Off-season production of melons (cucumis melo L.) in a closed system solar greenhouse

A solar system for greenhouse heating has been recently developed by the authors of this paper. Whenever possible the system operates as a "closed system", which typically results in above optimum air temperatures, very high air humidity and in low levels of CO₂ concentration. Experiments were conducted over three seasons to assess the performance of melons in this system. In all three seasons the experimental solar system raised night temperatures (mean and minimum) approx. 6°C above outdoor night temperatures. The effect of CO₂ enrichment (1000 vs 340 ppm) was investigated in the first (Nov. 1991-Feb. 1992) and second (Nov. 1992-Feb. 1993) experimental seasons. The first season was characterized by unusually low radiation regimes and minimal bee pollination. Carbon dioxide enrichment increased total fruit yield from 1.9 to 4.6 kg/m². Most of the fruit was parthenocarpic and therefore too small for the export market. Furthermore most of the fruit produced had no netting. In the second year CO₂ enrichment increased total yield from 2.5 to 3.4 kg/m² and exportable yield from 0.5 to 1.8 kg/m². The effect of planting date (28/9/93, 2/11/93 and 10/1/94) was evaluated during the third growing season. Carbon dioxide levels were kept at 1000 ppm. The first planting date produced a total yield of 2.9 kg/m² and exportable yield of 2.0 kg/m². Total yield of the second planting date was 3.2 kg/m² and exportable yield was 0.3 kg/m² (no netting). The third planting date gave a total yield of 5.5 kg/m² and exportable yield of 4.5 kg/m ². The results indicate that the new solar heating system supports melon production in Beer-Sheva during all winter and early spring months, and CO₂ enrichment enhances marketable yields. It was postulated that high humidity during fruit development has a strong negative effect on the development of netting on the fruit surface.