The growth of apical sprouts of potato tubers, var. Norland, kept in darkness at 15 deg;C, was significantly increased either by increasing the relative humidity (RH) from approximately 40 to 100 or by supplying water to the basal cut end of the tubers. These two treatments showed a significant interaction, the response to the external water supply being considerably greater at the higher RH. The growth response was closely correlated with increases in the dry weight and the water content of the sprouts, expressed on either a sprout or dry weight basis. The high RH plus water treatment also eliminated the inhibition of sprout growth by light, the length of the sprouts in this treatment being almost 160 greater than in the light at low RH and 60 greater than in the dark. These results are consistent with the hypothesis that when the tuber has no external supply of water, the growth of the sprouts is determined primarily by their water content, and thus by the effect and interaction of factors affecting the supply of water from the tuber and its rate of loss from the sprouts by transpiration.
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