Whole Plant Studies on Photosynthesis and Acetylene Reduction in Alnus glutinosa

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<ul><li><p>Whole Plant Studies on Photosynthesis and Acetylene Reduction in Alnus glutinosaAuthor(s): J. C. Gordon and C. T. WheelerSource: New Phytologist, Vol. 80, No. 1 (Jan., 1978), pp. 179-186Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2431649 .Accessed: 14/06/2014 20:26</p><p>Your use of the JSTOR archive indicates your acceptance of the Terms &amp; Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp</p><p> .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.</p><p> .</p><p>Wiley and New Phytologist Trust are collaborating with JSTOR to digitize, preserve and extend access to NewPhytologist.</p><p>http://www.jstor.org </p><p>This content downloaded from 185.44.77.82 on Sat, 14 Jun 2014 20:26:15 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/action/showPublisher?publisherCode=blackhttp://www.jstor.org/action/showPublisher?publisherCode=npthttp://www.jstor.org/stable/2431649?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>New Phytol. (1978) 80,179-186. </p><p>WHOLE PLANT STUDIES ON PHOTOSYNTHESIS AND ACETYLENE REDUCTION IN ALNUS GLUTINOSA </p><p>By J. C. GORDON* and C. T. WHEELER </p><p>Department of Botany, University of Glasgow, Glasgow </p><p>(Received 27 May 1977) </p><p>SUMMARY </p><p>Rates of photosynthesis and acetylene reduction by whole plants were measured on (a) twelve Alnus glutinosa clones and (b) A. glutinosa seedlings grown in low and high irradi- ances. Both genetic and environmental manipulation of photosynthetic rates produced concomitant changes in rates of acetylene reduction. For t-he clonal material, growing under closely similar environmental conditions, mean rate of net photosynthesis was significantly positively correlated with both the mean fresh weight of nodules formed per plant and with the mean rate of acetylene reduction. </p><p>INTRODUCTION </p><p>Previous studies of the relationship between carbohydrate supply and symbiotic nitrogen fixation in alder have relied on assays of acetylene reduction by detached nodulated roots for the estimation of nitrogenase activity (Wheeler, 1971 ; Wheeler and Lawrie, 1976). Subse- quent work has shown that not only is the rate of acetylene reduction decreased by the detopping procedures used in earlier experiments but also that the decrease in acetylene reduction does not always bear a constant relationship to the nitrogenase activity of the whole plant (Wheeler, Cameron and Gordon, 1978). These findings suggest that it would be of value to re-examine earlier data, using whole plants in the acetylene reduction assays, and also to include simultaneous measurements of photosynthetic rates by infra-red gas analysis. The omission of direct measurements of photosynthetic rate in previous studies of the dependence of nodule function on carbohydrate supply in non-legume species has prevented quantitative correlation of rates of carbon and nitrogen fixation, although recent studies on inter-relationships between photosynthesis, water potential and acetylene reduction in soy- bean have employed gas exchange techniques for measurements of photosynthetic rate (Huang, Boyer and Vanderhoef, 1975). </p><p>In alder (Wheeler, 1971; Wheeler and Bowes, 1974; Wheeler and Lawrie, 1976) as in other species (see Gibson, 1976; Sprent, 1976; Hardy and Havelka, 1976), rates of nitrogen fixa- tion or acetylene reduction are affected by treatments, such as shading, darkening or defolia- tion, which would be expected to have their greatest impact initially on photosynthesis. However, the rapidity of the response to changes in irradiance varies with species and de- pends also on plant size and development. Thus, although Akkermans (1971) found no significant changes over a 24-h period in the nodules of large, field-grown Alnus glutinosa, Wheeler (1969, 1971) reported significant diurnal fluctuations and a fairly rapid response </p><p>* P'resent address: Department of lorest Science, Oregon State University, Corvallis, Oregon 97331, U.S.A. </p><p>179 </p><p>This content downloaded from 185.44.77.82 on Sat, 14 Jun 2014 20:26:15 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>180 J. C. GORDON and C. T. WHEELER </p><p>(within 12 h) to continual darkness in young, glasshouse-grown alders. In young alder plants, radio-tracer experiments have shown that a small portion of the photoassimilate supply is translocated rapidly to the nodules, although it is not clear whether it can support nitro- genase activity (Wheeler and Lawrie, 1976). </p><p>In the present study, whole plants and infra-red gas analysis have been used to assay acetylene reduction and photosynthesis of young alder plants. To avoid possible effects of surgical treatments on acetylene reduction (Wheeler et al., 1978), both genetic and environ- mental methods have been employed to modify rates of photosynthesis in experiments de- signed to explore further relationships between carbon and nitrogen fixation in this species. </p><p>MATERIAL AND METHODS </p><p>Two sets of young plants of A lnus glutinosa (L.) Gaertn. were used. To produce plants with a high probability of exhibiting genetically based differences in photosynthetic capacity, many individual plants were grown from seed in the glasshouse under the same general cul- tural regime and environment (Bajuk, Gordon and l'romnitz, 1977). The plants were not inoculated, did not nodulate and were well supplied with combined nitrogen. Individuals showing high and low rates of growth in the favourable environment were vegetatively propagated from single-internode, greenwood cuttings and grown on in a growth chamber under 16 h, 20?C days and 8 h, 140C nights. Several ramets from each of twelve selected individuals were so grown to produce twelve clonal lines. Each ramet, after being rooted under mist, was inoculated with A. glutinosa endophyte from a single preparation of crushed- nodule inoculum. </p><p>After 8 weeks growth in the controlled environment chamber, rates of photosynthesis and acetylene reduction were measured on all clonal plants as described below and dry weights of plant parts were obtained. </p><p>The second group of plants was grown from seed in the glasshouse. After 10 weeks growth, the tops of the plants were removed and the detopped plants placed in one of two controlled environments: one with a higher irradiance (30 W m-2 photosynthetically active radiation-l'AR) and one with lower irradiance (8 W m-2 PAR). The new tops, which arose quickly from suppressed buds just above the root collar, were grown on for a further 10 weeks in the new light environment and then ten plants from each light environment were used for the measurements of rates of photosynthesis and acetylene reduction as described below. The use of cutback plants avoided the influence of lighting conditions on the pro- cesses of infection and early nodule development in seedlings, which are probably consider- able. Some nodule necrosis occurred during the regrowth of the tops, particularly in plants subjected to the lower irradiance, and only healthy nodules were included in the final weighing. </p><p>Rates of photosynthesis were measured on whole plants in a specially constructed 12-1 plexiglass container placed inside the growth chamber in which the alders were grown. An open-circuit system was used to monitor CO2 concentration in the chamber. It included, in series, these elements: 1. A compressed air source of known (c. 320 ppm) CO2 concentra- tion. 2. A humidification system comprising two constant-temperature water baths, one at 140C and one at 200C. Air from the source was saturated with water vapour in the 140C bath and then passed through a copper coil in the 200C bath, producing an air stream with a constant dewpoint of 140C. 3. The reference cell of a Grubb-Parsons differential infra-red CO2 analyser. 4. The plexiglass chamber into which the alder to be measured was sealed. </p><p>This content downloaded from 185.44.77.82 on Sat, 14 Jun 2014 20:26:15 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>Photosynthesis and acetylene reduction in alder 181 </p><p>Temperature during measurement was always within ? 1?C of growth chamber temperature. 5. The sample cell of the CO2 analyser. 6. A Rotameter flow rate meter. A flow rate of 21 min-' which was sufficient completely to change chamber air every 6 min was maintained. </p><p>After preconditioning in the chamber for 45 min, the rate of CO2 exchange was moni- tored for each plant for at least 1 h, during which time rates were stable, indicating a con- stant plant response to the experimental environment. For a set of plants from each light environment, photosynthetic responses to light intensity were determined using the two experimental irradiances (8 and 30 W m 2 PAR) and one lower irradiance (1.5 W m 2 PAR). Rates of acetylene reduction were measured by enclosing whole plants, undisturbed in the peralite-filled plastic pots in which they were grown, in plastic bags closed around subaseal stoppers with a twistum. Acetylene (0.51) was then injected into the 4-1 bags and 1-ml samples were withdrawn at 1 5-min intervals and analysed for ethylene. Empty bags and bags containing killed root systems in peralite and plastic pots were included as controls. A separate series of tests was conducted to determine rates of acetylene and ethylene leakage from the bags and one set of plants was measured both by the bag method as above and by enclosing only the pots in specially constructed glass jars. Rates determined by the two methods agreed closely. </p><p>RESULTS </p><p>Clonal coinparisons In addition to differing in growth, the alder clones differed in rate of net photosynthesis </p><p>per plant, rate of acetylene reduction per plant and in rate of acetylene reduction per g nodule fresh weight (Table 1). The clones did not differ, however, in rate of photosynthesis per dM2 leaf area (Table 2). The variance between clones in the rate of acetylene reduction per g nodule fresh weight (i.e. nodule efficiency) was not significantly different from the variance in acetylene reduction expressed on a per plant basis (Table 2). It is not possible to infer, therefore, whether nodule efficiency or the capacity of clones to form nodules was </p><p>Table 1. Rates of photosynthesis and acetylene reduction, nodule fresh weight, and total plant dry weight means for each of twelve A Inus glutinosa clones </p><p>Clone numbers are for identification only, with the first number designating the seedlot and the second the individual plant from which the clone was propagated. </p><p>Each tabular value is the mean of three observations. </p><p>Acetylene reduction (nmole h1 -) IPhotosynthesis Noduile fresh Total plant </p><p>Clone per g fresh per plant weight dry weight (number) per plant wt nodules (mg CO2 h-1) (g) (g) </p><p>5-50 335 386 12.8 0.98 5.29 3-25 266 473 10.9 0.55 4.30 3-23 188 533 8.8 0.40 3.79 1-23 179 332 9.3 0.54 4.28 2-50 157 161 11.8 1.02 4.89 2-58 151 275 8.8 0.52 4.10 3-13 142 352 2.9 0.26 2.11 6-15 133 228 6.8 0.58 3.25 6-12 127 232 7.6 0.57 3.21 5-37 123 171 8.6 0.67 3.05 4-40 38 276 6.6 0.14 2.81 3-21 25 92 6.5 0.15 2.79 </p><p>This content downloaded from 185.44.77.82 on Sat, 14 Jun 2014 20:26:15 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>182 J. C. GORDON and C. T. WHEELER </p><p>more important in explaining the observed differences in whole plant acetylene reduction rates. </p><p>Net photosynthesis rate per plant correlated well with acetylene reduction rate per plant and with nodule fresh weight per plant for the twelve clones (Table 1). The correlation co- efficients with acetylene reduction (r = 0.59) and with nodule fresh weight (r = 0.68) were both highly significant (P</p></li><li><p>Photosynthesis and acetylene reduction in alder 183 </p><p>(Table 3). Plants grown under the higher irradiance had approximately 50% greater leaf area and more than twice the total leaf dry weight. Thus, the low irradiance plants had about twice the specific leaf area (dm2 g-1) and half the specific leaf weight (g din2) of the high irradiance alders. Total shoot dry weight of the high irradiance alders was three times those grown under low irradiance but the former plants had only 70% more nodule dry weight, </p><p>Table 3. Oven dry weights and leafJarea ofJA lnus glutinosa plants grown at two irradiance levels </p><p>Fach tabular value is the mean of ten observations. </p><p>Irradiance Oven dry wt (g) Specific during growth Leaf area leaf area (W m 2 PAR) Top Leaves Nodules (dM2) (dM2 g-1) </p><p>30 1.58 1.10 0.15 2.38 2.18 8 0.52 0.39 0.10 1.65 4.19 </p><p>reflecting the influence of the similar light environment under which the root systems of both groups were formed initially. </p><p>The photosynthetic response to changes in irradiance of the two groups followed the pattern often reported previously for plants grown at different irradiance levels. Plants grown under 8 W mn2 had a lower rate of dark respiration, a lower apparent light compen- sation point and a lower rate of net photosynthesis (per plant and per unit leaf area) at the higher (30 W m 2) irradiance. At 8 W rn2, rates of photosynthesis per plant of both high and low light-grown plants were nearly equal and rates per unit leaf area exactly equal (Table 4). </p><p>Acetylene reduction rates, expressed on both a 'per plant' and a 'per g nodule dry weight' </p><p>Table 4. Rates of net CO2 exchange (- denotes net CO2 release) at three irradiance levels and in the dark fbr alder plants grown at </p><p>-high (30 W m 2) and low (8 W m 2) irradiances All CO2 exchange values are the means of four observations. </p><p>Irradiance (WX m2 PAR) CO2 exchange (mg CO2 h-1) D)uring growth l)uring assay per plant per dm 2 leaf area </p><p>30 0 -41.73 -0.60 1.5 -0.71 --0.46 8 2.04 0.70 </p><p>30 5.18 1.81 8 0 -0.32 -0.14 </p><p>1.5 0.12 0.05 8 1.61 0.71 </p><p>30 2.71 1.20 </p><p>basis, were increased 45 min after transfer of low light grown plants to higher irradiance levels, although the effect on ethylene production was considerably less than that noted between plants grown continually in the two irradiances (Table 5). A decrease in the rate of acetylene reduction also followed the transfer of plants, grown under the higher irradiance, to the lower irradiance levels, although the change in activity was much less than that which followed transfer of low-irradiance-grown plants to the higher irradiance conditions. </p><p>This content downloaded from 185.44.77.82 on Sat, 14 Jun 2014 20:26:15 PMAll use subject to JSTOR Terms and Conditions</p><p>http://www.jstor.org/page/info/about/policies/terms.jsp</p></li><li><p>184 J. C. GORDON and C. T. WHiEELER </p><p>ITable 5. Rates of/acetylene reduction for Alnus gluti- nosa plants grown either under low (8 W m 2) or h...</p></li></ul>

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