were less than in plants grown in ProMix in the media study. Fertilization regimes were slightly different. Plants in the substrate study received a weekly application of 100 mg·L-1N, whereas plants in the bottom heat study received twice weekly applications of 50 mg·L-1N fertilizer. Also, warmer ambient greenhouse temperatures, 29.1 ± 0.7 °C day and 21.2 ± 0.1 °C night, during the media experiment likely resulted in the greater growth of pawpaw seedling compared to those, 27 ± 0.8 °C day and 21 ± 0.2 °C night in the bottom heat experiment. Plants in the substrate experiment were subjected to warmer daytime temperatures, but lower temperatures at night. It is possible that there were higher losses of carbon during the night in bottom-heated plants compared to plants in the media experiment, due to increased nighttime respiration with bottom heat.
Hurewitz and Janes (1983) noted that tomato leaf carbon exchange rate and leaf area increased as soil temperature increased to 30 °C. They hypothesized that increased plant growth at warmer soil temperatures may have been due to the effect of increased root sink activity on leaf photosynthetic rate. Other factors that may have contributed to the growth enhancement noted for bottom-heated seedlings included changes in water use efficiency and production or availability of root-derived cytokines. Also, with such heating there are potential decreases in both the time to produce a saleable plant and the cost of heating greenhouses (Vogelezang, 1991). Bottom-heated seedlings also produced greater lateral and total root dry weight than ambient seedlings, which could result in a higher rate of establishment of bottom-heated seedlings in the field.
In conclusion, ProMix with a once weekly application of 100 mg·L-1 N as Peters water-soluble fertilizer served as an excellent substrate for the production of robust pawpaw seedlings for transplanting into the field or into larger containers. Pawpaw seedlings responded very favorably to root-zone warming to 32 °C, by speeding seedling emergence, and increasing shoot, root, and total dry matter production in developing seedlings.
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