EFFECTS OF CO2, ON LEAF STOMATAL CONDUCTANCE AND PRODUCTION OF LETTUCE AND RADISH

Abstract

Lettuce (cv. Waldmann's Green) and radish (cv. Giant White Globe) plants were grown hydroponically with a 18-hr photoperiod and 300 μmol m-2 s-1 PPF. Treatments consisted of 400, 1000, 5000 and 10000 μmol mol carbon dioxide (CO2). Leaf stomatal conductance was monitored with a steady-state porometer across one diurnal period at 21 days and all plants were harvested at 25 days. Conductance at 400 and 10000 was > 1000 μmol mol-1 for lettuce and conductance at 5000 and 10000 was > 1000 and 400 μmol mol-1 CO2 for radish. Carbondioxide treatments having the lowest leaf conductances also resulted in the highest yields, viz. 1,000 μmol mol-1 CO2 for radish and 5000 μmol mol-1 CO2, for lettuce. Dark-period conductance was higher at 5000 and 10000 μmol mol-1 CO2 compared to 400 and 1000 μmol mol-1 CO2. The higher dark-period conductances were 70% of the light-period rates for lettuce and 30% for radish. Water use efficiency (WUE) (g biomass kg water-1) was lowest at 400 μmol mol-1 CO2 for both lettuce and radish and was highest at 1000 μmol mol-1 CO2 for lettuce and 5000 μmol mol-1 CO2 for radish. The results suggest that WUE was improved with moderate CO2 enrichment but declined at very high concentrations, i.e. 10000 μmol mol-1 for lettuce and radish.