Photosynthesis and Respiration in Germinating Spores of Polytrichum

The rates of respiration and photosynthesis were plotted as a function of time after adding water to fresh spores of Polytrichum juniperinum. Respiration is limited by hydration, rising to a maximum after 10 minutes. Photosynthesis is limited by more than hydration, In continuous light the rate of photosynthesis is maximized by the time 1.5 hours has elapsed. The maximum photosynthetic capacity can be achieved by dark incubation, also. Experiments with O, deprivation lead to the suggestion that respiration-derived energy contributes to the buildup of photosynthetic potential in the dark. Directly after meiosis, the spores of the moss Polytrichum contain little or no chlorophyll. As a spore ripens, however, the single plastid that it contains regreens and divides (Paolillo, 1969). At the shedding stage, 2-4 typical chloroplasts are present in each spore (Reighard, 1967). Freshly shed spores of P. juniperinum germinate rapidly in a favorable environment. A variety of morphological changes can be detected with the light and electron microscopes within four hours of the time spores are placed on agar. One obtains the general impression that ripening and germination are two phases of a continuous vegetative development that is interrupted by dehydration as the spores ripen in the capsule (Reighard, 1967). The present study is an attempt to reveal whether the spores are dormant or physiologically active before and immediately after they come into contact with water. Rates of respiration and photosynthesis were determined for dry spores and during the early hours of germination, using infrared gas analysis. MATERIALS AND METHODS Spores of Polytrichum juniperinum Hedw. were collected from ripe capsules and stored airdry in a 30-dram snap-cap vial. The capsules were obtained from Portland Arch, Indiana, on 26 June 1968 at which time their lids had begun to fall. To release the spores from the capsules, the lid (if still present) and epiphragm were removed with forceps and the spores were pumped into the vial by repeatedly compressing the opened capsule between the tips of the forceps. The spores were accumulated over a period of several weeks before the experiments were started. For each experiment 49 mg of spores were weighed out to the nearest 0.1 mg on glazed weighing paper, transferred to an 11-cm circle of Whatman No. 1 filter paper and spread with the tip of a metal spatula. The finished preparation gave the appearance that the filter paper had been rubbed with a soft, yellow-green crayon. The waxiness of the spores caused them to stick to the weighing paper and the spatula, but total losses were probably less than 0.1 mg for each preparation. The size of the paper, the amount of spores, and the method of spreading 1 This study was accomplished with the aid of National Science Foundation grant GB-6520. 2 Department of Botany, University of Illinois, Urbana, Illinois 61801. 3 Department of Botany, University of Vermont, Burlington, Vermont 05401. This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 07:10:52 UTC All use subject to http://about.jstor.org/terms 1969] PAOLILLO & JAGELS: PHOTOSYNTHESIS AND RESPIRATION 445 were chosen to assure minimum heaping of the spores and, consequently, uniform exposure of the spores to their environment. Filter paper offered two distinct advantages as a substrate for these short-term experiments: 1) dry spores became lodged in the paper and were not blown free by the moving airstream in the assimilation chamber, and 2) the paper could be wet uniformly within a few seconds. When the preparations were to be wet, the paper with the spores was laid upon two additional thicknesses of the same paper. This allowed for a reservoir of water while assuring that the spores would not be floated free of the uppermost layer of paper when the appropriate quantity of water was added. Most of the preparations were wet in the assimilation chamber itself, and the CO, exchange was followed continuously from that time. A few were wet and incubated at 25?C in glass petri plates before they were placed in the chamber. Respiration and photosynthesis were evaluated with a Grubb-Parsons (of England) SB-2 infrared gas analyzer, which continuously measures the concentration of CO2 gas in a circulating airstream. The assimilation chamber consisted of a short plexiglass cylinder with an inside diameter of 12 cm and a height of 2 cm. When the spores were wet, the plexiglass top of the chamber was fitted to a reflux condenser that returned evaporated water to the edge of the filter paper. The bottom of the chamber was a brass plate, which was immersed in a water bath during the experiments. Hence, moisture and temperature were held constant during the experiments. Except where noted, experiments were run at 250 ? 1C. Rates of CO, exchange are reported per mg air-dry spores. A light intensity of 1000 f.c., incident on the spores, was used for photosynthesis. This intensity yielded photosaturation in germinating spores. The heating effect of the light was reduced by imposing a water filter between the light source and the chamber.