Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol82.Issue3-4.Dec1999:
Scientia Horticulturae 82 (1999) 255±263
Female and male sterility cause low fruit set in a clone
of the `Trevatt' variety of apricot (Prunus armeniaca)
A.M. Lillecrapp, M.A. Wallwork, M. Sedgley*
Department of Horticulture, Viticulture and Oenology, Waite Agricultural Research Institute,
The University of Adelaide, Glen Osmond, S.A. 5064, Australia
Accepted 9 April 1999
Abstract
This study investigated ovule and anther structure of the `Trevatt Blue' variety of apricot (Prunus
armeniaca) using bright field microscopy following reports of low fruit set. Ovules and anthers
from fertile `Moorpark' and `Trevatt Knight' flowers were compared with those of the `Trevatt
Blue'. In the `Moorpark' and `Trevatt Knight' ovules, all reproductive structures were present
including embryo sacs with a complete set of eight nuclei and the anthers contained mature pollen
grains. Multiple ovules which were small and retarded in development were present in the `Trevatt
Blue' apricot flowers and the anthers contained degenerated microspores, with some failure in
tapetal breakdown. This is the first report of a simultaneous mutation in both female and male
function in apricot. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Apricot; Fertility; Prunus; Microscopy; Ovule; Embryo sac; Anther; Pollen grain
1. Introduction
`Trevatt' is an old self-fertile cultivar of apricot (Prunus armeniaca) and high
yielding clones have been developed to produce large fruit with desirable
characteristics. The clone, `Blue' was planted over wide areas by many growers
because the original selection was high yielding with fruit of excellent canning
quality, but the trees failed to set fruit for several seasons after the juvenile phase
had ended.
* Corresponding author. Tel.: +61-8-8303-7401; +61-8-8303-7249; fax: +61-8-8303-7116
E-mail address: [email protected] (M. Sedgley)
0304-4238/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 6 1 - 8
256
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Some varieties of apricot have been found to be self-sterile or have pollination
problems (McLaren and Fraser, 1996), but it seemed that in `Blue' female
sterility may have been a possible contributor to low fruit set. Very low levels of
set were achieved even when pollen from a different cultivar was introduced to
the trees via bouquets from fertile orchards. It was also possible that male sterility
contributed to the low fruit set. Female and male sterility have been reported
independently in apricot (Eaton and Jamont, 1964; Nakanishi, 1983; Medeira and
Guedes, 1991; Burgos and Egea, 1994) as well as in many other tree crops
(Sedgley and Griffin, 1989), but have been attributed to adverse environmental
conditions.
In this study, the female and male structures of `Trevatt Blue' were observed
microscopically and compared with those of `Trevatt Knight' which is known to
be fertile. A comparison was also made with the cultivar `Moorpark' in case there
was a problem inherent in the `Trevatt' cultivar.
2. Materials and methods
Flower samples of the low yielding `Trevatt Blue' were randomly collected
from a single variety apricot orchard in Griffith, New South Wales. Fertile
`Trevatt Knight' flowers were obtained from a mixed planting at Loxton, South
Australia, and fertile `Moorpark' flowers were obtained from the Waite orchard
of the University of Adelaide, South Australia.
Flowers were collected at the late balloon stage just prior to anthesis and fixed
in FPA50 (90% ethanol at 50%, 5% propionic acid and 5% formaldehyde).
Ovules and anthers were dissected from each flower under a microscope and the
samples dehydrated using a tertiary-butyl alcohol series and embedded in GMA
(glycol methacrylate). Serial longitudinal 4.0 mm sections through each ovule (up
to 150 sections per ovule) and 48 sections of 4.0 mm through each anther were
collected onto a microscope slide, stained with periodic acid-Schiffs reagent
(PAS) and toluidine blue O (TBO) stain and mounted in methyl methacrylate in
xylene (O'Brien and McCully, 1981). The sections were observed under a Zeiss
Axiophot photomicroscope with bright field illumination. Ovules and anthers
from 36 `Trevatt Blue', 15 `Trevatt Knight' and 9 `Moorpark' flowers were
examined. The structure of male and female reproductive structures was recorded
for each flower.
3. Results
Two ovules were present in each `Moorpark' and `Trevatt Knight' flower.
Between one and four ovules were present in each `Trevatt Blue' flower, most
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Table 1
Ovule structure for fertile `Moorpark' and `Trevatt Knight', and sterile `Trevatt Blue' apricot
flowers at anthesis
`Moorpark'
Mean number ovules (SD)
Range
20
2±2
`Trevatt Knight'
20
2±2
`Trevatt Blue'
2.88 0.79
1±4
Percentage ovules with
outer integument
inner integument
nucellus
micropyle
embryo sac
100
100
100
100
100
100
100
100
100
100
100
100
100
88.5
50
Embryo sac with
0 nuclei
1 nucleus
2 nuclei
4 nuclei
5±8 nuclei
0
0
0
0
100
0
0
0
0
100
19.2
19.2
8.7
2.9
0
smaller in size than the fertile `Moorpark' and `Trevatt Knight' ovules but all
similar in size to each other. One hundred and four ovules were obtained from 36
`Trevatt Blue' flowers; 2.8% of flowers had one ovule, 27.8% had two, 47.2% had
three and 22.2% had four (Table 1).
Of the ovules examined of the `Moorpark' and `Trevatt Knight', all had an outer
integument, inner integument, nucellus, micropyle and embryo sac (Fig. 1(A))
with an egg cell (Fig. 1(B)), two synergids (Fig. 1(C)), two polar nuclei (Fig. 1(D))
and three antipodals (Fig. 1(E)), which had degenerated in some ovules.
All `Trevatt Blue' ovules had an outer integument, inner integument and
nucellus, and 88.5% had a micropyle (Fig. 2(A)). Some had mis-shapen nucellus
(Fig. 2(B)), and embryo sacs were degenerated or not present in 50% of ovules
(Table 1). The other 50% contained embryo sacs with 0, 1 (Fig. 2(C)), 2
(Fig. 2(D)) or 4 (Fig. 2(E)) nuclei. No `Trevatt Blue' apricot ovules observed
contained eight nuclei, and 19.2% of ovules were at the megaspore mother cell
stage. Other abnormalities included small, spherical ovules, ovules joined
together and ovules with underdeveloped or abnormal nucellus (Fig. 2(B)) or
nucellus incompletely surrounded by the integuments.
The anthers of the `Moorpark' and `Trevatt Knight' flowers were bright yellow
in colour and plump compared to those of the `Trevatt Blue' flowers which were
red-brown and shrunken. Anthers from `Moorpark' and `Trevatt Knight' apricot
flowers had an endothecium and degenerated tapetum (Fig. 3(A)). Fully
developed pollen grains with an exine, intine, germination pores, vegetative
258
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Fig. 1. (A±E) Longitudinal sections of `Moorpark' (figures A, B, D and E) and `Trevatt Knight'
(figure C) apricot ovules stained with periodic acid-Schiff's reagent (PAS) and toluidine blue O
(TBO) and photographed using bright field optics. (A) Ovule showing embryo sac (es), nucellus (n),
outer integument (oi), inner integument (ii) and micropyle (m). (B) Embryo sac with egg cell (e). A
polar nucleus is also visible. (C) Embryo sac showing two synergids (s). (D) Embryo sac showing
two polar nuclei (pn). An egg cell and antipodal are also visible. (E) Embryo sac showing three
antipodals (a). A polar nucleus and a synergid are also visible. Bar represents 200 mm in A, 700 mm
in B, 750 mm in C and 600 mm in D and E.
and generative nuclei were present in anthers from all flowers (Fig. 3(B) and
(C)). `Trevatt Blue' anthers were shrunken (Fig. 4(A)) with an endothecium
(Fig. 4(B)), 72.2% with degenerated tapetum (Fig. 4(C)) but the remainder
without tapetal degeneration (Fig. 4(B)). No pollen grains were observed in the
anthers and only degenerated lumen contents were visible (Table 2).
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
259
Fig. 2. (A±E) Longitudinal sections of `Trevatt Blue' apricot ovules stained with PAS and TBO
and photographed using bright field optics. (A) Ovule showing nucellus (n), outer integument
(oi), inner integument (ii) and micropyle (m). (B) Ovule showing an abnormal and underdeveloped
nucellus (n). (C) Embryo sac with one nucleus. (D) Embryo sac with two nuclei. (E) Embryo
sac with four nuclei. Bar represents 200 mm in A, 100 mm in B, 450 mm in C, 600 mm in D and
650 mm in E.
4. Discussion
The results show that both female and male sterility contributed to low fruit set
in `Trevatt Blue' apricot trees. `Trevatt Knight' was both female and male fertile
and there are no other reports of sterility in the cultivar indicating a simultaneous
mutation in female and male function. `Trevatt Knight' was structurally similar to
260
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Fig. 3. (A±C) Transverse sections of `Moorpark' (figures A and B) and `Trevatt Knight' (figure C)
apricot anthers stained with PAS and TBO and photographed using bright field optics. (A) Anther
lobe showing endothecium (en), degenerated tapetum (dt) and pollen grains (pg). (B) Pollen grain
showing intine (i), exine (ex) and nuclei (nc). (C) Pollen grain showing germinaton pores (gp). Bar
represents 200 mm in A and 750 mm in B and C.
Table 2
Anther structure for fertile `Moorpark' and `Trevatt Knight', and sterile `Trevatt Blue' apricot
flowers at anthesis
Percentage anthers with
endothecium
degenerated tapetum
degenerated lumen contents
pollen grains
intine
bexine
germination pore
vegetative and generative nuclei
`Moorpark'
`Trevatt Knight'
`Trevatt Blue'
100
100
0
100
100
100
100
100
100
100
0
100
100
100
100
100
100
72.2
100
0
0
0
0
0
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
261
Fig. 4. (A±C) Transverse sections of `Trevatt Blue' apricot anthers stained with PAS and TBO and
photographed using bright field optics. (A) Anther showing four shrunken lobes (arrows). (B)
Anther lobes showing endothecium (en) and non-degenerated tapetum (t) with degenerated lumen
contents (dc). (C) Anther lobes showing degenerated tapetum (dt) and degenerated content (dc). Bar
represents 100 mm in A and 200 mm in B and C.
`Moorpark' showing no inherent problem in the `Trevatt' cultivar. Female sterility
was due to multiple ovules and retarded development, and the clone was male
sterile due to microspore degeneration and some failure in tapetal breakdown.
The findings explain the lack of success achieved using pollinator bouquets and
pollen mixtures, although occasional fruit were produced. This suggests that
some fertile embryo sacs were formed despite none being observed by
microscopy.
Since the clone was both female and male sterile and since similar observations
were noted for all flowers, there must have been a mutation in the original
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A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
`Trevatt Blue' tree from which the budwood was taken and from which the trees
were clonally propagated. Trees maintained for budwood are generally prevented
from flowering to limit spread of pollen-borne viruses, so a mutation would not
have been recognised until the progeny flowered. The magnitude of the problem
suggests that a single mutation or mutational event affected both the female and
male fertility of the clone.
Sterile plants can show both male and female sterility as many processes during
microsporogenesis and megasporogenesis are under combined genetic control
(Chaudhury, 1993). In particular, disruptions during meiosis result in mutations
that can cause sterility of both male and female gametophytes (Reiser and Fischer,
1993). This research into the `Trevatt Blue' apricot is the only report of both male
and female sterility occurring in the same variety of apricot but the phenomenon
has been reported in other crops. Abnormal embryo sacs of the female
gametophytes and failure in gametogenesis of both male and female gametophytes
was the cause of unfruitfulness in the `Swan Hill' variety of olive (Rallo et al.,
1981), and a gene has been identified in the male gametophyte of soybean which
reduces male and female fertility due to failure of cytokinesis following meiosis
(Kennell and Horner, 1985). Failure in meiosis is suggested in this study by the
low number of embryo sacs with nuclei, and the lack of pollen grains.
Sterility of apricots has previously been attributed to factors other than a
mutation, such as adverse environmental conditions (Eaton and Jamont, 1964).
Multiple ovules, as described here, have been described in apricot (Egea and
Burgos, 1995), and it was suggested that there may be a relationship between the
number of ovules per ovary and temperature (Egea and Burgos, 1998). In other
studies, abnormalities such as degeneration of ovules which had developed
normally, ovules with twin nucellus and shortened integuments were reported
(Burgos and Egea, 1994; Egea and Burgos, 1994). Some authors have found that
male sterile apricot trees have shrunken anthers with little or no pollen
(Nakanishi, 1983; Medeira and Guedes, 1991), but there are no previous
microscopy studies.
There has been an increase in apricot breeding activity around the world in
recent years, which has exposed previously unrecognised fertility problems
(J. Witherspoon, personal communication 1998). Failure of female and male
fertility, and self-incompatibility problems were selected against in the old
apricot cultivars which are widely planted, but there is now a need for new high
yielding firm fruited selections. This breeding activity is unmasking fertility
problems, such as the one reported here, and understanding these problems
is fundamental to successful apricot breeding in the future. `Trevatt Blue' and
other genotypes showing fertility failure are located at the Loxton Research
Station in South Australia, and further research is required to characterise
the genetic basis of these problems. In the meantime, similar failures can be
avoided by ensuring the establishment of multiple budwood mother trees of a
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
263
particular cultivar or clone, so that a mutation in one will not result in widespread
loss of yield.
Acknowledgements
The authors acknowledge Peter Burn for reporting the problem, John Zirilli for
supply of the `Trevatt Blue' flowers, Jenny Witherspoon for comments on the
problem and supply of the `Trevatt Knight' flowers and Mike Harms for
assistance.
References
Burgos, L., Egea, J., 1994. Apricot embryo-sac development in relation to fruit set. J. Hortic. Sci.
68, 203±208.
Chaudhury, A.M., 1993. Nuclear genes controlling male fertility. The Plant Cell 5, 1277±1283.
Eaton, G.W., Jamont, A.M., 1964. Embryo sac development in the apricot, Prunus armeniaca L. cv
Constant. J. Amer. Soc. Hortic. Sci. 86, 95±101.
Egea, J., Burgos, L., 1994. Year-to-year variation in the developmental stage of the embryo sac at
anthesis in flowers of apricot (Prunus armeniaca L.). J. Hortic. Sci. 69, 315±318.
Egea, J., Burgos, L., 1995. Supernumerary ovules in flowers of apricot. Acta Hortic. 384, 373±377.
Egea, J., Burgos, L., 1998. Fructification problems in continental apricot cultivars growing under
Mediterranean climate. Ovule development at anthesis in two climatic areas. J. Hortic. Sci. and
Biotech. 73, 107±110.
Kennell, J.C., Horner, H.T., 1985. Influence of the soybean male-sterile gene (ms1) on the
development of the female gametophyte. Can. J. Genet. Cyt. 27, 200±209.
McLaren, G.F., Fraser, J.A., 1996. Pollination compatibility of `Sundrop' apricot and its progeny in
the `Clutha' series. N.Z. J. Crop Hortic. Sci. 24, 47±53.
Medeira, M.C., Guedes, M.E., 1991. Flower bud abscission and male sterility in apricot. Acta
Hortic. 293, 311±318.
Nakanishi, T., 1983. Morphological and ultraviolet absorption differences between fertile and
sterile anthers of Japanese apricot cultivars in relation to their pollination stimuli. Sci. Hortic.
18, 57±63.
O'Brien, T.P., McCully, M.A., 1981. The study of plant structure: principles and selected methods.
Termacarphi, Melbourne.
Rallo, L., Martin, G.C., Lavee, S., 1981. Relationship between abnormal embryo sac development
and fruitfulness in olive. J. Amer. Soc. Hortic. Sci. 106, 813±817.
Reiser, L., Fischer, R.L., 1993. The ovule and the embryo sac. The Plant Cell 5, 1291±1301.
Sedgley, M., Griffin, A.R., 1989. Sexual reproduction of tree crops. Academic Press, London.
Female and male sterility cause low fruit set in a clone
of the `Trevatt' variety of apricot (Prunus armeniaca)
A.M. Lillecrapp, M.A. Wallwork, M. Sedgley*
Department of Horticulture, Viticulture and Oenology, Waite Agricultural Research Institute,
The University of Adelaide, Glen Osmond, S.A. 5064, Australia
Accepted 9 April 1999
Abstract
This study investigated ovule and anther structure of the `Trevatt Blue' variety of apricot (Prunus
armeniaca) using bright field microscopy following reports of low fruit set. Ovules and anthers
from fertile `Moorpark' and `Trevatt Knight' flowers were compared with those of the `Trevatt
Blue'. In the `Moorpark' and `Trevatt Knight' ovules, all reproductive structures were present
including embryo sacs with a complete set of eight nuclei and the anthers contained mature pollen
grains. Multiple ovules which were small and retarded in development were present in the `Trevatt
Blue' apricot flowers and the anthers contained degenerated microspores, with some failure in
tapetal breakdown. This is the first report of a simultaneous mutation in both female and male
function in apricot. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Apricot; Fertility; Prunus; Microscopy; Ovule; Embryo sac; Anther; Pollen grain
1. Introduction
`Trevatt' is an old self-fertile cultivar of apricot (Prunus armeniaca) and high
yielding clones have been developed to produce large fruit with desirable
characteristics. The clone, `Blue' was planted over wide areas by many growers
because the original selection was high yielding with fruit of excellent canning
quality, but the trees failed to set fruit for several seasons after the juvenile phase
had ended.
* Corresponding author. Tel.: +61-8-8303-7401; +61-8-8303-7249; fax: +61-8-8303-7116
E-mail address: [email protected] (M. Sedgley)
0304-4238/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 6 1 - 8
256
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Some varieties of apricot have been found to be self-sterile or have pollination
problems (McLaren and Fraser, 1996), but it seemed that in `Blue' female
sterility may have been a possible contributor to low fruit set. Very low levels of
set were achieved even when pollen from a different cultivar was introduced to
the trees via bouquets from fertile orchards. It was also possible that male sterility
contributed to the low fruit set. Female and male sterility have been reported
independently in apricot (Eaton and Jamont, 1964; Nakanishi, 1983; Medeira and
Guedes, 1991; Burgos and Egea, 1994) as well as in many other tree crops
(Sedgley and Griffin, 1989), but have been attributed to adverse environmental
conditions.
In this study, the female and male structures of `Trevatt Blue' were observed
microscopically and compared with those of `Trevatt Knight' which is known to
be fertile. A comparison was also made with the cultivar `Moorpark' in case there
was a problem inherent in the `Trevatt' cultivar.
2. Materials and methods
Flower samples of the low yielding `Trevatt Blue' were randomly collected
from a single variety apricot orchard in Griffith, New South Wales. Fertile
`Trevatt Knight' flowers were obtained from a mixed planting at Loxton, South
Australia, and fertile `Moorpark' flowers were obtained from the Waite orchard
of the University of Adelaide, South Australia.
Flowers were collected at the late balloon stage just prior to anthesis and fixed
in FPA50 (90% ethanol at 50%, 5% propionic acid and 5% formaldehyde).
Ovules and anthers were dissected from each flower under a microscope and the
samples dehydrated using a tertiary-butyl alcohol series and embedded in GMA
(glycol methacrylate). Serial longitudinal 4.0 mm sections through each ovule (up
to 150 sections per ovule) and 48 sections of 4.0 mm through each anther were
collected onto a microscope slide, stained with periodic acid-Schiffs reagent
(PAS) and toluidine blue O (TBO) stain and mounted in methyl methacrylate in
xylene (O'Brien and McCully, 1981). The sections were observed under a Zeiss
Axiophot photomicroscope with bright field illumination. Ovules and anthers
from 36 `Trevatt Blue', 15 `Trevatt Knight' and 9 `Moorpark' flowers were
examined. The structure of male and female reproductive structures was recorded
for each flower.
3. Results
Two ovules were present in each `Moorpark' and `Trevatt Knight' flower.
Between one and four ovules were present in each `Trevatt Blue' flower, most
257
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Table 1
Ovule structure for fertile `Moorpark' and `Trevatt Knight', and sterile `Trevatt Blue' apricot
flowers at anthesis
`Moorpark'
Mean number ovules (SD)
Range
20
2±2
`Trevatt Knight'
20
2±2
`Trevatt Blue'
2.88 0.79
1±4
Percentage ovules with
outer integument
inner integument
nucellus
micropyle
embryo sac
100
100
100
100
100
100
100
100
100
100
100
100
100
88.5
50
Embryo sac with
0 nuclei
1 nucleus
2 nuclei
4 nuclei
5±8 nuclei
0
0
0
0
100
0
0
0
0
100
19.2
19.2
8.7
2.9
0
smaller in size than the fertile `Moorpark' and `Trevatt Knight' ovules but all
similar in size to each other. One hundred and four ovules were obtained from 36
`Trevatt Blue' flowers; 2.8% of flowers had one ovule, 27.8% had two, 47.2% had
three and 22.2% had four (Table 1).
Of the ovules examined of the `Moorpark' and `Trevatt Knight', all had an outer
integument, inner integument, nucellus, micropyle and embryo sac (Fig. 1(A))
with an egg cell (Fig. 1(B)), two synergids (Fig. 1(C)), two polar nuclei (Fig. 1(D))
and three antipodals (Fig. 1(E)), which had degenerated in some ovules.
All `Trevatt Blue' ovules had an outer integument, inner integument and
nucellus, and 88.5% had a micropyle (Fig. 2(A)). Some had mis-shapen nucellus
(Fig. 2(B)), and embryo sacs were degenerated or not present in 50% of ovules
(Table 1). The other 50% contained embryo sacs with 0, 1 (Fig. 2(C)), 2
(Fig. 2(D)) or 4 (Fig. 2(E)) nuclei. No `Trevatt Blue' apricot ovules observed
contained eight nuclei, and 19.2% of ovules were at the megaspore mother cell
stage. Other abnormalities included small, spherical ovules, ovules joined
together and ovules with underdeveloped or abnormal nucellus (Fig. 2(B)) or
nucellus incompletely surrounded by the integuments.
The anthers of the `Moorpark' and `Trevatt Knight' flowers were bright yellow
in colour and plump compared to those of the `Trevatt Blue' flowers which were
red-brown and shrunken. Anthers from `Moorpark' and `Trevatt Knight' apricot
flowers had an endothecium and degenerated tapetum (Fig. 3(A)). Fully
developed pollen grains with an exine, intine, germination pores, vegetative
258
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Fig. 1. (A±E) Longitudinal sections of `Moorpark' (figures A, B, D and E) and `Trevatt Knight'
(figure C) apricot ovules stained with periodic acid-Schiff's reagent (PAS) and toluidine blue O
(TBO) and photographed using bright field optics. (A) Ovule showing embryo sac (es), nucellus (n),
outer integument (oi), inner integument (ii) and micropyle (m). (B) Embryo sac with egg cell (e). A
polar nucleus is also visible. (C) Embryo sac showing two synergids (s). (D) Embryo sac showing
two polar nuclei (pn). An egg cell and antipodal are also visible. (E) Embryo sac showing three
antipodals (a). A polar nucleus and a synergid are also visible. Bar represents 200 mm in A, 700 mm
in B, 750 mm in C and 600 mm in D and E.
and generative nuclei were present in anthers from all flowers (Fig. 3(B) and
(C)). `Trevatt Blue' anthers were shrunken (Fig. 4(A)) with an endothecium
(Fig. 4(B)), 72.2% with degenerated tapetum (Fig. 4(C)) but the remainder
without tapetal degeneration (Fig. 4(B)). No pollen grains were observed in the
anthers and only degenerated lumen contents were visible (Table 2).
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
259
Fig. 2. (A±E) Longitudinal sections of `Trevatt Blue' apricot ovules stained with PAS and TBO
and photographed using bright field optics. (A) Ovule showing nucellus (n), outer integument
(oi), inner integument (ii) and micropyle (m). (B) Ovule showing an abnormal and underdeveloped
nucellus (n). (C) Embryo sac with one nucleus. (D) Embryo sac with two nuclei. (E) Embryo
sac with four nuclei. Bar represents 200 mm in A, 100 mm in B, 450 mm in C, 600 mm in D and
650 mm in E.
4. Discussion
The results show that both female and male sterility contributed to low fruit set
in `Trevatt Blue' apricot trees. `Trevatt Knight' was both female and male fertile
and there are no other reports of sterility in the cultivar indicating a simultaneous
mutation in female and male function. `Trevatt Knight' was structurally similar to
260
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
Fig. 3. (A±C) Transverse sections of `Moorpark' (figures A and B) and `Trevatt Knight' (figure C)
apricot anthers stained with PAS and TBO and photographed using bright field optics. (A) Anther
lobe showing endothecium (en), degenerated tapetum (dt) and pollen grains (pg). (B) Pollen grain
showing intine (i), exine (ex) and nuclei (nc). (C) Pollen grain showing germinaton pores (gp). Bar
represents 200 mm in A and 750 mm in B and C.
Table 2
Anther structure for fertile `Moorpark' and `Trevatt Knight', and sterile `Trevatt Blue' apricot
flowers at anthesis
Percentage anthers with
endothecium
degenerated tapetum
degenerated lumen contents
pollen grains
intine
bexine
germination pore
vegetative and generative nuclei
`Moorpark'
`Trevatt Knight'
`Trevatt Blue'
100
100
0
100
100
100
100
100
100
100
0
100
100
100
100
100
100
72.2
100
0
0
0
0
0
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
261
Fig. 4. (A±C) Transverse sections of `Trevatt Blue' apricot anthers stained with PAS and TBO and
photographed using bright field optics. (A) Anther showing four shrunken lobes (arrows). (B)
Anther lobes showing endothecium (en) and non-degenerated tapetum (t) with degenerated lumen
contents (dc). (C) Anther lobes showing degenerated tapetum (dt) and degenerated content (dc). Bar
represents 100 mm in A and 200 mm in B and C.
`Moorpark' showing no inherent problem in the `Trevatt' cultivar. Female sterility
was due to multiple ovules and retarded development, and the clone was male
sterile due to microspore degeneration and some failure in tapetal breakdown.
The findings explain the lack of success achieved using pollinator bouquets and
pollen mixtures, although occasional fruit were produced. This suggests that
some fertile embryo sacs were formed despite none being observed by
microscopy.
Since the clone was both female and male sterile and since similar observations
were noted for all flowers, there must have been a mutation in the original
262
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
`Trevatt Blue' tree from which the budwood was taken and from which the trees
were clonally propagated. Trees maintained for budwood are generally prevented
from flowering to limit spread of pollen-borne viruses, so a mutation would not
have been recognised until the progeny flowered. The magnitude of the problem
suggests that a single mutation or mutational event affected both the female and
male fertility of the clone.
Sterile plants can show both male and female sterility as many processes during
microsporogenesis and megasporogenesis are under combined genetic control
(Chaudhury, 1993). In particular, disruptions during meiosis result in mutations
that can cause sterility of both male and female gametophytes (Reiser and Fischer,
1993). This research into the `Trevatt Blue' apricot is the only report of both male
and female sterility occurring in the same variety of apricot but the phenomenon
has been reported in other crops. Abnormal embryo sacs of the female
gametophytes and failure in gametogenesis of both male and female gametophytes
was the cause of unfruitfulness in the `Swan Hill' variety of olive (Rallo et al.,
1981), and a gene has been identified in the male gametophyte of soybean which
reduces male and female fertility due to failure of cytokinesis following meiosis
(Kennell and Horner, 1985). Failure in meiosis is suggested in this study by the
low number of embryo sacs with nuclei, and the lack of pollen grains.
Sterility of apricots has previously been attributed to factors other than a
mutation, such as adverse environmental conditions (Eaton and Jamont, 1964).
Multiple ovules, as described here, have been described in apricot (Egea and
Burgos, 1995), and it was suggested that there may be a relationship between the
number of ovules per ovary and temperature (Egea and Burgos, 1998). In other
studies, abnormalities such as degeneration of ovules which had developed
normally, ovules with twin nucellus and shortened integuments were reported
(Burgos and Egea, 1994; Egea and Burgos, 1994). Some authors have found that
male sterile apricot trees have shrunken anthers with little or no pollen
(Nakanishi, 1983; Medeira and Guedes, 1991), but there are no previous
microscopy studies.
There has been an increase in apricot breeding activity around the world in
recent years, which has exposed previously unrecognised fertility problems
(J. Witherspoon, personal communication 1998). Failure of female and male
fertility, and self-incompatibility problems were selected against in the old
apricot cultivars which are widely planted, but there is now a need for new high
yielding firm fruited selections. This breeding activity is unmasking fertility
problems, such as the one reported here, and understanding these problems
is fundamental to successful apricot breeding in the future. `Trevatt Blue' and
other genotypes showing fertility failure are located at the Loxton Research
Station in South Australia, and further research is required to characterise
the genetic basis of these problems. In the meantime, similar failures can be
avoided by ensuring the establishment of multiple budwood mother trees of a
A.M. Lillecrapp et al. / Scientia Horticulturae 82 (1999) 255±263
263
particular cultivar or clone, so that a mutation in one will not result in widespread
loss of yield.
Acknowledgements
The authors acknowledge Peter Burn for reporting the problem, John Zirilli for
supply of the `Trevatt Blue' flowers, Jenny Witherspoon for comments on the
problem and supply of the `Trevatt Knight' flowers and Mike Harms for
assistance.
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