Crowding in Brassica rapa. Deanna Hall

Transcription

1 Crowding in Brassica rapa Deanna Hall Bio 493 March 24, 26

2 Crowding in Brassica rapa Deanna Hall Abstract Wisconsin Fast plants (Brassica rapa) were grown in four different densities of one, two, three and four plants per pot to determine the effects of crowding on reproduction. Each density was separated by trays and each tray held 12 pots. The plants were provided with equal amounts of light, water and fertilizer for the entire growing process. Measurements were taken of the plant height, the amount of light available, the number of flowers, seed pods and seeds produced. The weight per seed was also recorded. Significant differences were found in the number of seeds pods, seeds and the weight per seed between each density. There was no significant difference found in the height per plant or in the number of flowers produced by each plant. INTRODUCTION Brassica rapa, commonly known as the Wisconsin Fast Plant. Fast plants have a short life-cycle of about days with the tallest plants reaching about 3cm. The Fast Plants are members of the crucifer family of plants, which include cabbage, turnips, broccoli and other vegetables. According to Harley and Bertness (1996) clumping in plant distribution is a common ecological phenomenon. Nearly all of the research found on both interspecific and intraspecific competition from crowding suggests that when plants are crowded, they result in taller, thinner plants. Shading by neighboring plants reduces energy for photosynthesis and lowers the ratios of red: far-red light which triggers a stem elongation response in herbaceous plants (Collins and Wein 2). Plant stems are kept short, unless the plant faces competition for light. Phytochrome refers to a group of photoreceptors that absorb light and control developmental effects in plants. 1

3 Phytochrome inhibitis cell elongation when stimulated by red-light (78-62nm). When a plant is shaded it receives primarily far-red light and cell elongation resumes (Nabors 24). Smaller plants within areas of high local densities were more likely to die than larger plants due to competition (Suzuki et al 23, Bender et al 22). Cresswell et al. (21) found that plant size was strongly affected by plant density; plants in the lowest density pots developed ten-times more biomass per plant than individuals in the highest density pots. Sanders et al. (1998) found that plant productivity is indirectly correlated with plant density. Plants can have different responses to crowding. Reproduction in some plants is stimulated by stress (personal conversation with Dr. Winget). In the Lemna minor, it was found that size of the reproductive parts is more important for success in competition than is the number of the parts produced (Vasseur et al. 1995). Cresswell et al. (21) found that none of the observed individual attributes of Brassica napus L. flowers varied with density, but the number of flowers per plant declined as density increased. In resource scarcity the plants conserved flower size rather than number (Cresswell et al. 21). Harley and Bertness (1996) found that marsh plants that developed in a crowded area became structurally dependent upon one another; when the crowded plants were thinned they fell over. Crowding of Brassica rapa was found to affect plant size and the number of flowers produced (Gurevitch et al 1996). The purpose of this research was to find how crowding effected the reproductive capability of Brassica rapa as gauged by average height per plant, number of seed pods and seeds produced per plant, and the weight per seed. 2

4 MATERIALS AND METHODS Wisconsin Fast Plants were grown in five and a half centimeter pots with varying densities of one, two, three, or four plants per pot using a commercial vermiculite/perlite/peatmoss mix potting soil. Each pot was provided three granules of slow release fertilizer ( ) containing trace elements. The pots were arranged in four different trays corresponding with plant density, twelve pots for each plant density, for a total of forty-eight pots. All plants were provided with the same amount of light which was provided by four 3-watt florescent Grow-Lux lamps (1.2m), placed twenty centimeters above the trays. The light was provided 24 hours a day throughout the entire growing process. Water was provided in the tray underneath the pots. Each pot had a hole in the bottom to allow the roots to have constant contact with the water to ensure that moisture was not a limiting factor to the plants growth. The soil was kept damp, but not soggy. Height, number of flowers, number of seed pods, number of seeds produced in each pod and weight per seed (mg) on multiple occasions. Light intensity was measured above the plants and at soil level twice during the growing process. The plants were hand-pollinated daily during the flowering stage. A dried bee glued to a stick was used for pollination. The bee was rubbed on the anthers of several open flowers and then rolled on the stigmas of flowers of other plants. Plants were crosspollinated only with other plants from the same density. Pollination continued every day until three to four days after the last flowers opened. 3

5 Seeds were harvested approximately 2 days after the last pollination. Water was removed from the plants when the ends of the pods began to change color from green to brown. The plants were allowed to dry for about seven days or until the seed pods were brown and crisp. The seed pods were broken open and the seeds counted and weighed to the nearest.1 mg using a Mettler analytical balance. Analysis of variance (ANOVA) for each of the densities were performed for the height per plant, the number of seed pods per plant, the number of seeds per plant and the average weight per seed. RESULTS AND DISCUSSION The number of seed pods per plant tended to be inversely related to the number of plants per pot. In the one plant per pot density the average number of seed pods per plant was 8.8 +/ For the two plants per pot density the average number was 5.8 +/-2.48 pods, in the three plant density the average was 4.1 +/- pods and in the four plant density the average number of seed pods per plant was 3.2 +/ (Figure 1). The ANOVA showed significant differences (p<.1) in average number of seed pods per plant between one plant per pot and all three other densities, between two plants per pot and four plants per pot, and between three and four plants per pot. The difference between two and three plants per pot was not significant 4

6 14 Average number of seed pods per plant Number of plants per pot FIGURE 1 The average number of seed pods per plant for the densities of one, two, three and four plants per pot. +/- SD are also given. The number of seeds per plant tended to be inversely related to the number of plants per pot. The average number of seeds per plant was 11 +/-2.73 for the one per pot density, 68 +/-2.25 for the two plants per pot density, 38 +/-2.14 in the three per pot density, and 3 +/-1.41 in the four per pot density (Figure 2). The ANOVA showed significant differences (p<.1) in the average number of seeds per plant between the one per pot density and all three other densities, and between the two plants per pot and both the three and four plants per pot. There was no significant difference between the three and four plants per pot in the average number of seeds per plant. 5

7 16 14 Average number of seeds per plant plants per pot FIGURE 2 The average number of seeds per plant for densities of one, two, three, and four plants per pot. +/- SD is given. The weight per seed was recorded at the completion of the growing cycle and was found to be inversely related to the number of plants per pot. In the one plant per pot density the average weight per seed was 185mg +/-37.2, 117mg +/ for the 2 plants per pot density, 62mg +/ for the 3 plants per pot density, and 42mg +/ for the four plants per pot density (Figure 3). The ANOVA showed a significant difference (p<.1) between the one per pot density and the two three and four per pot densities and between the two per pot and the three and four per pot densities. There was no significant difference in the weight per seed of the three and four per pot densities. 6

8 25 2 Weight per seed (mg) Number of plants per pot FIGURE 3 The average weight per seed in mg for the densities of one, two, three and four plants per pot. +/- SD are shown. Heights of the plants were measured on days 13, 17, and 21 of the growth cycle (figure 4). There was no significant difference found for the height of the plants in relationship to crowding. 7

9 3 25 day13 day 17 day /pot 2/pot 3/pot 4/pot density FIGURE 4 Average height (cm) per plant measured at three different intervals throughout the growth cycle for the densities of one, two, three, and four plants per pot. Standard deviations of +/- SD are given. The number of flowers per plant was measured on four different occasions throughout the growth cycle (Figure 5). The ANOVA showed there was no significant difference between the number of flowers per plant on any of the days counted perhaps due to the extreme variation within each density. Day 17 8

10 Ave. No. Flowers Per Plant Day 15 Day 21 Day 23 Day Plants Per Pot FIGURE 5 The average number of flowers per plant per density of one, two, three and four plants per pot on four different intervals throughout the growth cycle. +/- 1 SD is given. Light intensity was measured at the canopy level of all plants on days 13 and 21. On day 13 the average lux at the canopy level was 466. and on day 21 it was lux. The increase was due to the rise of the canopy with relation to the light source. Light intensity was also measured at the soil level for all of the plant densities. Averages were calculated for each density. The one plant per pot density had an average lux of 281 on day 13 and on day 21. The two plants per pot density had an average of 285 lux on day 13 and 98.9 lux on day 21. The average lux for the three plants per pot density on day 13 was 211 and 86.5 on day 21. The four per plant density had an average of 188 lux on day 13 and 65.4 on day 21. (Figure 6) 9

11 12 Day canopy Lux 12 1 Day Number of plants per pot canopy FIGURE 6 Bars one through four represent the average lux per density of plants measured at soil level. Bar five represents the light at the canopy level. +/- 1 SD is given for day 21. 1

12 DISCUSSION The results of this study show a significant correlation between crowding and the number of seed pods and seeds produced per plant and the weight per seed for the Wisconsin Fast plant. When faced with competition the plants produced fewer numbers of seed pods, seeds and the weight per seed decreased. There was no significant difference found between the number of flowers produced for each density, but the number of flowers that set pods and matured seeds was greater in the lower densities. This study has shown that the height and number of flowers produced per plant was not significantly different between the densities used in this study. This is inconsistent with the study performed by Gurevitch et al. (1996) who found that the number of flowers produced and plant size were negatively affected by crowding. Perhaps if densities were higher in this study, results might have agreed with Gurevitch s findings. 11

13 SOURCES CITED Bender, M.H., J.M. Baskin, and C.C. Baskin. 22. Role of intraspecific competition in mass seeding and senescence in Polymnia canadensis, a primarily monocarpic species. Journal of the Torrey Botanical Society 129(2): Collins, B., and G. Wein. 2. Stem elongation response to neighbor shade in sprawling and upright Polygonum species. Annals of Botany 86(4): Cresswell, J.E., C. Hagen, and J.M. Woolnough. 21. Attributes of individual flowers of Brassica napas L. are affected by defoliation but not by intraspecific competition. Annals of Botany 88(1): Gurevitch, J., D.R. Taub, T.C. Morton, P.L. Gomez, and I.N. Wang Competition and genetic background in rapid-cycling cultivar of Brassica rapa (Brassiacaceae). American Journal of Botany 83(7): Harley, C.D.G., and M.D. Bertness Structural interdependence: An ecological consequence of morphological responses to crowding in marsh plants. Functional Ecology 1(5): Nabors, M.W., 24. Introduction to Botany. San Francisco: Pearson Benjamin Cummings. P Sanders, D.C., J.D. Cure, W.J. Sperry, J.C. Gilsanz, C.A. Prince, and O. Bandele Long-term effects of rows per bed and i-row spacing on yield and spear size of asparagus. Hortscience 33(4): Suzuki, R.O., H. Kudoh, and N. Kachi. 23. Spatial and temporal variations in mortality of the biennial plant, Lysimachia rubida: Effects of intraspecific competition and environmental heterogeneity. Journal of Ecology 91(1): Winget, R. Professor of Biology at Brigham Young University Hawaii (25). Personal Communication. Vasseur L., D.L. Irwin, and L.W. Aarssen Size versus number of offspring as predictors of success under competition in Lemna minor (Lemnaceae). Annales Botanici Fennici 32(3):