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Fig. 1. Response of pawpaw [Asimina triloba (L.) Dunal] seedlings to an ambient root-zone temperature at 24 °C (O) and bottom heating at 32 °C (●) for seedling leaf number for (A) fully expanded leaves and (B) height after seedlings had emerged from the growing substrate. Mean ± se, n = 3.
because the amount of soil is determined by the volume of the container. On a volume basis, CEC of ProMix was greater than other media. Interestingly, the CEC values of 1 sand : 1 sphagnum peat and 4 pine bark : 1 sand : 1 sphagnum peat substrate were similar, suggesting that the lower water holding capacity of 4 pine bark : 1 sand : 1 sphagnum peat resulted in the reduced seedling growth in this substrate. With daily watering, it is also likely that the nutrients (applied weekly) were leached more rapidly from the 6 pine bark : 1 sand (v/v), 1 sand : 1 sphagnum peat, and 4 pine bark : 1 sand : 1 sphagnum peat substrate, resulting in greater plant growth in ProMix. As the pine bark decomposed in containers during the experiment, some applied nutrients may have been used by microorganisms. If seedlings in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat were irrigated and fertilized more frequently, growth of seedlings in these substrates might have been en-
more rapidly from the 6 pine bark : 1 sand (v/v), 1 sand : 1 sphagnum peat, and 4 pine bark : 1 sand : 1 sphagnum peat substrate, resulting in greater plant growth in ProMix. As the pine bark decomposed in containers during the experiment, some applied nutrients may have been used by microorganisms. If seedlings in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat were irrigated and fertilized more frequently, growth of seedlings in these substrates might have been en-
Table 3. Influence of bottom heating on growth characteristics and leaf chlorophyll (Chl) concentration on greenhouse grown pawpaw seedlings 114 days after sowing.
| Ambient (24 °C) | Bottom heat (32 °C at 8 cm depth) | Independent t test P value | |
|---|---|---|---|
| Leaf number | 9.4 | 12.3 | <0.001 |
| Plant height (cm) | 15.7 | 20.8 | <0.002 |
| Whole plant leaf area (cm2) | 305 | 593 | <0.001 |
| Specific leaf dry weight (SLW; g· m-2) | 0.24 | 0.20 | <0.001 |
| Shoot dry weight (g) | 1.10 | 2.25 | <0.001 |
| Total root dry weight (g) | 0.48 | 0.72 | <0.001 |
| Lateral root dry weight (g) | 0.12 | 0.27 | <0.001 |
| Total plant dry weight (g) | 1.58 | 2.97 | <0.001 |
| Root:shoot ratio | 0.43 | 0.32 | <0.001 |
| Chl a (g · m-2) | 41.5 | 57.9 | <0.001 |
| Chl b (g · m-2) | 11.6 | 15.5 | <0.001 |
| Chl p (g · m-2) | 1.6 | 2.3 | <0.001 |
| Total chl (g · m-2) | 53.1 | 73.4 | <0.001 |
| Chl a:b ratio | 3.5 | 3.8 | <0.001 |
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hanced. However, extra labor required for the additional watering would not make these substrates attractive to small nurseries that do not have automated watering systems. A substrate that contains less pine bark and more sphagnum peat might also serve as a successful container substrate for production of paw-paw seedlings. ProMix contains dolomitic and calcitic limestone, and Ca concentration was almost 2-fold greater for ProMix than 1 sand: 1 sphagnum peat, and ≈5-fold greater in 4 pine bark: 1 sand: 1 sphagnum peat, and 6 pine bark: 1 sand (v/v) substrates. The irrigation water used in this study contained Ca at 35 mg·L-1. It is possible that low Ca substrate levels may have limited plant growth in the 4 pine bark: 1 sand: 1 sphagnum peat, and 6 pine bark: 1 sand (v/v) substrates; however, Ca deficiency symptoms were not observed with plants on these substrates. Overall, ProMix with a once weekly application of 100 mg·L-1N as Peters water-soluble fertilizer served as an excellent substrate for the production of robust pawpaw seedlings for transplanting. Seedlings in the 1 sand: 1 sphagnum peat substrate preformed well, but the heavy weight of this medium would also make containers heavy and more cumbersome to handle.
Pawpaw seedlings responded very favorably to root-zone warming to 32 °C by increasing shoot, root, and total dry matter production. Seedling emergence from the substrate 9 d before their ambient substrate temperature counterparts gave seedlings with bottom heat a significant advantage in their development. The positive response of seedlings to bottom heat indicates that pawpaw is similar to other tree species, such as thornless honey locust (Klock et al., 1996), that tolerate elevated root-zone temperatures. Further research will be required to determine the optimum soil temperature for pawpaw seedling growth and development.
Elevated soil temperatures have been reported to significantly increase nutrient uptake and dry weight production in passionfruit (Menzel et al., 1994), and they have increased bloom yield and stem length for hydroponically grown roses (Moss, 1984). Elevated soil temperatures significantly increased nutrient uptake and dry weight gain in passionfruit (Menzel et al., 1994) and N accumulation by apple trees (Toselli et al., 1999). Lopez-Cantarero et al. (1994) reported that leaf chl levels were directly related to N and P fertilization levels in Solanum melongena L. Based on the significantly higher leaf chlorophyll levels, whole plant leaf area, and biomass accumulated for bottom-heated vs. ambient seedlings, it is likely that seedlings with bottom heat had a higher nutrient uptake and quantity and greater photosynthetic activity. The Peters 20N–8.6P–16.6K plus soluble trace elements used in this study contained 3.94% NH4-N, 6.05% NO3-N, and 10.01% urea-N; we did not include any other fertilizer formulations as treatments; therefore it is unclear if pawpaw prefers to utilize one N form over another.
The shoot, root, and total dry weight of pawpaw seedlings in the bottom heat study
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