The effects of three levels of relative humidity (40 %, 65 %, and 90 %) and two types of containers (clay and plastic) on the seedling growth of three F, hybrid annuals were determined after 14 days of controlled-environment treatment. Forty percent relative humidity was severely limiting to the seedling growth of 'Blue Blazer' ageratum (Ageratum houstonianum Mill.), 'Pink Cascade' petunia (Petunia hybrida Vilm.), and 'Double Eagle' marigold (Tagetes erecta L.). Raising the relative humidity to 65 % resulted in striking increases in fresh weight, dry weight, and leaf area, especially when clay containers were used. Height of the main shoot was increased significantly at 65 % relative humidity but node number was influenced only slightly. Increasing the relative humidity further to 90 % had no significant effect on fresh weight, dry weight, or percent dry weight for any of the three species, in either container. Leaf area was increased significantly at 90 % only in ageratum seedlings grown in clay pots. Each species responded differently to the type of container used. The fresh weight and dry weight of petunia seedlings were significantly greater in plastic pots at every level of humidity while those of marigold seedlings were unaffected by the type of container used. Ageratum seedlings, on the other hand, had significantly greater fresh weights and dry weights in clay pots only at 90 % relative humidity. THE IMPORTANCE OF relative humidity in controlled-environment studies on plant growth and development is often not realized, although careful attention is generally paid to other environmental factors, such as temperature, light intensity, and photoperiod. Went (1957) has indicated that plants grow best when the relative humidity of the air is somewhere in the range of 50-88 %. But precise information is lacking on the level of atmospheric moisture required for optimum plant growth (Wangermann, 1961). Some information on the effects of relative humidity is available for crops such as tomatoes (Went, 1944), peanuts (Fortanier, 1957), strawberries (Went, 1957; Pareek, Sivanayagam, and Heydecker, 1969), coffee (Went, 1957), several grasses (Went, 1957), kale, sugar beet, and spring wheat (Watson, 1964), barley (Natr and Kousalova, 1965), wild white clover, dwarf bean, broad bean, peas, cucumber, sweet pepper, cauliflower, kohlrabi, turnip, radish, beet, raspberries (Pareek et al., 1969), or experimental plants such as sunflower (Demidenko and Golle, 1939; Pareek et al., 1969) and Mimosa (Wallace, 1931; Went, 1957). 1 Received for publication 7 January 1971. The authors are indebted to E. J. Koch, Biometrician, Division of Biometrical Services, Agricultural Research Service, U. S. Department of Agriculture, Be tsville, Maryland, for assistance with the statistical analysis of the data, and to George J. Ball Inc., West Chicago, Ill., for kindly furnishing the seed. Preliminary studies conducted in the Phy toEngineering Laboratory with selected F1 hybrid annuals indicated that relative humidity may be of considerable importance in influencing seedling growth of these plants. The present study was, therefore, conducted to determine the effects of three levels of relative humidity (40 %, 65 %, and 90 %) on the growth and development of selected F1 hybrid annuals under controlled-environment conditions. Since a previous experiment had shown a striking influence of containers on seedling growth under controlled environments, the possible interactions of relative humidity and containers were also examined. This report is one of a series of single-factor and multiple-factor studies on optimum environments for seedling growth of selected F1 hybrid annuals. MATERIALS AND METHODS Plant materialPlant material consisted of three F1 hybrid annuals-'Blue Blazer' ageratum (Ageratum houstonianum M\lill.), a day-neutral plant; 'Pink Cascade' petunia (Petunia hybrida Vilm.), a long-day plant; and 'Double Eagle' marigold (Tagetes erecta L.), a short-day plant. These cultivars were chosen because: (1) they are widely used as bedding plants; (2) they exhibit the major types of photoperiodic responses; and (3) they were previously found responsive to controlled-environment treatment (Cathey, 1967). Seed were planted in 7.5-cm plastic and clay pots containing a synthetic soil mix of peat and
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