Young and Yavitt (1987) reported that proximal pawpaw leaves, which developed before the overstory forest canopy closed, were 76% smaller than distal leaves that developed after canopy closure. In the present investigation, seedlings in the outdoor study displayed an even greater increase in leaf area with low to moderate shading than noted by Young and Yavitt (1987). Partial shading has been reported to increase leaf area in both shade-preferring (Vladimirova et al., 1997) and sun-preferring plants (Givinish, 1988; Kappel and Flore, 1983). Pawpaw seedlings under 28% to 51% shading in the outdoor study had more leaves than nonshaded plants. Moderate shading increased the number of leaves in the shade preferring plant Belgian evergreen (Vladimirova et al., 1997), whereas shading of developing apple trees reduced the number of leaves produced (Maggs, 1960). In sun-preferring plants, SLW usually declines with shading (Buisson and Lee, 1993; Kappel and Flore, 1983). Although pawpaw responded in many other ways as a shade-preferring plant, average plant SLW declined with shading.
Shade leaves generally have a higher concentration of chl, especially chl b, than sun leaves, thus allowing the shade leaves to use the limited amount of light reaching them (Anderson, 1986; Givinish, 1988). The resulting lower chl a to b ratio with shading may represent a change in stoichiometry of photosystems I and II (Anderson, 1986;
Shade (%)
t al., 1992). Moderate shading outdoors significantly
Fig. 3. Effect of shading and treatment of containers with (open symbols) and without (closed symbols) cupric hydroxide [Cu(OH)2] on leaf chlorophyll (chl) concentration in pawpaw seedlings grown in the greenhouse. (A) Leaf chl a in plants with (y = 50.67–0.14x, R2 = 0.71, P = 0.002) and without Cu(OH)2, (B) chl b in plants with (y = 17.02–0.03x, R2 = 0.72, P = 0.001) and without Cu(OH)2, and chl p for plants with and without Cu(OH)2, and (C) total chl for plants with (y = 67.70–0.17x, R2 = 0.71, P = 0.001) and without Cu(OH)2, and (D) chl a to b ratio for plants with (y = 2.958–0.003x, R2 = 0.70, P = 0.001) and without Cu(OH)2 (y = 3.200–0.004x, R2 = 0.83, P = 0.001). Each symbol is the mean of two blocks ± se.*Indicates that means were significantly different between Cu(OH)2 treated and nontreated plants using an unpaired t test (P < 0.05, n = 2).
Hidema, et al., 1992). increased chl a and b concentration in paw-paw seedlings, but interestingly raised chl a to b ratio (Fig. 6A and B).
Seedlings subjected to low to moderate shading (33%, 56%, or 81%) in the greenhouse study also showed increased total leaf area. However, shading of 33% failed to significantly increase root and total plant DW compared to controls, and shading above 56% actually decreased root, shoot, and total plant DW production. Total
lars of the zebra swallowtail butterfly (Eurytides marcellus Cramer). The caterpillars were removed weekly from any plant, thereafter. The seedlings in this block recovered well and by the time of destructive harvest were similar in size and other attributes to seedlings in the other block (data not presented).
The structure and physiology of a developing plant can be affected profoundly by the quantity and quality of light incident upon the plant (Buisson and Lee, 1993; Givinish, 1988; Rajapakse et al., 1999). When pawpaw seedlings were grown outdoors, they responded to low to moderate shading with increased production of both root and shoot biomass, as reported in other shade-preferring plants at similar shade levels (Vladimirova et al., 1997; Whitcomb, 1988). Prior to this study, common thought suggested that full sun was lethal to pawpaw seedlings. Nonshaded seedlings survived in this study; however, they accumulated about half the biomass of seedlings receiving 28% to 81% shading. Only heavy shading outdoors altered root to shoot partitioning in pawpaw, with 81% shading lowering the root to shoot ratio. A reduction in the root to shoot ratio is believed to maximize growth in light-limited environments and confer shade tolerance to a plant (Abrams and Kubiske, 1990; Givinish, 1988).
Young and Yavitt (1987) reported that proximal pawpaw leaves, which developed before the overstory forest canopy closed, were 76% smaller than distal leaves that developed after canopy closure. In the present investigation, seedlings in the outdoor study displayed an even greater increase in leaf area with low to moderate shading than noted by Young and Yavitt (1987). Partial shading has been reported to increase leaf area in both shade-preferring (Vladimirova et al., 1997) and sun-preferring plants (Givinish, 1988; Kappel and Flore, 1983). Pawpaw seedlings under 28% to 51% shading in the outdoor study had more leaves than nonshaded plants. Moderate shading increased the number of leaves in the shade preferring plant Belgian evergreen (Vladimirova et al., 1997), whereas shading of developing apple trees reduced the number of leaves produced (Maggs, 1960). In sun-preferring plants, SLW usually declines with shading (Buisson and Lee, 1993; Kappel and Flore, 1983). Although pawpaw responded in many other ways as a shade-preferring plant, average plant SLW declined with shading.
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