Control of Leaf Expansion by Nitrogen Nutrition in Sunflower Plants

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Abstract
N nutrition strongly affected the growth rate of young sunflower (H. annuus L.) leaves. When plants were grown from seed on either of 2 levels of N availability, a 33% decrease in tissue N of expanding leaves was associated with a 75% overall inhibition of leaf growth. Almost all of the growth inhibition resulted from a depression of the daytime growth rate. Measurements of pressure-induced water flux through roots showed that N deficiency decreased root hydraulic conductivity by .apprx. half. N deficiency lowered the steady-state water potential of expanding leaves during the daytime when transpiration was occurring. N-deficient leaves were unable to maintain adequate turgor for growth in the daytime. No deficiency decreased the hydraulic conductivity for water movement into expanding leaf cells in the absence of transpiration, but growth inhibition at night was much less than in the daytime. N nutrition had no detectable effects on plastic extensibility or the threshold turgor for growth.