Effect of carbon dioxide enrichment on diffusive resistance for gas exchange, water use, and water use efficiency of greenhouse tomatoes

Abstract

Three adjacent ventilated mini-greenhouses (MGH) made of clear polyethylene film, transmitting natural solar radiation, were enclosed in a conventional inflated polyethylene greenhouse. In each MGH, an equal number of tomato plants were grown in the Spring and the Fall of 1984, and kept at optimum levels of moisture and nutrients. From 10 to 98 days after emergence, CO2 levels in the mini-greenhouses were maintained at about 340, 700, and 1000 ppmv during the daytime. As CO2 levels in the MGH air increased from 340 to 1000 ppmv, the crop surface resistance, measured with a porometer, increased from about 30 to 100 s/m. CO2 enrichment also increased the ratio between the internal and the external CO2 levels of the leaves from 0.70 to 0.85. From the Fall 1984 data, a linear equation was derived to relate surface resistance to the internal CO2 level with an R-square value of 0.8. At an air exchange rate of 30 m^3/m^2/h in the MGH, the aerodynamic resistance, measured using a heated brass plate, or as computed by the residual method, averaged 225 s/m. This parameter dominated gas exchange by the plants at all CO2 levels. Therefore, the water use as measured by weighing mini-lysimeters (pots) on clear days, decreased only slightly, 15-20%, as result of the CO2 enrichment. This occurred in spite of an increase in leaf temperature of about 1.5 °C. The leaf area and stomatal density were not markedly affected by CO2 enrichment. Both the instantaneous and the seasonal water use efficiency increased markedly, by about 70%, by growing the plants at a CO2 level of 1000 ppmv rather than 340 ppmv. In part, this was due to the reduction of water use, but mainly to the increase in assimilation rate, in total dry matter, and in the mass of fresh fruit harvested, being 70%, 31%, and 50%, respectively.