PHOTOSYNTHETIC ACCLIMATION OF PLANTS OF DIVERSE ORIGIN

Plants of 5 species were acclimated for periods of not less than 3 weeks in a desert and subalpine environment. Subsequent measurements of photosynthesis over a range of temperatures indicated both rate modifications and shifts in temperatures of optimal photosynthesis. Plants of all species acclimated in the coldest environment were more efficient in terms of percentage of maximum photosynthetic activity at colder temperatures. Conversely, plants acclimated in the desert environment were more efficient at warmer temperatures. LIMITED attention has been directed toward the study of temperature-controlled photosynthetic acclimation of plants. The scant literature on this subject has been reviewed recently by Semikhatova (1960). Present knowledge can be summarized by the statement that the after-effect of exposure of plants to high or low temperatures may result in a depression of photosynthetic rates and a shifting of the point of optimal photosynthetic temperature. The latter phenomenon is apparently common in aquatic plants but is more limited in terrestrial plants (Evans, 1963). The objective of this study was to determine the photosynthetic plasticity of a number of terrestrial higher plant species from diverse climatic regions. The study area is located in the White Mountains of California, a high desert range in the eastern part of the state. The seed, collected from these mountains, represents species native to environmental extremes from the desert to the alpine. MATERIAL AND METHODs-Plant material utilized in this study was grown from seed in Los Angeles, California, during the fall of 1962. The elevation at which the seeds were collected as well as the actual and total altitudinal limits of the 5 species are given in Table 1. Plants were established in a uniform soil in 10-cm clay pots and cultivated in an air-conditioned greenhouse under a 20-hr photoperiod. During June, 1963, the plants were transported to Big Pine, California, at the western base of the White Mts. These were subsequently taken from this staging area to stations at various elevations on the White Mts. for acclimation (Table 2). At all stations, plants were continuously subirrigated utilizing an autoirrigator device. Thus, for all plants, water was non-limiting, and soil type was a constant. All other environmental elements that are elevation-dependent were variables. After acclimation periods of not less than 3 weeks (Table 2), 2 or 3 plants of each species were I Received for publication January 19, 1964. This study was supported by National Science Foundation Grant G-17880. We express our appreciation to the White Mt. Research Station personnel for assistance and use of facilities. brought from each station to the White Mt Research Station, Crooked Creek Laboratory (elev. 3094 m) for analysis. At this time all plants possessed leaves which were formed both prior to and during the acclimation period. For any given species, measurements on plants from all stations were performed during a period of not greater than 48 hr. All measurements of gas exchange rates were performed in the same routine manner. The entire shoot of a plant was sealed within an environmental chamber where temperature was controlled. Water at constant temperature was circulated through a radiator within the chamber. A fan, also mounted within the chamber and behind the radiator, provided air turbulence and heat exchange. A General Electric "Cool Beam" 300-w floodlamp was used for the light source for photosynthesis measurements. This lamp yielded 8,000 ft-c at the top glass surface of the chamber. A closed system was utilized for gas exchange measurements. An air stream was circulated through the environmental chamber to a Model 15A Beckman Infrared Gas Analyzer, and then returned to the chamber. The time interval in which the plant reduced or increased the CO2 content of the air stream between the levels of 0.50 and 0.55 mg CO2 per liter was recorded, and from this, photosynthesis and respiration rates were calculated. For any given plant the following measurement sequence was used: net photosynthesis at 10, 15, 20, 25, and 30 C, and then dark respiration at 30, 20, and 10 C. All results are given as averages and are expressed as mg CO2 per gram leaf dry weight per hour. RESULTS AND DISCUSSION-The mean photosynthetic rates of plants of 3 of the 5 species acclimated at the subalpine station was greatly depressed when compared with plants grown at lower elevations (Table 3). The remaining 2 species, Haplopappus apargioides and Artemisia tridentata, however, had fairly uniform rates regardless of where they were grown. In general, the dark respiration rates of plants of all species from all stations were quite similar.