PHYSIOLOGICAL ADJUSTMENT OF 2 FULL-SIB FAMILIES OF PONDEROSA PINE TO ELEVATED CO2

Seeds from two full-sib families of ponderosa pine (Pinus ponderosa) w ith known differences in growth rates were germinated and grown in an ambient (350 mul l-1) or elevated (700 mul l-1) CO2 concentration. Gas exchange at both ambient and elevated CO2 concentrations was measured 1, 6,39, and 112 days after the seed coat was shed. Initial stimulati on of CO2 exchange rate (CER) by elevated CO2 was large (> 100%). On D ay 1, CER of seedlings grown in elevated CO2 and measured at ambient C O2 was significantly lower than the CER of seedlings grown and measure d at ambient CO2, indicating physiological adjustment of the seedlings exposed to elevated CO2. Physiological acclimation to elevated CO2 wa s complete by Day 39 when there was no significant difference in CER b etween seedlings grown and measured at ambient CO2 and seedlings grown and measured at elevated CO2. After 4 months, the light response of s eedlings in the two treatments was determined at both ambient and elev ated CO2. Light compensation point, CER at light saturation, and appar ent quantum efficiency of seedlings grown and measured at ambient CO2 were not significantly different from those of seedlings grown and mea sured at elevated CO2. With a short-term increase in CO2, CER at light saturation (5.16 +/- 0.52 versus 3.13 +/- 0.30 mumol CO2 m-2 s-1 ) an d apparent quantum efficiency (0.082 +/- 0.011 versus 0.045 +/- 0.003 mumol CO2 mumol-1 quanta) were significantly increased. Leaf C/N ratio was significantly increased in the elevated CO2 treatment. There were few significant differences between families for any response to elev ated CO2. Under the experimental conditions, high growth rate was not correlated with a greater response to elevated CO2.