REEKIE, J.Y.C., HICKLENTON, P.R., and REEKIE, E.G. 1994. Effects of elevated CO, on time of nowering in four short-day and four long-day species. Can. J. Bot. 72: 533-538. This study was undertaken to determine if the effect of elevated CO, on flowering phenology is a function of the photoperiodic response of the species involved. Four long-day plants, Achillea millefolium, Callistephus chinensis, Campanula isophylla, and Trachelium caeruleum, and four short-day plants, Dendranthema grandtjlora, Kalanchoe blossfeldiana, Pharbitis nil, and Xanthium pensylvanicum, were grown under inductive photoperiods (9 h for short day and 17 h for long day) at either 350 or 1000 pLIL CO,. Time of visible flower bud formation, flower opening, and final plant biomass were assessed. Elevated C 0 2 advanced flower opening in all four long-day species and delayed flowering in all four short-day species. In the long-day species, the effect of CO, was primarily on bud initiation; all four species formed buds earlier at high CO,. Bud development, the difference in time between flower opening and bud initiation, was advanced in only one long-day species, Callistephus chinensis. Mixed results were obtained for the short-day species. Elevated C 0 2 exerted no effects on bud initiation but delayed bud development in Dendrantherna and Kalanchoe. In Xanthiuttz, bud initiation rather than bud development was delayed. Data on bud initiation and development were not obtained for Pharbitis. The negative effect of CO, upon phenology in the short-day species was not associated with negative effects on growth. Elevated CO, increased plant size in both long-day and short-day species.

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Young, greenhouse-grown tomato plants were transferred to growth cabinets where they were maintained in normal air (0.03% CO2) or in air enriched to 0.1 or 0.5% CO2. CO2 enrichment increased net assimilation rate but decreased leaf area ratio. As a result, relative growth rate was greatest at 0.1% CO2 and was less in 0.5% CO2 than in 0.03% CO2. Gas exchange measurements were made on the third true leaf of plants from different CO2 regimes. They indicated that growth under conditions of CO2 enrichment affected photosynthesis at an early stage of leaf development (leaf plastochron index (PI) = 5) but not at a later stage (PI = 10.5). The effects were linked to changes in mesophyll resistance, not stomatal resistance. At PI = 5 and under equivalent test conditions of irradiance and CO2 concentration, net photosynthesis tended to be increased following growth in 0.1% CO2 but was decreased or unchanged by 0.5% CO2. Young leaves developed in 0.1% CO2 were less subject to photosynthetic inhibition by atmospheric oxygen and had low CO2 compensation points. CO2 enrichment also affected the activities of the enzymes ribulose-1,5-bisphosphate carboxylase and glycolic acid oxidase and the enzyme responses corresponded well with the gas exchange responses. The results indicate that photosynthetic adaptations may occur in response to the concentration of CO2 present during growth, and that enrichment to concentrations much above 0.1% CO2 may be detrimental to net photosynthesis and growth rate.

Laboratory measurements of net carbon dioxide exchange in relation to light and temperature were made on Dicranum fuscescens Turn, at Schefferville, Quebec (latitude 55° N), during the summer of 1974. Net CO2 exchange was measured using an open-flow infrared gas analysis system. Moss samples were collected from two field sites (a lowland lichen woodland and highland semitundra region) immediately before the experiments. Temperature optima for photosynthesis in plants from both sites showed acclimation to higher temperatures in the middle of the season. Measured maximum rates of photosynthesis, attained in early July, equalled 2.1 mg CO2∙g dry weight−1∙h−1 in plants from the highland site and 0.74 mg CO2∙g dry weight−1∙h−1 in those from the lowland lichen woodland. Dark respiration rates showed no seasonal temperature acclimation. Radiation levels required for saturation of photosynthesis at optimum temperatures showed an increase from early season through midseason in samples from both populations. A reverse trend towards lower light requirements for saturation was detectable in the late season. Field-collected plants were exposed to different temperature regimes for [Formula: see text] months in growth chambers. During this period, temperature acclimation of photosynthesis to ambient temperature conditions elicited a rapid shift in optimum temperatures for photosynthesis over periods as short as 48 h. All results are discussed in relation to measured environmental parameters in the two study sites throughout the 1974 growing season.

Many plants grown at elevated CO2 concentrations exhibit enhanced photosynthetic rates. However, this increase in photosynthesis is often reduced after prolonged exposure to elevated CO2. This reduction may be related to the capacity of plants to utilize the extra photosynthate produced at elevated CO2. This study examined the effect of source to sink ratio on the capacity of plants to respond to elevated CO2. Seven species or cultivars within the genus Brassica were germinated and grown at either 350 or 1000 ppm CO2. Broccoli (Brassica oleracea L.) and cauliflower (B. oleracea L.) have large carbon sinks in the reproductive structures; Chinese broccoli (Brassica campestris L.) and marrow stem kale (B. oleracea) have carbon sinks in the stem; turnip (B. campestris) stores carbon in the root; rape (Brassica napus L.) and white mustard (Brassica alba (L.) Rabenh.) have no obvious carbon storage structures and were assumed to have a lower sink strength relative to the above cultivars. Plants were harvested at three stages of development and total plant weight, leaf area ratio, and allocation to leaf, root, and stem determined. As young seedlings, all cultivars responded positively to elevated CO2. The long-term growth response of different cultivars to CO2 was independent of sink location, but was dependent on sink size. Cultivars with no obvious carbon storage structures showed no significant growth enhancement by elevated CO2 by the end of the experiment. However, neither leaf area ratio nor biomass allocation pattern were reliable predictors of response to CO2 suggesting that assessing differences in source to sink ratio is not necessarily straightforward.Key words: biomass allocation, sink strength, functional groups, elevated carbon dioxide, leaf area ratio.