Voi 5 (1996): 285-298. The effects of prolonged exposure to elevated temperatures and elevated C02 levels on the growth, yield and dry matter partitioning of field-sown meadow fescue Kaija Hakala and Timo Mela Agricultural Research Centre ofFinland, Institute ofCrop and Soil Science, FIN-3 1600 Jokioinen, Finland Field-sown meadow fescue (Festuca pratensis, cv. Kalevi) stands were exposed to elevated temper- atures (+3°C) and elevated CO, (700 ppm) levels in two experiments conducted in 1992-1993 (ex- periment 1) and in 1994-1995 (experiment 2). Total aboveground yield was, on average, 38% higher at elevated than at ambient temperatures. At ambient temperatures elevated C0 2 increased the number of tillers by 63% in 1992, 24% in 1993, 90% in 1994 and 14% in 1995. At elevated temperatures, the increase in tiller number in elevated C0 2 was seen only in the first growing seasons after sowing. The total yield in a growing season was about 10% higher in elevated C0 2 in experiment 1. In experiment 2 the yield was more than 20% higher in elevated CO, at elevated temperatures, whereas at ambient temperatures the rise in C0 2 level had no effect on the yield; the root biomass, however, increased by more than 30%. In elevated C0 2 at ambient temperatures the root biomass also increased in experi- ment I, but at elevated temperatures there was no consistent change. The soluble carbohydrate con- tent of above-ground biomass was 5-48% higher in elevated CO, at most of the measuring times during the growing season, but the nitrogen content did not show a clear decrease. The reasons for the lack of a marked increase in biomass in elevated CO, despite a 40-60% increase in photosynthe- sis are discussed. Key words: carbon dioxide, grass, Festuca pratensis, climate change, roots, carbohydrate, nitrogen ntroduction The mean global atmospheric concentration of carbon dioxide (C02 ) has risen from about 270 ppm in pre-industrial times to almost 360 ppm in 1994 (IPCC 1996). This figure has been pro- jected to increase to between about 485 and 850 ppm by 2100, the exact level depending largely on future anthropogenic emissions of CO, into the atmosphere (Carter 1996).The increased C02 concentration is expected to enhance the photo- synthetic capacity of C 3 plants, because the net photosynthesis is currently often limited by the less than optimal C02 concentrationand by pho- torespiration (see reviews by e.g. Lawlor and Mitchell 1991 and Bowes 1993). Higher photo- synthetic rates are expected to lead to an increase © Agricultural and Food Science in Finland Manuscript received July 1996 285 AGRICULTURAL AMD FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=yoNeS4G0sTdSK3lq.UNUkTXjCjUUpgjTXytZV3w.n9LulDkB1JeCCT76PAnt9LoPffcsUIH33pLoUlAdAaBk35lVv3Y9_layQPVODDlDWU6GiQOT6GMNMLwyp7s9-e8W9P_2UyptwqnAQpJkboOfpCnb0Cbq63LssXsPQ0jLPS8v1EM2eHRYbiTcqM0QhBxGsdF7jTYEJKNbdv-Yb0z_nGAgWB3zRH-_pul_wZKkiwsXEDBQKT2Zlbktjz73UvGzuqjtKw0nD5lRSDG_ioVy5ifiOfPJo0wzwFCWhhxSIk6LvIYsXRdsYBBeuZLdT9k in the dry matter production of C 3 plants, an ef- fect that has already been observed in many ex- periments (Lawlor and Mitchell 1991, Bowes 1993). Increases in photosynthesis are seldom trans- lated into equivalent increases in crop yields, however, because of other constraining factors in the environment or genetic factors at plant level, which limit plant growth (Nijs et al. 1989, Lawlor and Mitchell 1991, Ryle et al. 1992, Bowes 1993, Gay and Hauck 1994). Alternative- ly, the extra photosynthate may be translocated to roots (Nijs et al. 1988, 1989, Bowes, 1993, Luoetal. 1994, Schenk et al. 1995) or extracted to the soil as carbohydrate (Diazet al. 1993, Zak et al. 1993).A more extensive root system would be beneficial for grass and for cereal crops through the improvement in nutrient and water availability, and also as a carbohydrate storage for regrowth of grass during the first days after cuts (Nijs et al. 1988). Moreover, increased ex- cretion of carbohydrate through roots to the soil might increase the microbial biomass of the soil, thus either improving (Zak et al. 1993) or di- minishing nitrogen availability (Diaz et al. 1993), probably depending on the soil type and the chemical composition (the C:N ratio) of the litter being digested by the microbes. Increases in concentrations ofCO, along with changes in other atmospheric constituents are also known to affect the global climate. Recent projections for Finland indicate a mean annual warming ofbetween 1.1 and 6.6°C by 2100, with a central estimate of 4.4°C (Carter 1996). High- er growing-season temperatures could be expect- ed to enhance the yield of crops with continu- ous growth, such as grass, mainly through a lengthening of the growing season. Gains in yield might be especially great in the spring, when the light intensity is high but temperatures are now too low for crop growth. The aim of the investigation described here was to evaluate, through direct experimentation, the impacts of future climate changes on the growth and yield of a grass crop, meadow fes- cue (Festuca pratensis). Although many experi- ments have been conducted to study the effects of C02 on individual or several grass plants in pots (e.g. Nijs et al. 1988, 1989,Ryle et al. 1992, Baxter et al. 1994),few investigations have been made of the effects of both C02 and temperature on crop stands, especially under the long pho- toperiod conditionsprevailing during the grow- ing season at high latitudes. As the sward was sown directly on the field at normal sward den- sity, the arrangement of the experiments simu- lated natural conditions closely with respect to canopy structure and intraspecific competition, and there was no pot size limitation for root growth, as may have been the case in many pre- vious experiments (Arp 1991,Thomas and Strain 1991). Material and methods The experiments were carried out in southern Finland, Jokioinen (60°49’N, 23°30’E). Mead- ow fescue (Festuca pratensis Hudson) cv. Ka- levi was grown according to the following four treatments: a) ambient air temperature and am- bient C02 concentration; b) increased tempera- ture (3°C above ambient) and ambient C02 ; c) ambient temperature and elevated CO, (700 ppm), and d) increased temperature and elevat- ed C02 . To maintain the experimental tempera- tures at a constant 3°C higher than the ambient temperatures, a greenhouse (20 m x 30 m) was built over an experimental field (Hakala et al. 1996).The experimental field outside the green- house, at ambient temperature, was covered at a height of 3-4 m with the same plastic film that was used in the construction of the greenhouse to achieve radiation conditions comparable to those in the greenhouse and to protect the area from rain. The temperatures in the open field under the film cover were recorded, and the greenhouse temperatures were regulated so that they were constantly 3°C higher than these. The mean temperatures during the day (from 6 a.m to 6 p.m.) under the cover in the open field were from I°C lower to I.5°C higher than those in 286 Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 285-298. the adjacent field without a cover (average tem- perature difference from I June to 31 August O.O°C, standard deviation 0.62°C) . The cover over the open field was removed during the win- ter to allow snow to protect the grass naturally from freezing temperatures. In the greenhouse the grass crop was covered with a light cloth for periods of extreme low temperatures in winter. The C0 2 experiments were conducted in open-top chambers (OTCs) 3 m in diameter and 2 m high (Hakala et al. 1996). The OTCs were divided into two halves, the southern half being occupied by the meadow fescue stand, and the other half being used for experiments with spring wheat. Two experiments were conducted, each covering two growing seasons: those of the year of sowing and those of the following year, which is considered as the main yield producing sea- son. Figure 1 shows the temperature regimes of the four growing seasons 1992-1995. Experiment 1 was conducted on a heavy clay soil. Before the experiments the soil was mixed with 1000 rrvVha of peat containing 35% sand. The peat was limed and fertilized with a peat fertilizer containing 11% nitrogen, 11% phos- phorus, 20% potassium, 6.2% sulphur, 1% cop- per, 0.6% sodium, 0.6% chloride, 0.5% iron, 0.4% manganese, 0.2% zinc, 0.1% molyb- denium, 0.1 % magnesium and 0.08% boron. The peat-sand mixture was mixed with the clay soil at a depth of 25-30 cm, after 500 kg/ha ofa fer- tilizer containing 20% N, 4% P and 8% K had been added. The soil was treated similarly both in the greenhouse and in the open field. At the end of 1993, when experiment 1 was finished, the clay-peat soil of the experimental site was Fig. 1. Mean daily temperatures in the open field (heavy line) during 1992, 1993, 1994 and 1995 and the average tempera- tures at nearby Jokioinen Observatory during 1961-1990(light line), for the period 15April until the last cut of the grass in September (for exact dates of last cuts, see Table 1). The beginning of the thermal growing seasons is marked with * (for exact dates, see Table 1). s 1 = sowing in the greenhouse, s 2 = sowing in the open field. *2 = beginning of growing season in the open field in 1995. The temperatures inside the greenhouse were 3"C higher than those in the open field (see Hakala et al. 1996 for details). 287 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table I. Beginning of thermal growing season (temperatures permanently over 5 °C) in 1992-1995, sowing density and sowing dates of meadow fescue stands in 1992 and 1994,and cutting dates in 1992-1995. experiment/gr. season I/lst I/2nd 11/Ist 11/2nd year 1992 1993 1994 1995 treatment amb. T elev. T amb. T elev. T amb. T elev. T amb. T, elev. T, amb. C02 amb. C02 amb. C02 amb. C02 amb. CO, amb. CO, amb, CO, elev, C02 amb. CO, elev. CO, elev. C02 elev. C02 elev. CO, elev. CO, elev. C02 elev. C02 soil clay/peat clay/peat clay/peat clay/peat sandy sandy sandy sandy sandy sandy loam loam loam loam loam loam beginning of thermal 27April 25 April 22April 22April 22April 19April 20April 20April 15April 15April growing season sowing density, 1250 1250 750 750 germinating seeds/m2 sowing 14May 29April 9 May 15April Lent 9July 15 June 1 June 18May 29 July 17 June 6June 6June 22May 22May 2. 14Aug, ISJuly BJuly 24June 24Aug. ISJuly 12July 3July 15June 16Jline 3. 24Sept. 20Aug. 16Aug. 2Aug. 21 Sept 15Aug. 14Aug, 24 July 3 July lOJuly 4. 22Sept. ISSept. 13Sept. 12Sept. 20Sept, 14Aug. 19 July 27July 5. cut 18 Sept 14Aug. 6 Sept. 6. cut 11 Sept. replaced with a lighter sandy loam soil brought to the experimental plots from another field in Jokioinen, and the drainage of the soil was im- proved by placing new drainage pipes under the soil at a depth of about 40 cm. Because nutrient analysis of the soil showed a deficiency in Ca and Mg, the soil was limed with 8000kg/ha of a fertilizer containing 35% calcium and 3% mag- nesium. Before sowing began, 500 kg/ha (green- house) or 550 kg/ha (open field) of a fertilizer containing 20% N, 6% P and 6% K was added to the soil surface. With the different fertiliza- tion rates, the nitrogen levels inside the green- house and in the open field were adjusted to about 120 kg N/ha. The experimental plots were sown directly on the field at 1250germinating seeds/m2 in 1992 (experiment 1) and 750 germinating seeds/m2 in 1994 (experiment 2), row width 12.5 cm. Inside the greenhouse, at elevated temperatures, sow- ing was completed shortly after the thermal growing season had started, i.e. when the tem- perature could be expected to stay constantly above 5°C. The sowing dates inside the green- house were thus 29 April in 1992 and 14 April in 1994. In the open field there was a delay in sowing, due partly to the later onset of the ther- mal growing season and partly to the need for the soil to dry sufficiently for sowing. Sowing thus proceeded some 2-3 weeks later in the out- side field than in the greenhouse (14 May in 1992 and 9 May in 1994,Table 1). After emergence, the seedlings were enclosed in OTCs: four in the greenhouse and four in the open field. Two of the four OTCs were main- tained at elevated C02 levels and the other two at ambient levels, giving two independent repli- cates per treatment (Hakala et al. 1996). The CO, levels in the high-CO, OTCs were not elevated during the winter, but the C02 treatments of the grass were started in April, as soon as the grow- ing season began. Irrigation was applied using 288 Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth AGRICULTURAL AND FOOD SCIENCE IN FINLAND drip-irrigation in experiment I and manually in experiment 2, to ensure that soil moisture levels would not limit growth. Fertilizer was adminis- teredboth in the open field and in the greenhouse at such a rate that the N level of the soil was 120 kg before sowing and in the spring. Thereafter in the first growing seasons the grass was ferti- lized with 80 kg N after the first cut and with 60 kg N after the second cut both in the open field and in the greenhouse, and with 60 kg N after the third cut but only in the greenhouse. In the second growing seasons the grass was fertilized with 80 kg N after the first cut and 40 kg N after the second and third cuts both in the open field and in the greenhouse.The fertilizer used con- tained 20% N. 6% P, 6% K, 5% Ca, 2.1 % S, 0.5% Mg, 0.03% B, 0.1% Fe, 0.1% Na and 0.016% Se. Pest and disease control were applied when needed. In 1992-1994, meadow fescue was cut at ap- proximately monthly intervals (Table 1). Dur- ing the second growing seasons it was cut for the first time shortly after it began to flower. This occurred in both experiments 2 weeks earlier at elevated temperatures than at ambient tempera- tures (Table 1). In 1995, following the first cut at the beginning of flowering, the grass was al- ways cut after the leaf area index (LAI) of a stand had reached a value of 5, as measured with an automatic LAI meter (Licor, USA). This result- ed in a different number of cuts in each treat- ment (Table 1). Biomass samples were collect- ed during the cuts from 10randomly chosen sam- pling plots in each of the chambers, each sam- pling plot comprising 15 cm of planted row. For biomass, nitrogen and yield determinations, the samples were oven-dried (2 hours at 100 °C, then overnight at 60 °C). The samples for soluble car- bohydrate measurements were freeze-dried and stored at -20 °C before analysis. Root samples were taken at a depth of 20 cm during the first and second cuts in the first grow- ing season of experiment 1 (1992), and during the last cuts in the other growing seasons (1993- 1995). The sample size was 15 cm of row, and both tiller number and root biomass were deter- mined from the samples. The nitrogen content of the above-ground biomass was determined in 1992 with the Kjel- dahl method with a Kjeltec System 1026 Dis- tilling Unit (Tecator AB, Sweden), and with CuS0 4 *SH,O as the catalyst. For details of the procedure, see manual of the Kjeltec System 1026 Distilling Unit. In 1993, the nitrogen lev- els were not measured because insufficient re- sources were available for the laborious Kjel- dahl procedure. In 1994-1995,the nitrogen con- tent was determined with an automatic nitrogen analyser, LECO FP-428 (LECO corp., USA). For the soluble carbohydrate analysis, the plant sam- ples were first inverted overnight at +5O °C in 0.1 N HCI. They were then filtered, and the fil- trate was mixed with ion-exchangers, shaken for one hour and refiltered; 2-ml samples of this fil- trate were then used for the analysis of soluble sugars (glucose, fructose, saccharose and fruc- tosans). The analysis was performed photomet- rically (Schimadzu, Japan, 540 nm), according to the method of Nelson (1944) and Somogyi (1945), but modified in such a manner that after the addition of copper reagent the test tubes were filled with distilled water to 7 ml before the ar- senomolybdate reagent was added. To give an idea of the variation between sam- ples, standard errors of the mean were calculated over all the samples taken from each treatment at each sampling time. The “n” in the tables and figures refers to the total number of samples of both replicates. Results Number of tillers and yield The number of tillers was counted in connec- tion with the first and second cuts in the first growing season of experiment 1 (1992) and in connection with the last cuts in all the other growing seasons (1993-1995). The number of tillers/15 cm of row was 1.7-2.7 times higher (depending on the treatment) in the first grow- 289 Vol. 5 (1996): 285-298. AGRICULTURAL AND FOOD SCIENCE IN FINLAND ing season and 2.0-2.2 times higher in the sec- ond growing season of experiment 2 than in the first and second growing seasons of experiment 1 (Fig. 2). There was no consistent effect of the temperature treatment on the tiller number of meadow fescue, but the increase in C02 concen- tration increased the tiller number by 63% and 90% at ambient temperatures and 20% and 55% at elevated temperatures during the first grow- ing seasons of experiments I and 2, respective- ly (counted on 14 July-14 August 1992 and 30 September 1994) (Fig. 2). The tillernumber also increased in elevated C02 during the second growing seasons of experiments 1 and 2 at am- bient temperatures (by 24% and 14%,respective- ly), but no increases were recorded at elevated temperatures (Fig. 2). The total above-ground yield (cumulative agricultural yield of the growing season, exclud- ing stubblebiomass) of meadow fescue was 32% (experiment 1) and 41% (experiment 2) higher at elevated temperatures in ambient CO, than at ambient temperatures and ambient C02 in the first growing seasons (Fig. 3). During the sec- ond growing season ofexperiment 1, when there were four cuts both inside the greenhouse and in the open field, the yield was 15% higher in the greenhouse in ambient CO, than in the open field. As the crop was cut whenever it attained a LAI of 5 during the second growing season of experiment 2, there were six cuts inside the greenhouse in ambient CO,, and four cuts on the equivalent plots in the open field, and the total above-ground yield of the plots inside the green- house was 65% higher than in the open field (Fig. 3). The total yield in a growing season was about 10% higher in elevated than in ambient C02 at both ambient and elevated temperatures in the first growing season of experiment 1. In the sec- ond growing season, the total yield was still 10% higher in elevated C02 at elevated temperatures, but there was no difference in total cumulative yield at ambient temperatures (Fig. 3). In exper- iment 2 the yield of meadow fescue was 29% (first growing season) and 22% (second grow- ing season) higher in elevated than in ambient C02 at elevated temperatures, but more or less the same in both CO, treatments at ambient tem- peratures (Fig. 3). In experiment 1, at elevated temperatures, the increase in the total yield in elevated CO, was achieved before the first cut, after which CO, enrichment had no effect on growth rate (Fig. 3). At ambient temperatures, an increase in yield in Fig. 2. Tiller number/15 cm of planted row in meadow fes- cue from the first and second cuts of the first growing sea- son (1992) and the last cut of the second growing season (1993) of experiment 1 (top) and from the last cuts of the two growing seasons of experiment 2 (1994-1995) (bot- tom). n is the number of 15-cm samples. 290 Hakala, K. & Mela, T: Effects ofelevated temperature and CO, on grass growth AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 285-298. elevated C02 also occurred early in the growing season, i.e. between the first and the second cuts during the first growing season and before the first cut during the second growing season. Dur- ing the second growing season, however, the growth rate in ambient C02 increased markedly relative to that in elevated C02 after the second cut, resulting in an equal total yield in both C02 treatments. In experiment 2, at elevated temper- atures, the favourable effect of C02 enrichment on growth rate was evident until late in the sea- son (August), after which the effect was slight (first growing season) or negative (second grow- ing season). At ambient temperatures the growth rate was more or less the same in both ambient and elevated CO, (Fig. 3). Root biomass Root biomass was measured both in the open field and in the greenhouse at the end of the 1993-1995 growing seasons, and during the sec- ond cut in 1992 (Fig. 4). Root biomass increased markedly towards the end of both experiments. It was approximately the same in ambient C02 treatments both at ambient and at elevated tem- peratures except in the second growing season of experiment 1, when the root dry weight was higher at ambient temperatures (Fig. 4). At ambient temperatures, the root dry weight was 67% (first growing season) and 38% (sec- ond growing season) higher in elevated C02 in experiment 1 and over 30% higher in experiment Fig. 3. Cumulative yield (g dry weight of above-ground biomass/15 cm of planted row) during the growing seasons of experiment 1 (1992-1993) and experiment 2 (1994-1995). The numbers on the lines represent the daily growth rate of the grass during the period between cuts. The number of samples (n) was 17-20 in all cuts except at ambient temperatures in 1993, when it was 7, and at ambient temperatures in 1995, when it was 14. The standard error of the mean of the biomass samples at different cuts was in general around or less than 10% of the mean, but in 1993, at ambient temperatures, it averaged 19% of the mean. 291 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth weight, being generally higher in experiment 2 than in experiment 1 (Table 2), possibly because of the different soil types. The nitrogen content was 5-22% higher in elevated C02 than in am- bient CO, in experiment 1 (measured only in the first growing season) at both temperatures. In experiment 2 the nitrogen content of the above- ground biomass of the grass was 2-17% higher in elevated C0 2 than in ambient C02 at ambient temperatures, and 3-12% lower at elevated tem- peratures (Table 2). Soluble carbohydrate content of shoots and roots The soluble carbohydrate content of meadow fescue above-ground biomass was 5-48% high- er in elevated than in ambient C02 most of the measuring times during summer, but there was no effect of elevated C02 on the carbohydrate content in the autumn (Table 3). The effect of the elevation of CO, levels on the carbohydrate content of theroots was inconsistent; in the first growing season of experiment 2 the carbohydrate content of the roots was lower in elevated C02 at both temperatures, but in the second growing season it was the same at ambient temperatures and higher at elevated temperatures in elevated CO, (Table 3). 2 (Fig. 4). At elevated temperatures, the rise in CO, did not increase root weight in experiment 1; in experiment 2, however, it increased root weight by 29% (first growing season) and 8% (second growing season) (Fig. 4). Nitrogen content of above-ground biomass The nitrogen content of meadow fescue above- ground biomass ranged from 2% to 5% of dry Discussion The total cumulative agricultural yield of meadow fescue, cv. Kalevi during the growing season was high compared to the long-term average (1988-1995) of the official variety trials of the same cultivar from the same area (Järvi et al. 1996). The yield of the stands at ambient tem- peratures and ambient CO, levels was already 10% higher in the years of sowing (1992 and 1994) than the long-term average for cv. Kalevi achieved in variety trials in the same agricultur- al area. In the year after sowing, the yield was Fig. 4. Root weight/15 cm of planted row in experiment 1 and experiment 2. n is the number of 15-cm samples. 292 AGRICULTURAL AND FOOD SCIENCE IN FINLAND AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 285-298. Table 2. Nitrogen content (% of dry weight) of meadow fescue above-ground biomass in 1992(experiment 1, first growing season) and in 1994-1995 (experiment2). n is the number of 15-cm sampling plots. date amb. T, amb. C0 2 amb. T, elev. C02 date elev. T, amb. C02 elev. T, elev. C02 experiment 1 1992 1992 9 July 1.88 (±0.10) n=2o 2.29 (±0.13) n= 17 15 June 2.75 (±0.14) n=2o 3.31 (±0.09) n=2o 14Aug. 2.68 (±0.10) n= 17 2.83 (±0.13) n= 15 15 July 2.10 (±0.15) n=l6 2.40 (±O.l l)n=l6 24 Sept. 3.15 (±0.19) n=l7 3.34 (±0.14) n= 18 20 Aug. 2.67 (±0.08) n=2o 3.18 (±0.10) n=2o 22 Sept. 3.65 (±0.14) n=l9 4.21 (±0.10) n=22 experiment 2 1994 29 July 3.32 (±0.08) n=2o 3.38 (±0.10) n=2o 17 June 4.47 (±0.10) n=2o 4.06 (±0.15) n=2o 24 Aug. 4.82 (±O.l l)n=2o 5.23 (±0.10) n=2o 18 July 4.10 (±0.06) n=2l 3.97 (±0.08) n=2o 21 Sept. 5.06 (±0.10) n=2o 5.34 (±0.05) n= 17 15Aug. 4.56 (±0.06) n=2o 4.38 (±0.07) n=2o 12 Sept. 4.93 (±0.09) n=2o 5.04 (±0.07) n=2o 1995 6June 3.41 (±0.11) n=l9 6June 3.46 (±0.12) n=2o 22 May 3.26 (±0.12) n=2o 22 May 3.19 (±0.15) n=2l 12 July 4.09 (±0.12) n=2o 3 July 4.78 (±0.09) n=2o 15 June 4.61 (±0.07) n=2o 16 June 4.42 (±0.10) n=2l 14Aug. 3.62 (±0.09) n= 19 24 July 4.13 (±0.12) n=2o 3 July 5.12 (±0.06) n=2o 10 July 4.50 (±0.06) n=2l 20 Sept. 2.39 (±0.10) n=2o 18 Sept. 2.12 (±0.04) n=l9 II Sept. 2.57 (±0.13) n=2o 6 Sept. 2.68 (±0.07) n=l9 from 63% (in 1993) to 105% (in 1995) higher than the average yield of the grass in the variety trials. The totalabove-ground yield ofmeadow fes- cue was higher at elevated than at ambient tem- peratures in all the growing seasons studied. In the first growing seasons, the higher total yields at elevated temperatures were partly due to the longer growing period, since the grass was sown 2-3 weeks earlier in the greenhouse than in the Table 3. Soluble carbohydrate content (% of dry weight) of meadow fescue above-ground (shoots) and below-ground (roots) biomass in 1992-1995. n is the number of 15-cm sampling plots. date amb. T, amb. C02 amb. T, elev. C02 date elev. T, amb. C02 elev. T, elev. C02 shoots experiment 1 9.7.1992 15.4(±1.2)n=5 21.2 (±1.7) n=4 15.6.1992 9.9 (±0.6) n= 11 11.6 (±1.0)n=10 8.7.1993 8.2 (±1.05) n=3 12.1 (±0.9) n=3 5.8.1993 8.2 (±0.2) n=2 9.4 n=l 15.9.1993 17.9 (±1.1) n=s 16.5 (±1.0) n=3 10.9.1993 13.6 (±1.0) n=lo 12.7 (±1.3) n=9 experiment 2 29.7.1994 10.6 (±1.2) n=2 13.5 (±0.5) n=2 18.7.1994 7.2 (±0.4) n=6 9.5 (±0.4) n=6 25.8.1994 13.6 (±0.1) n=2 12.7 (±1.6) n=2 25.8.1994 11.3 (±0.1) n=2 11.9 (±0.6) n=2 21.9.1994 9.4(±0.5)n=3 9.1 (±1.1) n=3 12.9.1994 9.3 (±0.2) n=6 9.6 (±0.8) n=6 6.6.1995 5.1(±0.2)n=6 4.4 (±0.2) n=6 22.5.1995 6.1(±0.5)n=6 7.8 (±0.8) n=6 roots experiment 2 21.9.1994 11.9 (+0.5) n=3 9.9(±1.4)n=3 12.9.1994 10.1 (±0.6) n=6 8.7 (±0.6) n=s 20.9.1995 15.0(±0.5) n=2o 15.6 (±0.6) n= 19 11.9.1995 5.9 (±0.6) n=2o 7.5 (±0.6) n= 19 293 Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth open field, but higher growth rates at elevated temperatures also contributed to the increase in yield, especially in 1994 (Fig. 3). In the second growing season ofexperiment 1, the yield in the greenhouse was only 15% higher than that in the open field. Because the grass was not cut accord- ing to LAI, but at about monthly intervals dur- ing the second growing season of experiment 1, the small difference between yields in the open field and inside the greenhouse may have been caused by stronger self-shading and, consequent- ly, a lower growth rate of the grass in the green- house. When the crop was cut each time it at- tained a LAI of 5 (in 1995), the total yield in the greenhouse was about twice that in 1993, and the growth rate between the cuts was clearly higher than in 1993, resulting in 65% higher yields than at ambient temperatures. Although favourable, the effect of C02 on the yield of meadow fescue was less favourable than expected. Earlier investigations and reviews have reported increases in plant biomass of between 20% and 209% at CO, levels double those of the ambient level (Lawlor and Mitchell 1991,Bowes 1993). Even though the photosynthetic rate of meadow fescue in the present experiment was 40-60% higher in elevated than in ambient C02 at both ambient and elevated temperatures (Heliö et al. 1995), the increase in photosynthesis was not translated into an equal increase in biomass. Similar results have been reported previously with grasses, and the lack of a greater effect of elevated C02 on the yield has been attributed to low sink strength due to limited tiller formation capacity (Ryle et al. 1992, Gay and Hauck 1994. Schenk et al. 1995). After experiment 1, control of C02 levels in the C02 -enriched OTCs was improved; the clay- peat soil was replaced with sandy loam, a soil with good moisture properties, and the drainage of the soil was improved. Moreover, the sowing rate was lowered from 1250 to 750 germinating seeds/m2 , and cuts were adjusted to take place according to the LAI of the grass (in 1995). These changes were made to promote the effect of C02 on the tiller number and yield, as it is known that tillerdevelopment and yield are en- hanced in soils with good nutrient availability, and when the light penetration into the canopy is improved (by lowering the sowing rate and by more frequent cuts) (Williams 1982). The till- er number/sample increased markedly in all treatments after these changes were made, and the effect of elevated CO, on the number of till- ers was greater in the first growing season of experiment 2 than in the first growing season of experiment 1. However, in the course of experi- ment 2, justas in experiment 1, the effect of el- evated C0 2 on the tiller number declined, being even smaller at the end of experiment 2 than at the end of experiment 1. Because the tiller number was approximate- ly 2-3 times higher in each treatment during ex- periment 2 than during experiment 1, there should have been enough room for the tillers to develop. Thus canopy closure cannot explain the lack of effect of elevated C02 on the tiller number at elevated temperatures in the second growing season of experiment 1. In the second growing season of experiment 2, in contrast, canopy closure may have been responsible for the decline in the effect of C02 on tiller devel- opment. Even when the tiller number was greater in elevated than in ambient C02 (Fig. 2), the total yield per cut did not necessarily increase (Fig. 3). The failure to produce higher yields in ele- vated C02 , even if the tiller number increases, has been observed with Phleum pratense and Lolium perenne (Saebo and Mortensen 1995), Agrostis capillaris, Dactylis glomerata and Fes- tuca pratensis (Saebo and Mortensen 1996) and has been attributed to the lower dry weight of individual tillers, due at least partly to reduced plant height. At elevated temperatures, the tiller number was the same in the CO, treatments at the end of the second growing seasons in both experiments, even if the yield was higher in ele- vated C02 (Figs 2 and 3). As the tiller number was counted only at the end of the second grow- ing seasons, the gain in biomass may have been due to an increase in the tillernumber earlier in the season, as suggested by the increase in the yield in elevated C02 that took place early in 294 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 285-298. the season (before the first cut) in 1993, and by the reduced daily growth rate in elevated C02 relative to that in ambient C02 late in the season in 1995. Closure of the plant canopy leads to poorer light penetration and hence lower biomass pro- duction than expected on the basis of higher as- similation rates in elevated C02 measured at sat- urating light (Du Cloux et al. 1987, Nijs et al. 1989, Schenk et al. 1995). When the grass was cut during the second growing season of exper- iment 2 (1995) according to the LAI of the indi- vidual replicates, canopy self-shading was re- duced and made more or less similar in all treat- ments. The higher yields both in the open field (29% greater) and in the greenhouse (85% great- er) in ambient CO, in 1995 than in 1993 (the second growing season of experiment 1) (Fig. 3) may have been partly due to reduced self- shading and better use of light for photosynthe- sis and growth at canopy level, although the ef- fects of different soil type and differentclimatic conditions should not be discounted. Neverthe- less, even withconditions apparently favouring a positive effect of enriched CO, on yield in ex- periment 2, such an effect was still only observed in the greenhouse. In cases when the above-ground biomass does not increase in proportion to the increase in photosynthesis, there have been reports of the extra photosynthate being translocated to the roots (Nijs et al. 1988, Luo et al. 1994, Schenk et al. 1995) or to the stubble (Saebo and Mortensen 1995). There was evidence of this occurring here, too, as theroot fraction increased in elevated C02 even though the elevationofC02 had no effect on the above-ground biomass at ambient temperatures in experiment 2 (Figs 3 and 4). As there was no decrease in the nitrogen con- tent in the shoots of meadow fescue at ambient temperatures in either experiment 1 or 2, it is improbable that the root fraction would have increased mainly to make more of the nitrogen and other nutrients available for the plant, as proposed in another study on grass swards (Schenk et al. 1995), but the response seems to be rather a way of translocating extra photosyn- thate to roots when the above-ground biomass cannot use it efficiently enough (Luo et al. 1994). Even if the yield of the grass did not increase at ambient temperatures in elevated C02 in exper- iment 2, the increase in root biomass could have been beneficial for grass growth during drought or if nutrients had become scarce, and could also have served as a bigger carbohydrate reserve for the grass to help it survive the winterbetter. Accumulation of carbohydrate in the leaves and/or an increase in the number of mesophyll cell layers have been interpreted as another sign of the reduced sink strength (Ryle et al. 1992, Bowes 1993,Baxter etal. 1994, Luo et al. 1994) often associated with a decrease in the nitrogen content of the leaves (Campbell et al. 1988,Ryle etal. 1992,Bowes 1993,Luo et al. 1994, Schenk et al. 1995). The decrease in the nitrogen level in the leaves has been explained, at least partly, by dilution of nitrogen with the carbohydrates (Campbell et al. 1988, Luo et al. 1994) or by translocation of the nitrogen from rubisco and photorespiratory enzymes to other proteins more limiting to photosynthesis, or to other parts of the plant to increase the sink strength in elevat- ed CO, (Hocking and Meyer 1991, Stitt 1991, Conroy and Hocking 1993). The carbohydrate content of the leaves was not measured separate- ly here, but that of the total above-ground bio- mass was higher in elevated CO, at both ambi- ent and elevated temperatures at most of the measuring times during summer (Table 3). According to the official variety trials con- ducted in 1988-1995, the average protein con- tent of the above-ground biomass of meadow fescue cv. Kalevi is 15%, or 2.3% N of dry weight (Järvi et al. 1996). The nitrogen levels in meadow fescue in the present experiments were thus approximately average or slightly above average in 1992, and clearly above average in 1994-1995. Despite the increase in the carbo- hydrate content of the above-ground biomass in elevated C02 , the nitrogen content of the above- ground biomass was not reduced in elevated C02, except for a slight decrease at elevated tempera- tures in experiment 2. Rather, there seemed to be an increase in the uptake of nitrogen in ele- 295 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth vated CO, at ambient temperatures in all the years that nitrogen content was studied, and at elevated temperatures in experiment 1 (in 1992). It has been noted that the effect of elevated C02 on photosynthesis is lower at low light in- tensity (Gay and Hauck 1994) and may even be reduced relative to that in ambient C02 (Nijs et al. 1989). Lowered photosynthesis rates at low light intensities, e.g. inside the grass canopy or on cloudy days, would not have been detected here, because the measurements reported by Heliö et al. (1995) were always performed at saturating light intensity. The measurements were also usually conducted in the morning or early afternoon; thus any late-afternoon decreas- es in the photosynthetic rate (expected to take place because of sink size limitation and the con- sequent accumulation ofphotosynthetic products in the leaves) would not have been detected (Nijs et al. 1989). It is possible that the relatively mi- nor effect of CO, on the yield of meadow fes- cue, especially at ambient temperatures, was partly due to lower photosynthetic activity of meadow fescue in elevated than in ambient CO, at non-saturating light intensities and/or during the late afternoon if both the sink capacity and the capacity of the leaves to store carbohydrate were restricted. The increases in the dry weight of the root fraction at ambient temperatures with- out any increase in the above-ground biomass also suggests the restricted sink capacity of the above-ground biomass (Luo et al. 1994). Evi- dence further exists for significantly higher dark respiration rates in elevated COr causing a large part of the C02 assimilated during the day to be released during the night (Nijs et al. 1989, Wolfenden and Diggle 1995). This possible mechanism suppressing biomass increases in enriched CO, remains to be investigated. Diur- nal changes in the photosynthetic activity of meadow fescue in elevated and ambient CO, concentrations will also be the subject of future experiments. Acknowledgements. This work was part of the Finnish Re- search Programme on Climate Change (SILMU) and was supported, in part, by the Academy of Finland. The excel- lent technical assistance ofMs Leila Salo, Ms Marjo Vuo- rinen, Ms Ulla Helin and Ms Helena Ihamäki is gratefully acknowledged. 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Plant and Soil 151: 105-117. 297 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Hakala, K. & Mela, T: Effects ofelevated temperature and C02 on grass growth SELOSTUS Kohotetun lämpötilan ja kohotetun C0 2 -pitoisuuden vaikutukset peltoon kylvetyn nurminadan kasvuun, satoon ja kuiva-aineen jakautumiseen Kaija Hakala ja Timo Mela Maatalouden tutkimuskeskus Peltoon kylvettyä nurminataa (lajike Kalevi) kas- vatettiin normaalissa ja kohotetussa (+3 °C) lämpö- tilassa ja normaalissa ja kohotetussa C0 2-pitoisuudes- sa kahdessa kahden vuoden mittaisessa kokeessa vuo- sina 1992-1993 (koe I) ja 1994-1995 (koe 2). Kasvatus kohotetussa lämpötilassa lisäsi nurminadan kokonaissatoa keskimäärin 38 % normaalissa lämpö- tilassa kasvatettuun verrattuna. Kohotettu C0 2-pitoi- suus lisäsi normaalissa lämpötilassa nurminadan ver- sojen määrää 63 %, 24 %, 90 % ja 14 % vuosina 1992- 1995. Kohotetussa lämpötilassa kohotetun C02-pitoi- suuden vaikutus versomäärään näkyi vain ensimmäis- ten kasvukausien aikana molemmissa kokeissa. Nur- minadan kokonaissato nousi 10 % kohotetussa C0 2-pitoisuudessa kokeessa 1. Kokeessa 2 sadot nou- sivat C0 2-käsittelyn vaikutuksesta yli 20 % kohote- tussa lämpötilassa. Normaalissa lämpötilassa C02-kä- sittelyllä ei ollut vaikutusta nurminadan satoon, mutta juuriston paino lisääntyi yli 30 %. Juuriston paino nousi normaalissa lämpötilassa C0 2 - käsittelyssä myös kokeessa 1, mutta kohotetussa lämpötilassa C0 2 -käsittelyllä ei ollut selvää vaikutusta juuriston biomassaan. Liukoisen hiilihydraatin määrä nurmina- dan maanpäällisessä osassa oli useimmilla mittaus- kerroilla kasvukauden aikana 5-48 % suurempi ko- hotetussa C0 2:ssa, mutta typen määrä ei laskenut. Syitä siihen, miksi C02 -käsittely vaikuttaa vain vä- hän biomassaan, vaikka saman kokeen yhteydessä on aikaisemmin todettu fotosynteesin nousevan 40- 60 %, pohditaan. 298 AGRICULTURAL AND FOOD SCIENCE IN FINLAND