Influence of Handling and Nitrogen Nutrition on Flowering and Growth of Watermelon Transplants in the Greenhouse

Cucurbit Genetics Cooperative Report 12:64-67 (article 27) 1989

R.N. McArdle
Biological and Chemical Sciences, Central Research Division, General Foods USA, 555 S. Broadway, Tarrytown, NY 10591

Efficient, early production of female flowers in watermelon Citrullus lanatus (Thunb.) Matsum. and Nakai is of keen interest to those making controlled crosses or otherwise attempting to produce fruit in a greenhouse setting. I noted on numerous occasions that seedlings started in peat pellets and transplanted at a very late stage were early and precocious flowers. Manipulation of this stress response might prove useful to those requiring greenhouse-grown watermelon fruit, since earlier female formation could led to more rapid fruit set and moreover, better control of the vining habit.

Seeds of ‘Charleston Gray #5’ (CG) and ‘Bush Charleston Gray’ (BGG) were sown in Jiiffy-7 peat pellets held in plastic flats. The flats were watered to runoff daily until the pellets were removed for transplanting to 1-gallon pots (2:1 vermiculite:peat) at either the two-true-leaf stage (early) or 3 weeks afterwards (late); these constituted the 2 levels of the handling treatment. the third treatment was fertilization level, altered by adding 100 ml of 0, 200 or 400 ppm N three times a week as reagent-grade ammonium nitrate in double-distilled water. A completely randomized design of a complete 2x2x3 factorial, replicated 3 times, was used. Flower counts were made at 3 weeks following the delayed transplanting date (approximately 6 weeks from seeding) and again at 6 weeks, at which time the plants were also harvested for dry weight determination. Flower counts at 6 weeks excluded the first 8 nodes on each plant.

Results are presented in Table 1. The two cultivars clearly differed for flowering and growth. CG plants were, as expected, larger and had more flowers, but BCG seemed to produce earlier female flowers (significantly larger node). Average dry plant weight of the two cultivars was similar (although significantly different), but differences in the number of male flowers produced per plant indicate a different flowering response for the two types. Nonetheless, femaleness was not significantly different for these two cultivars. These results seem to suggest that the bush type produces fewer flowers than the vining type, but in the same male-female proportion, and on a shorter, stockier plant.

Time of transplanting had the most dramatic effect of any factor. Late planting significantly reduced growth and flower development of the plants. However, these same seedlings produced the earliest female flowers by a wide margin (fifth node as opposed to ninth). Transplant timing also seemed to reduce the numbers of flowers produced and femaleness (% female flowers) at the earlier measurement date. It seems likely that differences in the number of flowers produced was a direct result of plant development differences, as plant dry weight was severely lowered by delaying transplanting. Femaleness was not significantly different for the two timings at the later measurement date.

Increasing N resulted in small but statistically significant increases in male flowers at six weeks and female flowers at three weeks. Here again the flowering response may be attributable to plant growth, since plant dry weight was highly influenced by N level. Earliness of female development was increased significantly by increased N level, although not substantially. N level promoted femaleness at the three-week measurement, but not at the later date. Although significant nitrogen x time interactions were found for two variables, the data (not shown) merely tended to show a much stronger influence of nitrogen in the late planted seedlings, no doubt an outcome of their poor initial nutritive condition.

Response of earliness to N level was opposite to that indicated by the timing data, and suggests that delayed transplanting causes more than simple nitrogen stress. Certain environmental influences, such as daylength, temperature and application of growth regulators have well-documented influences on watermelon sex expression (2, 3, 4). A field study (1) on watermelon showed little difference in date of first female anthesis under N rates of 0-150 1b/A. Higher N rate did increase the number of females/plant, but percent females was not recorded. Sex expression in Cucumis is known to be influenced by environmental factors, but a recent report (5) showed no effect of increased fertilization on sex expression and earliness of gynoecious cucumber lines. It seems possible that differences between the handling regimes is attributable to more than nutrient stress. Moisture, which was undoubtedly less stable in the delayed transplants, may be involved.

The results suggest that female flowering can be accelerated by late transplanting, but probably at the expense of general plant vigor. Application of N appeared to alter flowering mainly by altering growth response, but the promotion of female earliness by increasing N contradicts earliness induced by late transplanting, a condition one might expect to be related to nutrition. Nutritional differences due to other (unmonitored) consequences of ammonium nitrate application (pH, etc.) are also plausible.

The previously mentioned field study (1) showed fruit set in watermelon to be reduced by low N application rates. A priority of further work will be to test whether stressed watermelon plants can set and produce fruit with appropriate late nutrition.

Table 1. Main treatment and interaction effects for cultivar, transplant timing and nitrogen regime on flowering and growth of watermelon transplants.

Main effect	# Males 3 wk	# Males 6 wk	# Females 3 wk	# Females 6 wk	Node bearing first female	% Female 3 wk	% Female 6 wk	Plant dry weight (g)
Cultivar (Cv)
Charleston Gray	12.4	16.9	1.3	1.6	9.5	7.4	12.1	8.8
Bush Ch Gray	9.3	7.2	1.7	1.4	6.4	13.2	16.4	8.2
Significant z	ns	**	ns	ns	**	ns	ns	*
Transplanting
Early	17.2	14.4	2,7	2.3	9.3	14.5	11.6	12.6
Late	4.6	9.8	0.3	0.8	5.0	6.1	17.2	4.5
Significant	**	**	**	**	**	*	ns	**
N level (ppm)
0	8.0	9.6	1.1	1.3	8.6	9.1	14.8	7.0
200	12.3	13.1	1.4	1.5	8.2	7.7	12.9	7.6
400	12.4	13.6	1.9	1.9	6.6	14.3	15.3	10.9
Significant linear	ns	*	**	ns	*	ns	ns	**
Significant quadratic	ns	ns	ns	ns	ns	*	ns	**
Interactions
Cv x Time	ns	ns	ns	**	ns	ns	ns	*
N x Time	*	ns	ns	ns	ns	**	ns	ns
N x Cv	ns	ns	ns	ns	ns	ns	ns	ns
N x Cv x Time	ns	ns	ns	ns	ns	ns	ns	ns

^z Separation by F-test, ns=not significant at 5% level, *=significant at the 5% level, **=significant at the 1% level. analysis performed on transformed data as needed to account for lack of homogeneity of treatment variance.

Literature Cited

1. Brantley, B.B. and G. F. Warren. 1960. Effect of nitrogen on flowering, fruiting and quality in watermelon. Proc. Amer. Soc. Hort. Sci. 75:644-649.
2. Buttrose, M.S. and M. Sedgley. 1978. Some effects of light intensity, daylength and temperature on growth of fruiting and non-fruiting watermelon (Citrullus lanatus). Ann. Bot. 42:339-344.
3. Christopher, D.A. and J.B. Loy. 1983. Influence on foliar-applied growth regulators on sex expression in watermelon. J. Amer. Soc. Hort. Sci. 107:401-404.
4. Rudich, J. and A. Peles. 1976. Sex expression in watermelon as affected by photoperiod and temperature. Sci. Hortic. 5:339-334.
5. Staub, J.E. and L. Crubaugh. 1987. Imposed environmental stresses and their relationship to sex expression in cucumber (Cucumis sativus L.). Cucurbit Genetics Coop. Rpt. 10:13-17.