Photosynthesis and nutrient supply in needles of Sitka spruce [Picea sitchensis (Bong.) Carr.]

Growth and photosynthetic development were measured for currently developing needles of young trees of Sitka spruce [Picea sitchensis (Bong.) Carr.] throughout their fourth growth season. Treatments included trees that were fully fertilized (control), trees deficient in phosphorus (-P), or nitrogen (-N), and trees initially deficient but then supplied with phosphorus (-PR) or nitrogen (-NR). Growth was measured in terms of needle projected area, and the photosynthetic components measured were pigment concentration, net photosynthetic rate, (P_N ), activity of ribulose-1,5-bisphosphate carboxylase, (RuBPC), stomatal conductance to CO₂ , (G_s ), and the intercellular partial pressure of CO₂ , (C_i ). Needle growth was rapid, beginning in early May and being complete by the end of the month or early in June. Free growth occurred in the -NR treatment. Photosynthesis increased throughout the season, reaching a peak in August, with some variables subsequently showing a decrease in value. P_N increased more rapidly during needle expansion than either chlorophyll concentration or RuBPC activity. Phosphorus deficiency led to a reduction in RuBPC activity, which was restored to the control value following Refertilization with P. Nitrogen deficiency severely reduced values of all variables studied, except C_i , which was higher than for the controls. Refertilization of -N trees caused a very rapid increase in values of all variables, with an increase in C_i , representing a larger increase in mesophyll conductance to CO₂ , (G_M ), than G_S , P_N and RuBPC activity were significantly correlated with total chlorophyll concentration for all treatments, but P_N was not correlated with G_S or RuBPC activity.