Productivity of Vegetable Crops in a Region of High Solar Input. IV. Field Chamber Measurements on French Beans (Phaseolus vulgaris L.) And Cabbages (Brassica oleracea L.)

Abstract

Net CO₂ uptakes have been measured for crop canopies of French beans and cabbages, sown at two plant densities, using a large field assimilation chamber and a semi-closed gas-analysis system.

For both species, the maximum rates of uptake were a little less than 40 mg CO₂ dm^-2 (ground area) h^-1, and light saturation of the canopy occurred at 600-650 W m^-2 (French beans) or about 800 W m^-2 (cabbages). Net CO₂ uptake decreased with leaf area index at values below about 5, but was relatively insensitive to temperature over the range used. Once this leaf area index was reached, the relationship between net uptake and solar radiation remained fairly constant throughout the growth period. For both species, dark respiration rates were markedly dependent on temperature, and also were lower at night than during the day when measured at the same temperature.

For both French beans and cabbages, growth analyses showed the maximum growth rates to be 18-19 g dry weight m^-2 (ground area) day^-1. The mean growth rate from emergence to harvest for an overwintered cabbage crop was 5.5 g m^-2 day^-1. It is suggested that the main advantage of the region in terms of plant productivity lies in the long frost-free growing season and the ability of frost-tolerant crops to maintain fairly high growth rates throughout a mild and comparatively sunny winter.