Cultivation of Lemma gibba under desert conditions. II: The effect of raised winter temperature, CO₂ enrichment and shading on productivity

The aim of this work was to increase the productivity of Lemna gibba ponds under desert conditions. In the winter season, the ponds were covered with transparent plastic tents which raised water temperature. This also allowed CO₂ to be added to the air in the tents to either the ambient, ∼ 340 μmol^-1, or to higher concentrations. The plastic covers attenuated photosynthetically active light by ∼ 30%. Winter-season yields in the covered ponds, maintained at ambient CO₂ concentration, were 39% higher than in the uncovered ponds. This could be ascribed to raised temperatures. Enrichment of the atmosphere with CO₂ further increased yields by as much as 28%. The different treatments did not affect protein content expressed as a percentage of dry weight. Laboratory experiments indicated that the shorter the photoperiod the larger is the growth response of Lemna gibba to CO₂ enrichment. Shading of the ponds during the June-August summer season reduced pond temperatures at midday by about 5-6°C and resulted in a 30-80% increase of growth. It was concluded that under desert conditions similar to those prevailing in this trial, high yields of Lemna gibba can be achieved throughout a growing season of 12 months per year by covering the ponds and raising ambient [CO₂] during the winter, and by shading in summer. Productivity of 7·4±1·0 g m^-2 day^-1 can be maintained throughout the year. Whether or not it is worthwhile to do so is a question of local economics.