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/s10265-003-0085-7

    Springer-VerlagTokyo10265

    0918-9440

    1618-0860

    30669031Journal of Plant Research

    J Plant Res008510.1007/s10265-003-0085-7

    Carbon autonomy of reproductive shoots of Siberian alder (

    Alnus hirsuta

    var.

    sibirica

    )

    ORIGINAL ARTICLE

    Received: August 8, 2002 / Accepted: February 3, 2003 / Published online: March 8, 2003

    Shigeaki Hasegawa

    Keisuke Koba

    Ichiro Tayasu

    Hiroshi Takeda

    Hiroki Haga

    S. Hasegawa

    1

    (*

    ) I. Tayasu

    2

    H. TakedaLaboratory of Forest Ecology, Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto University, Kyoto, Japan

    K. KobaDivision of Biosphere Informatics, Graduate School of Informatics, Kyoto University, Japan

    H. HagaLake Biwa Museum, Kusatsu, Japan

    Present addresses:

    1

    Laboratory 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.jp

    2

    Research Institute for Humanity and Nature, Kyoto, Japan

    Abstract

    Carbon 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,

    13

    C. The current-year shoot and 1-year-old shootsystems were fed

    13

    CO

    2

    and the atom% excess of

    13

    C 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 words

    Alnus hirsuta

    var.

    sibirica

    13

    C Current-yearshoot population Tracer experiment Translocation ofphotosynthate

    Introduction

    A 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

  • 184

    temporally 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 CO

    2

    , 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 methods

    Species studied

    Siberian 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 tree

    The 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 analysis

    In this study,

    13

    C a stable isotope of carbon was used forlabelling carbon dioxide. Using

    13

    C for tracing experimentsis more advantageous in cost and in handling security prob-lems than

    14

    C 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, by

    infusing

    13

    CO

    2

    onto the leaves and monitoring changes in

    13

    C content of reproductive organs.We selected flowering current-year shoots and fruiting

    1-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

    13

    CO

    2

    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 CO

    2

    from the air. Thus, pumping the cylinder, wecould feed the air containing a low concentration of CO

    2

    into the chamber. Then,

    13

    CO

    2

    (Syoko-Tsusho, Tokyo,Japan) was injected into the chamber at about 360 ppm.This procedure enabled us to provide a higher proportionof

    13

    C than is the normal state but with a concentration ofCO

    2

    equivalent to that of normal air.The current-year shoot or 1-year-old shoot system under

    study was exposed to

    13

    CO

    2

    during two successive sunnydays. During the daytime period (08001800) of

    13

    CO

    2

    expo-sure, to maintain the concentration of CO

    2

    at normal levelswe determined the concentration of CO

    2

    in the chamberby a one-time CO

    2

    analyzer, GASTEC No. 2LL (Gastech,Tokyo, Japan) and supplemented any decrease in CO

    2

    byinjecting

    13

    CO

    2

    at 1 h intervals.

    13

    CO

    2

    was not supplementedduring the nighttime period of

    13

    CO

    2

    exposure.Target flowers and fruits were sampled after exposure

    and 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

    13

    CO

    2

    addition were sampled ascontrols, to estimate the natural abundance of

    13

    C in flowersand fruits. Samples were dried at 40C for 48 h and groundto a fine homogeneous p