Production of Triploid Watermelon Transplants

Charles S. Vavrina

University of Florida, Southwest Florida Research and Education Center, 2686 SR 29 N,
Immokalee, FL 34142-9515

Donald N. Maynard

University of Florida, Gulf Coast Research and Education Center, 5007 60th St. E.,
Bradenton, FL 34203-9324

Additional index words. Citrullus lanatus, seedless watermelon

Abstract. Containerized production of triploid watermelon transplants is essential because of the special conditions required for seed germination, emergence, and early plant development not found in open-field situations. Furthermore, the extra cost of seedling production is justified because triploid watermelon seeds costs are six times greater than those of diploid hybrid seeds and 60 times greater than open-pollinated diploid watermelon seeds. One seed per cell should be planted 2-cm deep with the radicle (pointed end) up to reduce seedcoat adherence to the cotyledons. The tray is watered lightly to bring the seed and mix in contact. Stacked trays are placed in a germination chamber at 30 to 32 °C for 2 days or until radicles are visible in the cell drainage holes. The trays are then arranged on benches in a greenhouse with day temperature 21 to 27 °C and night temperature 18 to 21 °C where temperature control can be achieved. Plants are fertilized every 3 days with a solution containing 50 mg·L­1 N from Ca(NO3)2 and KNO3 from cotyledon expansion until the first true leaf is fully expanded, then with a 200 mg·L­1 N solution applied every other day until the second true leaf is fully expanded, finally the fertilizer is reduced for several days before transplanting to the field. Plants are ready for transplanting when the roots are sufficiently developed to permit removal from the cell with the entire growing mix volume intact. This will require 3 to 5 weeks depending on cell size and growing conditions. Using these production methods, transplant return efficiency has been obtained from trial data since 1990 for ten varieties grown in at least 5 of 8 years. The overall average transplant return was 79% with a range of 69% for 'Tiffany' to 89% for 'Queen of Hearts'.

Florida Agricultural Experiment Station contribution R-06348.

 

The guidelines outlined here define the requirements for production of good quality triploid watermelon transplant. However, it must be stressed that buyer preference often dictates final appearance. The buyer, more often than not, tends to err on the side of a more nutritionally hardened plant. Based on the research of Schultheis and Dufault (1994) and more recently Vavrina et al. (1998) buyer perceptions appear grounded in reality. While these guidelines have worked well for the authors, transplant growers are encouraged to develop guidelines corresponding to their local conditions and buyer preference.

Watermelon seed costs in the United States per 1000 seeds are about $3 for open-pollinated varieties, $30 for hybrid diploid varieties and $180

for hybrid triploid varieties. Comparable price relationships exist in other watermelon producing countries.

Diploid watermelon field plantings can be established by direct seeding or by transplanting containerized seedlings. As the use of hybrid varieties increases, there is a trend towards increased use of seedlings. This is due in part to higher seed costs for hybrid varieties and in part to the possibility of higher early and total production. An additional benefit of use of transplants is that they are exposed to the possibility of unfavorable conditions in the field for less time than direct-seeded crops, this is particularly important in temperate production areas.

 

 

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ling vigor, they frequently adhere to the cotyledons and inhibit their normal expansion. Maynard (1989) showed that orienting the seed with the radicle end up at planting greatly reduces seedcoat adherence (Figure 3) without affecting emergence (Table 3). Apparently, the seedcoat is sloughed off before emergence as the radicle moves in response to gravity from a superior to an inferior position with respect to the seed. Nascimento and West (1998) reported that muskmelon (Cucumis melo L.) seedcoat adherence was reduced not only by placement orientation, but by priming as well. Horizontal or radicle-down seed orientation also reduced seedling leaf area and dry matter accumulation whether seed was primed or not.

Standardized growing mix. Use of a standardized growing mix is essential for repeatable and predictable results. High quality prepared mixes are available and may be favored by small growers for the ease of use. Larger growers may favor fabrication of mixes from standardized components. Most commercial plant growing mixes contain peat moss (70%) in combination with a proportion of horticultural vermiculite (30%) or horticultural perlite with lime, small amounts of N­(predominantly NO3)­P­K and micronutrients together with a wetting agent (Boodley and Sheldrake, 1973).

Seed germination. Triploid watermelon seed are more susceptible to anoxia if over watered than diploid watermelon seed, therefore proper

Figure 1. Containerized transplants for field setting.

Triploid watermelon field establishment, on the other hand, must be made only with containerized seedlings (Figure 1). From the foregoing, it is obvious that high seed costs are a major consid eration in this decision. In addition, field seeding of triploid watermelons results in poor stands because of the dense seedcoat and comparatively low seedling vigor of triploids.

Transplant production practices

Cell volume. Watermelon yields usually increase with increased cell volume (Table 1). However, the additional greenhouse space required for production, extra growing mix necessary to fill larger cells, and longer production times required for sufficient root development to enable extraction of an intact root mass may make use of cells with volumes >36 to 40 cm3 impractical.

Age. It is generally advantageous to time the seedling production period so that weather conditions in the field are favorable for land preparation and transplanting when seedlings are ready. Excellent seedlings can be produced in 3 to 5 weeks with the preference for younger plants. The deciding factors are ease of root mass extraction from the cell and compatibility of seedling size with equipment or manual transplanting procedures. Seedlings may be held for extended periods (Figure 2) without adversely affecting yields (Table 2).

Seed protectant. Routine use of a seed protectant against plant pathogens such as Thiram 50W (tetramethylthiuram disulfide) is recommended because of the comparatively low vigor of triploid watermelon seeds which results in an extended period from seeding to emergence.

Seedcoat adherence. Because of the dense seedcoat of triploid watermelon and reduced seed

Table 1. Watermelon yield as affected by transplant cell volume.

Cell vol Yield

Investigator Year (cm3) (t·ha­1)

Hall 1989 19 46

40 74

Significance *

Vavrina et al. 1993 19 49

31 50

66 52

Significance ns

Liu and Latimer 1995 18 76

26 88

36 92

46 92

80 116

Significance L**

ns,*,**Nonsignificant or significant at P = 0.05 or 0.01; L = linear.

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the seed which can result in uneven germination. A good rule of thumb is to only stack trays 10 high before making a new stack and to allow sufficient space between stacks to provide for good air movement.

Plant production. Trays from the germination chamber are arranged on greenhouse benches or rails. Where possible, day temperatures are maintained at 21 to 27 °C and night temperatures are set at 18 to 21 °C. Plants are fertilized every 3 days with a solution containing 50 mg·L­1 N from Ca(NO3)2 and KNO3 from cotyledon expansion until the first true leaf is fully expanded, then with a 200 mg·L­1 N solution applied every other day until the second true leaf is fully expanded, finally the fertilizer is reduced for several days before transplanting to the field (i.e., hardening off). Plants are ready for transplanting when the roots are sufficiently developed to permit removal from the cell with the entire growing mix volume intact. This will require 3 to 5 weeks depending on cell size and growing conditions. A conditioning ap

Figure 2. Although not ideal, overgrown transplants perform well in the field.

Table 2. Watermelon yield as affected by transplant age.

Age Yield

Investigator Year (weeks) (t·ha­1)

Vavrina et al. 1993 3 99

5 103

7 97

9 96

11 86

13 96

Significance ns

Olson et al. 1995 3 50

4 50

5 52

Significance ns

nsNonsignificant.

soil moisture is critical at seeding. It is advised to let well watered trays sit for 24 h before seeding triploid watermelon seed. The seeded trays should be watered lightly to bring the seed and mix into direct contact before stacking and covering with a polyethylene sheet. The stacked, covered trays are placed in a germination chamber at 30 to 32 °C. Trays are removed from the chamber when roots are first visible in the cell drainage holes. This usually occurs in 2 or 3 days.

Covering with polyethylene is not recommended if seed is germinated directly in the greenhouse as excessive heat may be generated, especially in fall plantings. If a germination chamber is not available trays may be stacked in an environmentally conducive area (i.e., cool in summer, warm in winter) until germination occurs. Stacking trays, particularly styrofoam trays, may create major variations in temperature in the vicinity of

Figure 3. Orienting seeds at planting with the radicle (pointed) end up reduces seedcoat adherence to the cotyledons.

Table 3. Effect of triploid watermelon seed orientation on seedling emergence and seedcoat adherence.

Seed orientation Emergence (%) Adherence (%)

Radicle down, 90o 62 az 84 a

Radicle down, 45o 62 a 69 b

Radicle horizontal 63 a 62 b

Radicle up, 45o 63 a 19 c

Radicle up, 90o 61 a 19 c

zMeans in a column followed by the same letter are not significantly different, 0.05 level.

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Transplant return efficiency. Transplant return efficiency is a measure of the effectiveness of seedling production practices and of seed quality. It is obtained by calculating the proportion of seeds that produce high-quality transplants and multiplying by 100 to obtain a percentage. Using the production practices outlined here, the average transplant return efficiency of 10 triploid watermelon varieties assessed in at least 5 of the last 8 years was 79% with a range of 69% for 'Tiffany' to 89% for 'Queen of Hearts' (Table 4).

Literature cited

Boodley, J.W. and R. Sheldrake, Jr. 1973. Cornell peat-lite mixes for commercial plant growing. N.Y. Ext. Info. Bul. 43.

Hall, M.R. 1989. Cell size of seedling containers influences early vine growth and yield of transplanted watermelons. HortScience 24:771­773.

Liu, A. and J.G. Latimer. 1995. Root cell volume in the planter flat affects watermelon seedling development and fruit yield. HortScience 30:242­246.

Maynard, D.N. 1989. Triploid watermelon seed orientation affects seedcoat adherence on emerged cotyledons. HortScience 24:603­604.

Nascimento, W.M. and S.H. West. 1998. Priming and seed orientation affect seed coat adherence and seedling development of muskmelon transplants. HortScience 33:847­848.

Olson, S.M., G.J. Hochmuth, and R.C. Hochmuth. 1995. Using transplants in watermelon production, p. 30­31 In: G.J. Hochmuth and D.N. Maynard. Vegetable crops proceedings, Fla. Agr. Conf. and Trade Show.

Schultheis, J.R. and R.J. Dufault. 1994. Watermelon seedling growth, fruit yield, and quality following pretransplant nutritional conditioning. HortScience 29:1264­1268.

Vavrina, C.S., S. Olson, and J.A. Cornell. 1993. Watermelon transplant age: Influence on fruit yield. HortScience 28:789­790.

Vavrina, C.S., G.J. Hochmuth, J.A. Cornell, and S.M. Olson. 1998. Nitrogen fertilization of FL-grown tomato transplants: Seasonal variation in greenhouse and field performance. HortScience 33:251­254.

Figure 4. 'Leggy' transplants (r) are difficult to transplant and do not perform well.

Table 4. Transplant return efficiencyz of triploid watermelon varieties.

Variety Years Percent

Crimson Trio 8 88

Genesis 5 84

Millionaire 6 71

King of Hearts 6 85

Nova 5 76

Queen of Hearts 6 89

Scarlet Trio 7 77

Summersweet 5244 5 82

Tiffany 5 69

Tri-X-313 8 73

zNumber of usable transplants/no. of seed planted x 100.

plication of a water soluble complete fertilizer the day of field setting is suggested. Pest management is practiced rigidly throughout the seedling production period.

Watermelon transplants tend to become leggy (Figure 4) if over watered or over fertilized. Leggy transplants are more susceptible to breakage during transplanting and wind damage. Transplant growers should not irrigate or fertigate by schedule alone, but determine plant needs as related to weather (e.g., do not apply water or fertilizer during extended periods of cloudiness, etc.). Recently, Vavrina et al. (1998) showed that fertilization rates used to produce spring tomato (Lycopersicon esculentum Mill.) transplants actually reduced yields when used for fall transplant production. Schultheis and Dufault (1994) have shown that increasing watermelon transplant N from 25 to 75 to 225 mg·L­1 resulted in an increase in transplant shock and the incidence of hollow heart in `Queen of Hearts'.

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