walters, S. A.. 2005. Honey bee pollination requirements for tri-ploid watermelon. Hortscience 40:1268-1270.
18.    Wolf, S., Y. Lensky and N. Paldi. 1999. Genetic variability in flower attractiveness to honeybees (Apis mellifera L.) within the genus Citrullus. HortScience 34:860-863.
19.    U. S. Watermelon Statistics, 1950-2010. Stock No. 89029. Economic Research Service, USDA. Cited June 1 2012. Tables 8 and 34.