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Layne, H.O. Jackson, and G.A. Spear man. 1975. An improved exotherm method for measuring cold hardiness of peach flower buds. HortScience 10:521-523. 21. Snedecor, G.W. and W.G. Cochran. 1967. Statistical methods. 6 th ed. Iowa State Univ. Press, Ames. p. 6-7. 22. Steel, R.G.D. and J.H. Torrie. 1960. Principles and procedures of statistics. McGraw-Hill, New York. p. 232-251 (chapter 12). J. Amer. Soc. Hort. Sci. I08(5):750—754. 1983. Enhancement of Lettuce Yield by Manipulation of Light and Nitrogen Nutrition Sharon L. Knight1 and Cary A. Mitchell2 Department of Horticulture, Purdue University, West Lafayette, IN 47907 Additional index words, controlled-environment agriculture, hydroponics, Lactuca sativa Abstract. Several levels of photosynthetic photon flux density (PPFD) were tested for effects on growth of 4 cultivars of lettuce (Lactuca sativa L.) under controlled-environment conditions. Growth of ‘Salad Bowl’, ‘Bibb’, and ‘Ruby’ was greater at 932 pmol s_,m "2 than at ^ 644 pmol s -1m-2 under a 16-hour photoperiod. Thirty mM N 03“ or 5 him NH4+ + 25 mM N 03" increased leaf dry weight while reducing leaf chlorosis in ‘Salad Bowl’ and ‘Grand Rapids’ relative to that with 15 mM N 03~, and reduced leaf purpling in ‘Bibb’ and ‘Ruby’ with little or no effect on yield. Continuous illumination with 455 or 918 pmol s- , m~2 stimulated yield of ‘Salad Bowl’ and ‘Bibb’ when 30 mM N as NH4+ + N 03" was used relative to that with 15 mM N03“. Although production costs generally have prevented con trolled-environment agriculture from becoming a superior eco nomic alternative to open-field agriculture (15, 17), yield rate of crops grown in controlled environments is often greater than that in the field (4, 6, 19). For instance, it takes at least 55 days to produce a marketable leaf lettuce crop in the field, but only 22 days in a controlled environment (4, 8). Furthermore, fresh weight yield per cycle of 6.0 kg m-2 has been obtained in controlled environments vs. 2.4 kg m ~2 in the field. Although controlled-environment production holds considerable promise for further improvements in yield of lettuce (7), lack of economic incentive has delayed identification of optimum cultural con ditions for this and other crops. However, the U.S. National Received for publication March 3, 1982. Purdue Agricultural Experiment Station Journal Paper No. 8864. Project supported in part by NASA Cooperative agree ment NCC 2100. The authors thank P.A. Hammer for helpful discussion re garding experimental design and statistical analysis of data. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 'Project conducted in partial fulfillment of the MS degree. 2Associate Professor. Aeronautics and Space Administration has initiated a program in which photosynthetic higher plants may be of central impor tance in a space-deployed, regenerative life-support system (13, 15, 16). Ground tests in the Soviet Union indicated that plants can provide part of the calories and all of the 0 2 required by several humans over a 6-month period (9). Leaf lettuce is one candidate species being considered for a controlled ecological life-support system (CELSS) because it provides some of the minerals, vitamins, and fiber needed in the diet, has a high yield rate, has a favorable proportion of edible biomass, and sustains a high level of photosynthetic activity during production (11). In addition to seeking “ optimum environments” for leaf lettuce growth, selection or development of superior cultivars will be important to realize the goal of crops approaching their genetic potential for edible biomass production in a CELSS. Materials and Methods Cultivars. Four cultivars of lettuce were compared for their growth response to various light and N treatments: "Salad Bowl’, ‘Grand Rapids’, and ‘Ruby’ are loose-leaf types, and ‘Bibb’ is a butterhead type.