Predicting yield responses to different greenhouse CO 2 enrichment schemes: cucumbers and tomatoes

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Data from six years of carbon dioxide (CO 2) enrichment studies at North Carolina State University were analyzed in an attempt to develop predictive relationships for plant responses to different enrichment schemes and CO 2 levels (600–5000 μl l −1). Cucumbers ( Cucumis sativus L.) and tomatoes ( Lycopersicon esculentum Mill.) were enriched using: (i) closed-loop cooling to extend enrichment periods beyond that generally practicable and (ii) elevated CO 2 levels to compensate for short enrichment times normally encountered in conventional enrichment. Yields of nine cultivars of cucumber and seven of tomato, from both ground bed and bag culture, were regressed against solar energy, number of enrichment hours, fractional enrichment time, CO 2 set point concentration (i.e., target concentration), and actual daily CO 2 concentration. Absolute yields for cucumber were found to be strongly related to the solar energy received and, to a lesser degree, the number of enrichment hours. CO 2 concentration, either set point or actual, was significant only when included in quadratic form. The relationship developed suggests that the optimum concentration is inversely related to the length of the enrichment period and that the product of the number of enrichment hours and the set point concentration should equal 14 400 μl h l −1. Absolute yields for tomato were also highly dependent upon solar energy, and to a lesser degree, either actual CO 2 concentration, number of enrichment hours, or fractional enrichment time. Weight gain advantages for cucumber were found to be a linear function of fractional enrichment time (enrichment time divided by solar daylength), reaching a maximum value of 54% when continuously enriched during daylight hours. Weight gain advantages for tomato were found to be a non-linear function of fractional enrichment time with values of fractional enrichment time less than 0.5 producing little or no gain.