PHOTOSYNTHESIS AND DARK RESPIRATION OF BLACK SPRUCE CUTTINGS DURING ROOTING IN RESPONSE TO LIGHT AND TEMPERATURE

Photosynthesis and dark respiration of semihardened black spruce cutti ngs (Picea mariana (Mill.) B.S.P.) were periodically measured at a ran ge of light intensities at 10, 15, 20, 25, and 30-degrees-C over an 8- week period in rooting chambers and for 4 additional weeks after the c uttings were transferred to a greenhouse. Increases in the total dry w eight of the cuttings over the experimental period were due exclusivel y to increases in root biomass. The light-saturated photosynthetic rat es at 20-degrees-C decreased from 3.8 to 2.2 mumol CO2 . m-2 . s-1 dur ing the 8 weeks in the rooting chamber. At 15-degrees-C, the light-sat urated photosynthetic rate was about 2 mumol CO2 . m-2 . s-1 and no si gnificant change was observed during the experimental period. Maximum photosynthetic rates were generally attained at photosynthetic photon flux densities (PPFD) of 200-300 mumol. m-2. s-1. At the range of PPFD generally used in rooting chambers (0-50 mumol.m-2.s-1), the light us e efficiency of cuttings (net photosynthesis per cutting per PPFD) was greatest at 15-degrees-C. Furthermore, the light compensation point w as lowest at 15-degrees-C. The effect of light intensity and temperatu re on the photosynthesis and dark respiration of cuttings was modelled to predict the carbon balance of cuttings under different conditions of PPFD and temperature. This model should be useful in determining an appropriate set of environmental conditions to use inside rooting cha mbers and thus improve the overall physiological quality of this type of vegetatively propagated planting stock. The modelling approach desc ribed in this study could prove useful for the production of other con ifer species by rooted cuttings even when it is conducted using other methods of cultivation (e.g., cold-frames or greenhouses).