Nitrogen deficiency and fertilization effects on needle growth

(Bong.) Carr.) seedlings were subjected to nitrogen (N) deficiency for 1 year and then treated twice weekly with N fertilizer solutions (0 to 224 mg N l) for 20 weeks. Needle growth in terms of projected area and cell number, and photosynthesis in terms of pigment concentration, net photosynthetic rate (PN), stomatal conductance to CO2 (GS) and intercellular partial pressure of CO2 (Ci) were measured. Nitrogen deficiency caused significant reductions in needle size and needle number per shoot. Refertilization caused free growth, and increases in size and cell number of needles arising from primordia initiated during N deficiency. Nitrogen concentration did not affect mean cell size, indicating that N nutrition determined needle size through cell division and not cell expansion, and that favorable N supply during needle expansion was more important in determining final needle size than limitations imposed on the primordia during the N deficiency pretreatment. Nitrogen deficiency caused reductions in chlorophyll and carotenoid concentrations, PN and GS, and an increase in Ci. Refertilization caused a reversal of these changes, and the magnitude of the responses increased with increasing N supply. The large increases in many of the measured parameters in response to the small amounts of added N suggest that young Sitka spruce trees respond positively to N supply and are tolerant of all but severe N deficiency.