branching and stimulate formation of a more fibrous root system in other container-grown tree species (Arnold and Struve, 1989, 1993). Root to shoot ratio has been reported to decrease with use of copper compounds on the interior surface of containers (Arnold and Struve, 1989). The Cu(OH)2 did not promote development of a more fibrous root system in pawpaw seedlings, based on failure of the copper compound to increase lateral root DW production. In fact, in nonshaded control seedlings, lateral root weight actually declined with Cu(OH)2. Leaves of pawpaws grown in Cu(OH)2-treated containers yellowed by the end of the experiment. The leaf yellowing and reduced chl levels may have been the result of a toxic response of plants to absorbing high levels of Cu in the treated containers or reduced cytokinin production by pawpaw root tips (Allan and Jarrell, 1989; Arnold, 1992; Arnold and Struve, 1989). Cytokinins are produced in the root tips and are transported to the growing leaves, where they may delay senescence (Satoh et al., 1977). If seedling root tips of pawpaw were damaged when they came in contact with the Cu(OH)2 on the container wall, this may have impaired cytokinin production. Arnold and Young (1991) reported that CuCO3 treatment of containers increased root growth and nearly doubled the number of new root tips in container-grown apple seedlings; however, there was not a sig-

significant difference in xylem sap cytokinin level in CuCO3-treated, root pruned, and control seedlings. In this present study, we did not examine whether root tips were killed or whether new root tips were initiated when pawpaw root tips encountered the Cu(OH)2-treated containers.

shoot DW was greater in greenhouse-grown plants with 33% shading compared to nonshaded plants. This is in contrast to the response of seedlings to shading in the outdoor study, where 28%, 51%, or 81% significantly increased root, shoot, and total plant DW compared to the control.

In contrast to pawpaw seedlings grown outdoors, shading of 28%, 51%, 81%, or 98% did not increase chl a, b, and p leaf concentration in plants grown in the greenhouse, only the a to b ratio decreased significantly. Shade leaves generally have more chl,

(Givinish, 1988) and a lower a to b ratio than sun leaves (Anderson, 1986; Hidema, et al., 1992). Once again, alteration of light quality in terms of red to far-red light ratio and UV light level in the greenhouse environment may have resulted in the different response in chl and a to b ratio of greenhouse grown plants compared to those grown outdoors.

Although plants in the greenhouse study did not respond to shading as in the outdoor study, pawpaw seedlings in corresponding control and shade treatments (33%, 56%, and 81%) in the greenhouse with or without Cu(OH) 2, had more leaves and had greater leaf area, as well as larger shoot, root, and total plant DW (compare Figs. 2 and 5). The greater growth of greenhouse-grown plants may have been due to twice weekly fertigation compared to weekly fertigation of plants outdoors. Altered red to far-red light ratios can affect phytochrome-mediated processes and can profoundly affect plant growth and development (Hutchison and Matt, 1977; Lee and Graham, 1987; Morgan and Smith, 1976; Rajapakse et al., 1999; Tasker and Smith, 1976). UV light has been shown to decrease growth in other plants (Gonzalez et al., 1998; Sisson and Caldwell, 1977). In the present study, an artificial shade environment was established utilizing black woven fabric; however, the shade fabric used did not alter the red to far-red light ratio. The greenhouse glass had a small, but significant effect on the red to far-red light ratio, which may have affected plant growth. The greenhouse glass reduced the level of UV light encountered by plants growing in the greenhouse environment, which may also have enhanced growth of pawpaw seedlings in the greenhouse. The shade cloth also significantly reduced UV light levels outdoors, resulting in growth enhancement.

In conclusion, growth

Fig. 6. Effect of shading on leaf chlorophyll (chl) concentration of pawpaw seedlings grown outdoors. (A) Leaf chl a (y = 10.808 + 0.169x - 0.001x², R² = 0.97, P = 0.001), chl b, chl p, total chl (15.787 + 0.234x - 0.001x², R² = 0.98, P = 0.002), and (B) chl a to b ratio (y = 2.155 - 0.005x, R² = 0.93, P = 0.001). Each symbol is the mean of two blocks ± SE.

Use of copper compounds in containers can cause lateral branching and stimulate formation of a more fibrous root system in other container-grown tree species (Arnold and Struve, 1989, 1993). Root to shoot ratio has been reported to decrease with use of copper compounds on the interior surface of containers (Arnold and Struve, 1989). The Cu(OH) 2 did not promote development of a more fibrous root system in pawpaw seedlings, based on failure of the copper compound to increase lateral root DW production. In fact, in nonshaded control seedlings, lateral root weight actually declined with Cu(OH) 2. Leaves of pawpaws grown in Cu(OH) 2-treated containers yellowed by the end of the experiment. The leaf yellowing and reduced chl levels may have been the result of a toxic response of plants to absorbing high levels of Cu in the treated containers or reduced cytokinin production by pawpaw root tips (Allan and Jarrell, 1989; Arnold, 1992; Arnold and Struve, 1989). Cytokinins are produced in the root tips and are transported to the growing leaves, where they may delay senescence (Satoh et al., 1977). If seedling root tips of pawpaw were damaged when they came in contact with the Cu(OH) 2 on the container wall, this may have impaired cytokinin production. Arnold and Young (1991) reported that CuCO 3 treatment of containers increased root growth and nearly doubled the number of new root tips in container-grown apple seedlings; however, there was not a significant difference in xylem sap cytokinin level in CuCO 3-treated, root pruned, and control seedlings. In this present study, we did not examine whether root tips were killed or whether new root tips were initiated when pawpaw root tips encountered the Cu(OH) 2-treated containers.

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