Effects of nitrogen on the response of loblolly pine to water stress I. Photosynthesis and stomatal conductance



Photosynthetic and stomatal response of loblolly pine (Pinus taeda L.) to water stress were determined for seedlings grown under 4 different nitrogen levels. Photosynthetic rate of well–watered seedlings was linearly related to foliar N. There was no observed N effect on the relative reduction of photosynthesis due to water stress. N did not influence stomatal conductance before or during the water stress period. Therefore, instantaneous water use efficiency of well–watered seedlings was significantly increased with increased N. Significantly reduced internal CO2 concentration (Ci) associated with increased N in well–watered seedlings may be indicative of greater relative stomatal contribution to overall limitation of photosynthesis. Water stress caused an increase in Ci, the slope of the increase being related to N treatment. Stomatal conductance and photosynthesis correlated strongly within each N treatment, and increased N caused an increase in initial slope of the function. Therefore, stomatal control was uncoupled from photosynthetic capacity by N treatment in loblolly pine. These data fail to support the proposition that improved stomatal response to water stress can explain reported gains in water status of nitrogen–fertilized conifers in the field.