Journal of Experimental Botany, Vol. 51, No. 348, pp. 1319–1321, July 2000

SHORT COMMUNICATION

Anther dehiscence in two-rowed barley (Hordeumdistichum) triggered by mechanical stimulation
T. Matsui1,4, K. Omasa2 and T. Horie3
1 Experimental Farm, Kyoto University, 12–1 Hatchonawate, Takatsuki, Osaka 569–0096, Japan
2 Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113–8657, Japan
3 Graduate School of Agriculture, Kyoto University, Kyoto 606–8502, Japan
Received 17 November 1999; Accepted 21 March 2000

Abstract
Pricking or pressing an ovary can induce anther dehiscence in cleistogamous two-rowed barley in theabsence of lodicule swelling. This response is considered to be inherited from a chasmogamous ancestor
in which anther dehiscence is induced by the stimulus
given to the pistil by the lodicule swelling, whichcauses floret opening.
Key words: Anther dehiscence, floret opening, sensory
organ, trigger, two-rowed barley.

Introduction
In many plants, including some poaceous plants such as
rice and wheat, anthers dehisce at the time of flower
opening. Historically, it has been believed that antherdehiscence is a consequence of desiccation (Schmid,1976), probably because of the synchrony between antherexposure to ambient air and dehiscence. However, it hasbeen proposed (Cheng et al., 1979, 1986), on the basis of
SEM and TEM observations of the anther cuticle, thatin Zea mays the tensile force generated by a shorteningof the outer circumference of the microsporangia is thedriving force for septum rupture. Bonner and Dickinson
concluded that in Lycopersicon esculentum, anther dehiscence is not a simple desiccatory process (Bonner andDickinson, 1990) and Keijzer showed that in liliaceous
plants swelling of pollen, imbibition of anther wall cells,
and lysis of the septum are responsible for anther dehiscence ( Keijzer, 1987). Recently, Matsui et al. demonstrated that rapid swelling (within a few minutes) ofpollen grains in response to artificial floret opening is thedriving force to open the locules and results in anther
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dehiscence in Oryza sativa (Matsui et al., 1999a, b) andHordeum distichum (Matsui et al., 2000). Detailed studies
have clarified that desiccation of the anther is not thedirect driving force for anther dehiscence (see review byKeijzer, 1999) and the mechanism responsible forco-ordinating the phenomenon with the time of flower
opening remains unknown.
The observations by Matsui et al. (Matsui et al., 1999a,b, 2000) suggests that synchrony between anther exposureand dehiscence is due to some stimulus associated withfloret opening. A relationship between mechanical stimuli
and anthesis was first reported in the Poaceae over acentury ago ( Tschermak, 1890). It is reported here thata stimulus, similar to removal of the lemma, inducesanther dehiscence in cleistogamous two-rowed barley.
How this might trigger anther dehiscence and the phenomenon synchronized with floret opening is discussed.

Materials and methods
Plant materials
Cleistogamous two-rowed barley (Hordeum distichum L. emend.
LAM. cv. Saikaikawa-24), grown under field conditions ( Kyoto
University, Experimental Farm, Osaka, Japan) from November
to May was used. The heading period was from the end of
March to mid-April. Florets with indehisced anthers that candehisce in response to the removal of the lemma (referred to as
mature anthers hereafter) were selected for this experiment.
Such florets were selected by observing the sequence of anthesis
on a panicle. In the same panicle, anthesis occurs from the
middle position toward both apical and basal ends (Hoshikawa,1980). This sequence of anthesis corresponds with the order ofanther maturity: the anthers in the florets that pollinate earlier
have a higher ability to dehisce than those in the florets thatpollinate later (Matsui et al., 2000). Anther maturity wasdetermined as follows. A floret in which the anthers had notdehisced was detached from the panicle, the lemma of the floret

1320 Matsui et al.
was removed and the anthers that dehisced within 5 min wereregarded as mature anthers.
Search for triggers and sensory organs of the anther dehiscence
The following treatments were given to the florets with mature
anthers without detaching them from the panicle (Fig. 1). (1)Cut the glumes above the anthers with scissors, (2) pierce theglumes above the anthers with a needle, (3) pierce the glumes
beside the anthers with a needle, (4) prick the anther throughthe lemma with a needle, (5) prick the ovary through thelemma with a needle, and (6) press the pistil from the side overthe lemma at 10, 20, 30, 40, and 50 g per 28 mm2 (a circle 3 mm
in diameter). Five minutes after the treatments, the dehiscenceof the anthers was examined. The test was repeated 10 times
for each treatment.

Results and discussion
Matsui et al. reported that, in the florets of two-rowedbarley, the removal of the lemma induces the pollen
swelling that causes anther dehiscence (Matsui et al.,2000). This suggests that some stimulus given to the floralorgans at the time of lemma removal induces antherdehiscence. The stimulus may be classified into the following three categories. (1) Injury around the base of thepistil; (2) friction between the lemma and the palea; (3)changes of gaseous and light conditions inside the floret.
In the experiments in Table 1, pricking of an anther andthe ovary induced dehiscence whereas cutting or piercingof the glumes did not. These results rule out the possibility
that any change in exogenous gaseous or light conditionsin the floret or the mechanical stimulation given to theglumes per se induces dehiscence, and suggest that theFig. 1. The stimuli given to the floral organs.

removal of the lemma triggers the phenomenon throughinjury of the base of the pistil.
A maize anther dehisces at the time of floret opening
and it has been proposed that the endogenous carbon
dioxide concentration in the flower bud of maize determines the time of anthesis and that production of the gas
from the respiring pollen might provide an explanationof why the septum and stomium should be open justbefore anthesis ( Keijzer, 1999). Despite such a possible
role in anthesis, the present results suggest that carbon
dioxide would not be the trigger of anther dehiscence.
However, any role of ethylene, produced due to damaging
the ovular base, cannot be excluded.
Anther dehiscence induced by removal of the lemma
was observed also in rice (Matsui et al., 1999b). This
indicates that similar mechanisms may trigger antherdehiscence in rice and two-rowed barley. In natural
flowering of rice, swelling of the lodicules at the base of
the pistil is considered to force the glumes to open, butit also may press the pistil, causing anther dehiscence atthe time of floret opening. A similar mechanism may be
involved in the synchronous floret opening and antherdehiscence in wheat. To test the possibility that suchpressure to the base of the pistil can trigger antherdehiscence, the eﬀect of pressure applied to the pistil onbarley dehiscence was examined. As shown in Table 2,pressure applied to the pistil induced anther dehiscence.
Although the percentage of dehisced anthers increased
gradually as the pressure applied to the pistil wasincreased, the percentage of dehisced anthers in eachfloret was either 0% or 100%. This suggests that athreshold pressure applied to the pistil may trigger antherdehiscence. If the pressure given by the swelling of lodicules to the pistil is above the threshold level, it may
induce anther dehiscence. This may explain the synchrony
in rice between anther dehiscence and floret opening.
The mature anthers of rice dehisce in response to
cutting of the glume tip (Matsui et al., 1999a). In tworowed barley, cutting of the glume tip did not cause
anther dehiscence ( Table 1), but cutting with blunt scissors induced anther dehiscence (data not shown). This isprobably because the pulling of the glumes act as astimulus at the base of pistil. It is, therefore, supposedthat the rice floret is more susceptible to a mechanicalstimulus given to the base of pistil. It is generally known
that, just before anthesis, the lodicules of rice are very
sensitive to mechanical stimuli given to the glume, suchas rubbing, vibration and even air pressure and that thelodicules swell rapidly in these cases. The rice lodicules
may respond to a slight stimulus given to the glume, andtransmit the information to the pistil as pressure thus
causing anther dehiscence.
Lodicules in the cleistogamous two-rowed barley haveno swelling ability. Therefore, mechanical stimulationthat triggers anther dehiscence may not be given to theAnther dehiscence in barley 1321
Table 1. The eﬀect of injury to floral organs on anther dehiscence
Treatment/position

Percentage of dehisced anthers

Cutting glumes

Piercing glumes

Above anthers

Above anthers

Beside anthers

AntherOvary000100100

Table 2. The eﬀect of pressure applied to the pistil over lemmaon anther dehiscence
Pressure applied(g per 28 mm2)1020304050

Percentage of florets
All anthers
indehisced

All anthers
dehisced

Othersa

1009060300010407010000000

aFlorets of which anthers partly dehisced.

pistil or stamen in the two-rowed barley under naturalconditions. The two-rowed barley may not require rapid
anther dehiscence (because it is cleistogamous). Nevertheless, the anther of two-rowed barley dehisced rapidly
in response to the mechanical stimulus applied to thepistil or stamen. The ability of anthers to dehisce in
response to such mechanical stimuli seems to be inheritedfrom a chasmogamous ancestor. Actually, a floret offour-rowed barley (H. tetrasticum) opens and its anthers
dehisce just after opening.
In some poaceous autogamous crops, such as rice,anthers dehisce at the time of floret opening under naturalconditions probably by the mechanism mentioned above.
The synchrony between anther dehiscence and floret
opening would be favoured to maintain a high selffertilization rate ( Hoshikawa, 1993). Plants may have
evolved to increase their self-pollination rate if thedecrease of viability by inbreeding depression is small
relative to the costs of cross pollination, because themutation with a high self-fertilization rate is advantageous(Lande and Schemske, 1985; Schemske and Lande, 1985)
especially in anemophilous species (Aide, 1986). The
response of the anther to floret opening, therefore, may
have evolved to hasten the perfect self-fertilization.
However, there are few reports on the rapid action of thestamen in such gravity-pollination systems. The rapid
response may not be essential for the high self-pollination
rate because it can be maintained by another system, forexample, anther dehiscence before floret opening or cleistogamy. On the other hand, floral organs sensitive tomechanical stimuli are observed in entomophilous genera,such as Portulaca (Jaﬀe et al., 1977), Berberis andMahonia (Percival, 1965). The rapid response of thestamen to the mechanical stimulus seems to help thevector, such as wind and insects, to carry the pollen

Pricking through lemmaeﬃciently. The poaceous autogamous crops may haveinherited these functions from an ancestor with a crosspollination system.

Acknowledgements
We thank H Kagata and R Wada for their technical assistancein the field.

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