EFFECTS OF NITROGEN ON THE RESPONSE OF LOBLOLLY-PINE TO WATER-STRESS .1. 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 (C(i)) 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.