Rate of Respiration of Douglas-Fir Seedlings

The rate of photorespiration of Douglas-fir seedlings was measured under different light intensities by: (1) extrapolating the curve for CO., uptake in reilation to atmospheric CO, content to zero CO2, content, and (2) measuring CO, evolution of the plants into a CO2-free airstream. Different results, obtained from these techniques, were lelieved to be caused by 'a severe restriction of the photosynthetic activity when the latter was used. With the first method, CO, !evolution was lower than the dark respiration rate at low light intensity. For all temperatures studied (6°, 200, 28°) a further increase in light intensity raised the CO., evolution above dark respiration before it leveled off. The rate of CO, evolution was stimulated by increase in temperature at all light intensities. With the CO.free air method, CO, evolution in the light was less than dark respiration at all light intensities. In recent years considerable evidence has been prodtuced concerning the inftluen,ce of light on plant respira'tion. The magnitude of photorespiration varies between plant species. In some pliants the CO. evolved in pho,torespiirattion is a major proportiion of the CO, flixed in photosynithesis; in ottherls, such as maize, photorespilration a-ppears to be negliig.lble o,r non-existent (5). In explaining differences in ra,te of apparent photosynvthesis, photorespiirat,ion should be related to planot type anld environment. Iit has been suggested that the h!igh rate of apparent ph,oitosynthesis in maize is catused by iits low rate of photorespiration (5, 17). In a study of environmental influences onl rate of apparent photosynithesis of Douglas-fir [Pseudotsuga mienziesii (Mirb.) Franco] seedlings, effects of light intensity and temperature oln pho,toOrespirati,on were examined. Materials and Methods The rate of CO. exchange beitween intact shoots of Douglas-fiir seedlings and the surrounding atmosphere was mea,sured at different exlterna(l CO., concentirations between 600 pp,m and the CO2 compensation point, where th'ere is no neit uptake of CO,. The CO, concentraition was measturecl with a Beckman Model 15A CO., analyzer in a conitinuouis a'irsdtream o'f a closed system containing sho'ots of 3 Doutglas-fir seedlings grown in a 17.5 cm pot. The seedllings were 2 m,onithis olId, about 8 cm tall and had numerous fuldly deve,loped leaves. The 3 shoots were enclo,sed in a double-waille,d chamber withl a plexiglass top and sealedl airtight from the ro,ot-s (luring measturement. Air temperatuire in the 389 chamber was controlled to ± 0.50 by regulatdng a flow of cooled water beltween the walils of the plant chamiber. The leaf su.rface temperatu,re, measured with a thermocouple, was about 10 higher than the aiir temperature, wh,en a light intensity of 2500 ft-c was used. In experiments at this lii'ght intensity and at 3300 ft-c, the ai,r temperature was kept 1° lower than the desired leaf temperature. Adjustmenit wvas nolt made at lower ligh't iratensilties. Light was filtered through a 5 cm laver oif water from a 750 w G.E. reflector spot lamp. Tihe initenisity was regullated by adjusiteinig the distance to the light