Some Effects of Humidity on the Growth and Development of Cirsium arvense

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Abstract
Seedlings of Canada thistle Cirsium arvense (L.) Scop. were grown in sand culture under controlled conditions at low (30%-50%) and high (90%-100%) relative humidity (RH). High RH increased stem height by 50% and shoot and root dry weights by 50% and 80%, respectively, but had no significant effect on root bud growth. When the root buds were released from correlative inhibition by removal of the main shoot, increasing the RH from ca. 50% to 100% significantly increased their rate of emergence and resulted in a twofold increase in the length and dry weight of emergent shoots after 7 days of growth. At low RH, growth of the root buds was significantly inhibited not only by mature leaves but also by mature stem tissue. The root buds were released from the stem-induced inhibition by keeping the stem at a high RH. These results, and supporting evidence from previous investigations with other species, indicate that the correlative inhibition of root buds is caused primarily by competition for water between the root buds and the main shoot.