Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light (600-1000 ft-c), and CO₂, concentration, he estimated that 2.3 m² of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m²/hr would provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour. A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) considered _Wolffia_ as a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B₂, B₆, and C, with C the most abundant. One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions. In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly. At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions. Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft² of leaf surface are required to support an astronaut. Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.