Transpiration Efficiency in Relation to Nutrient Status 1

Many experiments have shown a linear relationship between plant biomass and water use. Despite this, it is thought that general nutrient deficiency lowers transpiration efficiency. Since little attention has been given to the effects of specific nutrients, a study was designed to examine the effects of a single nutrient, K, on the transpiration efficiency of alfalfa ( Medicago sativa L.). In addition, maize ( Zea mays L.) biomass and water‐use data obtained in 1911 by Montgomery and Kiesselbach, at a range of nutrient levels, were reanalyzed to further assess nutrient effects on transpiration efficiency. In a greenhouse study, ‘Banner’ alfalfa was grown, in pots, in a range of soils varying widely in K status but otherwise adequately fertilized. A sand mulch was used to minimize evaporation from the pots. Evapotranspiration (determined by weighing) and biomass data were collected over seven regrowth cycles. At every harvest there was a wide range in tissue K status, but only at harvest four did transpiration efficiency appear reduced by restricted K supply. Biomass and evapotranspiration remained well correlated and linearly related for every harvest, despite the increasing occurrence of severe K deficiency as the experiment progressed. Transpiration efficiency did fall substantially for the last two harvests, possibly in response to the lower (winter) light regime under which those harvests were made. Although not applicable to a specific nutrient, Montgomery and Kiesselbach's maize data yield similar results, and do not suggest that nutrient deficiency leads to lower transpiration efficiency.