Carbon autonomy of reproductive shoots of Siberian alder ( Alnus hirsuta var. sibirica )

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J Plant Res (2003) 116:183188 The Botanical Society of Japan and Springer-Verlag Tokyo 2003Digital Object Identifier (DOI) 10.1007/s10265-003-0085-7Springer-VerlagTokyo102650918-94401618-086030669031Journal of Plant ResearchJ Plant Res008510.1007/s10265-003-0085-7Carbon autonomy of reproductive shoots of Siberian alder (Alnus hirsuta var. sibirica)ORIGINAL ARTICLEReceived: August 8, 2002 / Accepted: February 3, 2003 / Published online: March 8, 2003Shigeaki HasegawaKeisuke KobaIchiro TayasuHiroshi TakedaHiroki HagaS. Hasegawa1 (*) I. Tayasu2 H. TakedaLaboratory of Forest Ecology, Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto University, Kyoto, JapanK. KobaDivision of Biosphere Informatics, Graduate School of Informatics, Kyoto University, JapanH. HagaLake Biwa Museum, Kusatsu, JapanPresent addresses:1Laboratory of Regional Ecosystems, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, JapanTel. +81-11-7062264; Fax +81-11-7064954e-mail: shasegaw@ees.hokudai.ac.jp2Research Institute for Humanity and Nature, Kyoto, JapanAbstractCarbon autonomy of current-year shoots in flow-ering, and of current-year shoots plus 1-year-old shoots (1-year-old shoot system) in fruiting of Siberian alder (Alnushirsuta var. sibirica) was investigated using a stable isotopeof carbon, 13C. The current-year shoot and 1-year-old shootsystems were fed 13CO2 and the atom% excess of 13C inflowers and fruits was determined. The majority of photo-synthate allocated to flower buds was originally assimilatedin the leaves of the flowering current-year shoots. Of all thecurrent-year shoots on fruiting 1-year-old shoots, only thosenearest to the fruits allocated the assimilated photosynthateto fruit maturation. These results indicate that the current-year shoots and 1-year-old shoot systems are carbon-autonomous units for producing flowers and maturingfruits, respectively.Key wordsAlnus hirsuta var. sibirica13C Current-yearshoot population Tracer experiment Translocation ofphotosynthateIntroductionA current-year shoot of a tree is regarded as a fundamentalunit in reproduction (Lovett Doust and Lovett Doust 1988;Tuomi et al. 1982; Newell 1991). Several studies have beenconducted to investigate the role of current-year shoots inthe reproduction of trees (Tuomi et al. 1982; Hoffmann andAlliende 1984; Cooper and McGraw 1988; Karlsson et al.1996; Obeso 1997; Hasegawa and Takeda 2001; Henriksson2001). These studies are based on the assumption that thecurrent-year shoot is resource-autonomous (Tuomi et al.1988a, 1988b; Sprugel et al. 1991). This assumption is sup-ported by evidence that damage limited to local foliage haspredominantly local effects (Tuomi et al. 1982, 1988a, 1988b,1989; Haukioja and Neuvonen 1985; Haukioja et al. 1990;Ruohomki et al. 1997).Many studies have also pointed out that reproductivetissues obtain photosynthates from the nearest vegetativetissues (Kozlowski and Clausen 1966; Hansen 1969;Mooney 1972; Stephenson 1981; Honkanen and Haukioja1994), though they have the ability to derive resources fromvegetative tissues more than 1 m apart (Mooney 1972).Thus, the photosynthates allocated to the reproductive tis-sues are probably assimilated in the leaves on the current-year shoot to which the reproductive tissue is attached.However, there are few studies that show the carbon-autonomy of current-year shoots in reproduction from thedirect evidence of the movement of photosynthate withinand between current-year shoots.In terms of resource allocation for reproductive activi-ties, two sequential events should be taken into account:flower production and fruit maturation (Stephenson 1981;Willson 1983). Siberian alder (Alnus hirsuta Turcz. var.sibirica [Fisch.] C. K. Schn.) is a deciduous early-successional tree species, with monoecious sex expression.Male and female flower buds develop on the top part of thecurrent-year shoots and flower in the following year, whichmeans that fertilized female flowers mature to fruit on 1-year-old shoots (Hasegawa and Takeda 2001). These find-ings suggest that flower production and fruit maturation are 184temporally separated, and the pattern of photosynthateallocation in both flower production and fruit maturationmay require to be checked in Siberian alder.In this study, we examined the carbon autonomy ofcurrent-year shoots in the reproduction of Siberian alder.Our specific questions are: (1) whether flowers obtain pho-tosynthates assimilated in the leaves on the current-yearshoots to which they are attached, (2) whether the fruitsobtain photosynthates assimilated in the leaves on thecurrent-year shoots developed on the fruiting 1-year-oldshoot, and (3) which current-year shoots on a fruiting 1-year-old shoot are effective for fruit maturation. To answerthese questions, we conducted a tracing experiment usingisotope-labelled CO2, a popular technique with which toexamine the movement of photosynthate in a plant body(Rabideau and Burr 1945; Hartt et al. 1963; Geiger andSwanson 1965; Hartt 1965; Hansen 1967, 1969; Davis andSparks 1974; Steer and Pearson 1976; Takeda et al. 1980;Cliquet et al. 1989, 1990; Delens et al. 1994).Materials and methodsSpecies studiedSiberian alder (Alnus hirsuta var. sibirica) is a deciduousearly-successional tree species. Male and female flowers areproduced from early August. A flowering shoot normallyhas both male and female flower buds at its terminal end.They overwinter and then flower in mid-April of the subse-quent year. Male flowers fall after flowering, whereas fertil-ized female flowers mature to fruit on 1-year-old shoots,and then seeds are dispersed by the wind in November(Hasegawa and Takeda 2001).Study site and target treeThe study was conducted in a secondary forest (approxi-mately 1,110 m in altitude) at Mt. Norikura in the easternpart of Takayama City, Gifu Prefecture, central Japan. Theonly tree species constituting the canopy layer was Siberianalder. The height of the canopy trees was about 15 m.Three sample trees were selected from the edge of thestudy site where light conditions appeared to be appropriatefor photosynthetic assimilation. The height of the targettrees was about 1112 m.Isotope labeling and carbon stable isotope analysisIn this study, 13C a stable isotope of carbon was used forlabelling carbon dioxide. Using 13C for tracing experimentsis more advantageous in cost and in handling security prob-lems than 14C a radioactive isotope of carbon commonlyused in previous tracing experiments (Delens et al. 1994).To examine the carbon autonomy of the current-year shootsand 1-year-old shoot systems in reproduction, we traced themovement of photosynthate to the reproductive organs, byinfusing 13CO2 onto the leaves and monitoring changes in13C content of reproductive organs.We selected flowering current-year shoots and fruiting1-year-old shoots from the first-order branches (branchesdirectly issuing from the trunk) of target trees. Thesebranches were located about 3 m above the ground, andwere accessible with the help of a ladder.A nylon bag (40 cm 28 cm) was used as a simple cham-ber. A small hole was opened at the closed side of the nylonbag and a short plastic tube (9 mm in diameter) wasattached, penetrating the hole with adhesives, to allow air-flow. The current-year shoots or 1-year-old shoot systems toreceive the 13CO2 were covered with the nylon bags. Theopen side of each nylon bag was sealed with adhesive tape.The air in the nylon bag was roughly purged by hand. Afterpurging the air, the nylon bag was connected to a cylinder(100 ml) via a glass tube (8 mm in diameter, length 27 cm)filled with soda-talc. The soda-talc in the glass tubeabsorbed CO2 from the air. Thus, pumping the cylinder, wecould feed the air containing a low concentration of CO2into the chamber. Then, 13CO2 (Syoko-Tsusho, Tokyo,Japan) was injected into the chamber at about 360 ppm.This procedure enabled us to provide a higher proportionof 13C than is the normal state but with a concentration ofCO2 equivalent to that of normal air.The current-year shoot or 1-year-old shoot system understudy was exposed to 13CO2 during two successive sunnydays. During the daytime period (08001800) of 13CO2 expo-sure, to maintain the concentration of CO2 at normal levelswe determined the concentration of CO2 in the chamberby a one-time CO2 analyzer, GASTEC No. 2LL (Gastech,Tokyo, Japan) and supplemented any decrease in CO2 byinjecting 13CO2 at 1 h intervals. 13CO2 was not supplementedduring the nighttime period of 13CO2 exposure.Target flowers and fruits were sampled after exposureand immediately brought to the laboratory in a cooler boxfilled with ice. At the same time, flowers and fruits that werefar distant from the site of 13CO2 addition were sampled ascontrols, to estimate the natural abundance of 13C in flowersand fruits. Samples were dried at 40C for 48 h and groundto a fine homogeneous powder using a sample mill TI-100(CMT, Tokyo, Japan). The abundance of 13C was analyzedusing an automatic nitrogen and carbon analyzer, Integra-CN (European Science, London, UK).Statistical analysisThe abundance of 13C was indicated by the index, atom%.Atom% of 13C is calculated as follows:(1)Under natural conditions, the atom% of 13C is about 1.1%(Delens et al. 1994). The increment of 13C in target flowersor fruits from controls was analyzed by the Mann-WhitneyU test, comparing atom% of 13C of target flowers or fruitswith atom% of 13C of control intact flowers or fruits. Theatom%amount of Camount of C amount of C1312 13=+ 100 185Mann-Whitney U test was conducted using SPSS ver. 7.5.1 Jfor Windows (SPSS, Chicago, Ill.).Experimental designEach experiment was conducted as follows (Fig. 1). Exper-iment A: (1) leaves of a current-year shoot producing flow-ers were provided with 13CO2, (2) leaves of a non-floweringcurrent-year shoot neighboring a flowering shoot were pro-vided with 13CO2. Experiment B: (1) leaves of a 1-year-oldshoot system with maturing fruits were provided with 13CO2,(2) leaves of a non-fruiting 1-year-old shoot system closestto a fruiting 1-year-old shoot system were provided with13CO2. Experiment C: (1) leaves of the current-year shootnearest to the fruit in a 1-year-old shoot system were pro-vided with 13CO2, (2) leaves of a current-year shoot furtheraway from fruits were provided with 13CO2.Experiment A was conducted in August 1999. Tencurrent-year shoots were selected for each treatment,including the control. Experiments B and C were conductedin August 1998. Ten current-year shoots and ten 1-year-oldshoot systems were selected for each treatment, includingthe control. To check the seasonal changes of translocationof photosynthate to fruits, Experiments B-2 and C were alsoconducted in May, July, August and October 1999. Five 1-year-old shoot systems were selected for each treatment.We omitted experiment B-1 in 1999 since the transportationof photosynthate from the leaves of 1-year-old shoot systemto fruits could be checked by experiment C-1 and C-2.ResultsCarbon autonomy of current-year shootsFlowers of current-year shoots whose leaves were providedwith 13CO2 showed a significant (P

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