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Role of natural self-pollination in self-fruitfulness of almond
Scientia Horticulturae, 27 (1985) 295--302 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
ROLE OF NATURAL SELF-POLLINATION ~ OF ALMOND
295
SELF-FRU~FULNESS
STEVEN A. WEINBAUM Department of Pomology, University of California, Davis, CA 95616 (U.S.A.) (Accepted for publication 6 July 1985)
ABSTRACT
Weinbaum, S.A., 1985. Role of natural self-pollination in self-fruitfulness of almond. Scientia Hortic., 27: 295--302. The capacity for self-fruitfulness in the almond cultivar 'Le Grand' is limited by (a) self-incompatibility and (b) insufficient capacity for natural self-pollination. Expression of self-incompatibility following self-pollination by hand reduced fruit-set by 67 and 89% in 2 crop-years in comparison with fruit-set percentages achieved following cross-pollination by hand. Pollen retention following natural self-pollination equalled about 8 pollen grains/stigma and reduced fruit set by 50% when compared with branches receiving non-limiting (>300 grains)quantities of self-pollen. These data indicate that 'Le Grand' is (a) unsuited for planting in monoculture and (b) highly dependent on insect-mediated pollen transfer. Blossom observations made at anthesis indicated that the relative capacity for natural self-pollination among clones may be anticipated by the spatial relationship between dehiscing anthers and receptive stigmas. Keywords: almond; incompatibility; pollination; self-fertility.
INTRODUCTION All c o m m e r c i a l l y i m p o r t a n t a l m o n d cultivars in California e x h i b i t gam e t o p h y t i c self-incompatibility (Socias i C o m p a n y et al., 1976). Thus, fertilization o f ova p r o d u c e d b y a p l a n t o f given g e n o t y p e requires t h a t pollen o f an i n t e r c o m p a t i b l e g e n o t y p e be d e p o s i t e d on receptive stigmas. This is a c c o m p l i s h e d in c o m m e r c i a l o r c h a r d s b y (a) interplanting interc o m p a t i b l e g e n o t y p e s t h a t f l o w e r s i m u l t a n e o u s l y , and (b) bringing h o n e y bees into t h e o r c h a r d t o e f f e c t t h e intercultivar t r a n s f e r o f pollen (Tufts, 1919; Griggs, 1953). I n c o m p l e t e p o l l i n a t i o n is a principal barrier to maxim u m a l m o n d p r o d u c t i v i t y (Kester and Griggs, 1959a). Thus, t h e r e has been recurring interest in self-fertile a l m o n d cultivars t h a t , if available, c o u l d be p l a n t e d in solid blocks, t h e r e b y reducing certain m a n a g e m e n t p r o b l e m s (e.g. i n a d e q u a t e b l o o m overlap o f cross-compatible cultivars).
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© 1985 Elsevier Science Publishers B.V.
296 The efficiency of almond culture could be improved further if the dependence on honeybees for pollen transfer could be reduced, as is the case with peach (Weinbaum and Erez, 1983). This is important because the foraging of honeybees is limited during inclement weather (Thorp, 1978). Two sources of self-fertility (the ability of a cultivar to produce fruit with viable seed following self-pollination; Griggs, 1953) have been recognized within the cultivated almond. Self-fertility has been reported in a number of almond cultivars (e.g. 'Falsa Barese', 'Ferrante', 'Filippo Ceo' and 'Tuono'} grown in the region of Puglia in Italy (Grasselly and Olivier, 1976). The fruit-set percentages of these self-fertile cultivars, however, were often substantially lower following self-pollination than following cross-pollination (Godini et al., 1977). The second source of selffertility in almond has been derived on a limited scale from the peach via peach--almond interspecific hybridization. Presumed self-fertile almond cultivars such as 'Le Grand' have been recovered from consecutive backcross populations to almond (Kester and Asay, 1975}. 'Le Grand', released in 1972 by F.W. Anderson (plant patent 3130), is currently grown on about 400 ha in California, but the degree and consistency of its self-fertility have not been assessed critically. My objective was (a) to estimate the degree of self-fertility in 'Le Grand' and (b) to estimate the degree to which self-pollination, i.e. the transfer of pollen from anthers to stigmas, in this cultivar depends upon insects. Fruit-set resulting from natural selfpollination (i.e. pollen transfer without insect mediation) is apparently not limited by the amount of pollen in peach (Weinbaum and Erez, 1983). Natural self-pollination is presumably the result of anther--stigma contact, perhaps influenced by wind-buffeting of branches. MATERIALS AND METHODS Cultivars. - - Investigations were carried out at Manteca, California, over
a 2-year period using trees in their sixth and seventh leaf. 'Le Grand', a presumed self-fertile almond cultivar, was of primary interest. 'Texas' ('Mission'), a commercially important self-incompatible almond, whose bloom period is coincident with that of 'Le Grand', was also studied for comparative purposes. Assessment of self-compatibility
The degree of self-compatibility in 'Texas' and 'Le Grand' was assessed by comparison of fruit-set following pollination by hand of blossoms with either self-pollen or pollen of a cross-compatible cultivar 'Ne Plus Ultra'. In vitro germinability of freshly collected 'Texas', 'Le Grand' and 'Ne Plus Ultra' pollen on 10% sucrose was tested prior to application, and all pollen tested was >80% viable. Test branches were enclosed in fiberglass insect screen bags prior to anthesis to preclude honeybee-mediated pollination
297
throughout the period of receptivity. Each treatment was applied to 1 branch (about 200 flowers) on each of 10 trees per cultivar. A s s e s s m e n t o f natural self-pollination and r e t e n t i o n Q u a n t i t a t i o n o f p o l l e n retention. -- Bees were excluded as described above.
Samples consisting of stigmas and attached distal portions of styles of 20 individual flowers were collected from both 'Texas' and 'Le Grand'. Completion of stigmatic receptivity was indicated by the drying of stigmatic exudate and browning of the stigmatic surface, and samples were stored at room temperature in ethanol:glacial-acetic-acid ( 3 : 1 v/v) until processed. Only germinated pollen grains were retained on the stigma following processing (Weinbaum et al., 1985). Pollen grains were observed and counted in squashed preparations using epifluorescence microscopy (Weinbaum and Polito, 1985). R e l a t i v e fruit-set. -- Relative fruit set is an expression of the relative success
of natural self-pollination and is defined as the ratio of fruit-set percentages resulting from natural self-pollination divided by the fruit-set percentages following self-pollination by hand. Fruit-set data were obtained following the second drop (Kester and Griggs, 1959b), i.e. after abscission of nonfertilized ovaries. 'Texas' and 'Le Grand' blossoms were observed during anthesis to determine whether their comparative capacities for natural self-pollination could be anticipated by the proximity of anthers and stigma within the flower. RESULTS
Percentage fruit-set following hand-cross-pollination was distinctly higher in both cultivars in 1984 as compared with 1983, and apparently reflected more favorable temperatures at anthesis (Table I). 'Le Grand' and 'Texas' TABLE I E f f e c t o f p o l l e n source o n p e r c e n t a g e fruit-set o f 10 b r a n c h u n i t s a n d s e l f - c o m p a t i b i l i t y (see " M e t h o d s " ) o f 'Le G r a n d ' a n d ' T e x a s ' a l m o n d s following h a n d - p o l l i n a t i o n Year
Cultivar
F r u i t - s e t (% ± SE) Hand-self-pollination b y p o l l e n of:
1983 1983 1984 1984
Le G r a n d Texas Le G r a n d Texas
'Texas'
Hand-crosspollination b y p o l l e n of 'Le G r a n d ' ' N e Plus U l t r a '
-2.8±0.5 -3.4±0.8
12.6±2.3 -8.1±2.2 --
37.9±2.3 41.0±2.5 74.5±3.2 69.5~4.9
M e a n daily air temperatures during 5 days a f t e r d a t e of pollination
Selfcompatibility (%)
(°C) 11.2 11.2 13.4 13.4
_+ 0.3 -+ 0.3 -+ 0.1 _+ 0.1
32.2 6.8 10,9 4.9
--' --
7.6-+3.9 0.3 +_ 0.1
1.3+_0.4 4 . 4 - + 3 . 3 0.6 -+ 0.2 5.2 -+ 0.8
1984 ---
1983
>300 >300
1984
1983
1983
1984
Pollen r e t e n t i o n (grains/stigma)
F r u i t set (% -+ SE)
Pollen r e t e n t i o n (grains/stigma _+ SE)
12.6_+2.3 2.8 -+ 0.5
1983
8.1-+2.3 3.4 -+ 0.8
1984
F r u i t set (% -+ SE)
H a n d self-pollination
Natural self-pollination
1 - - i n d i c a t e s data n o t c o l l e c t e d .
LeGrand Texas
Cultivar
0.10 0.21
1983
0.54 1.53
1984
Relative fruit-set
Significance o f natural self-pollination a n d r e t e n t i o n in the s e l f - i n c o m p a t i b l e cultivar ' T e x a s ' and t h e selffertile cultivar 'Le G r a n d ' e x p r e s s e d as pollen r e t e n t i o n and relative fruit-set. Details in " M e t h o d s "
T A B L E II
c£) O0
299
exhibited equivalent capacities for fruit set following hand-cross-pollination in both years (Table I). Fruit-set of 'Le Grand' was 2.4 (1984) to 4.5 (1983) times greater than fruit-set of 'Texas' (Table I). These data confirm that 'Le Grand' exhibits greater self-compatibility than 'Texas', a standard self-incompatible commercial cultivar. In 1983 and 1984, the percentage self-compatibility of 'Le Grand' was only 32.2 and 10.9%, respectively (Table I). As more than 300 pollen grains were deposited by hand, fruit set by self-pollination in 'Le Grand' was apparently limited by factors other than the availability of self-pollen. The percentage self-compatibility of 'Texas' was much lower. Hand-self-pollination of 'Le Grand' which deposited >300 grains/stigma also resulted in 10 and 2 times more fruit-set in 1983 and 1984, respectively, than did natural self-pollination (Table II). Thus, the 7.6 grains retained per 'Le Grand' stigma following natural self-pollination (Table II) were insufficient to support maximum fertilization. The higher relative fruit-set of both cultivars in 1984 (Table II) may reflect higher temperatures during bloom in 1984 as compared with 1983 (see Table I). A relative fruit-set value >1 in 'Texas', in 1984 only, indicated that the amount of self-pollen deposited on the stigma was not the primary factor limiting fruit-set in that cultivar. The greater proximity of anthers and stigma within blossoms of 'Le Grand' allowed us to anticipate the greater capacity of this cultivar for natural self-pollination as compared with 'Texas'. 'Le Grand' blossoms exhibited frequent contact between receptive stigmas and dehiscing anthers. The blossom phenotype of 'Texas' differed greatly, i.e. stigmas were positioned above all anthers prior to anther dehiscence, and elongation of the style persisted following anthesis. There was no apparent possibility for natural self-pollination in 'Texas'. DISCUSSION
Self-fertility, the ability of a cultivar to produce fruit with viable seed following self-pollination (Griggs, 1953), is desirable but perhaps insufficient to meet the demands of commercial almond culture. Commercial cultivars, even if self-fertile, should exhibit fruit-set percentages which approximate 25--30% or higher (Kester and Griggs, 1959a). Over a 2-year period, 'Le Grand' was incapable of approaching this level of set with its own pollen even following self-pollination by hand (Table I), and appears unsuited for monoculture. That 'Le Grand' is only partially self-compatible is supported by the reduced fruit-set percentage following self-pollination by hand as compared with cross-pollination by hand. The inhibition appeared greater in 1984 than in 1983 (Table II), and is consistent with a reduction of stylar penetration by incompatible pollen tubes with increasing temperatures (Socias i Company et al., 1976). The percentage fruit
300 set of 'Le Grand' following self-pollination by hand was distinctly greater than that of 'Texas', but does not appear commercially adequate. Self-set of both cultivars following natural self-pollination was greater in 1984 (a warmer year) than it was in 1983 (Table II). The fruit-set percentage achieved by 'Texas' following natural self-pollination in 1984 was equivalent to that achieved in 'Le Grand'. This small (5+%) self-set may be attributed to the phenomenon termed pseudo-self-compatibility (Pandey, 1959). This phenomenon reflects both environmental and genetic influences (Pandey, 1959) and is widespread within the species (R. Socias i Company, personal communication, 1984). Self-compatibility has been the only trait associated with self-fertility in the past (Socias i Company et al., 1976). The desirability of transferring "the self-fertility gene of the peach into the almond genome ..... " has been discussed by Hesse (1975). It is not clear whether Hesse intended a literal interpretation and referred only to self-compatibility, or whether he referred to a complex of undefined traits which contributed to self-fruitfulness and were presumably controlled by a single gene. I propose that the capacity for natural self-pollination may be integrally related to the level of selffruitfulness in these cultivars--particularly under inclement weather conditions when honeybee activity is severely reduced. The possibility of eliminating the dependence on insect vectors for pollination is intriguing, as California almond growers spend an estimated $14 million a year to rent honeybees (E.C. Mussen, Extension Apiculturist, University of California, Davis, CA, U.S.A., personal communication, 1984). Although only one pollen grain is theoretically necessary to set an almond fruit, there is increasing evidence that quantities of viable pollen must be deposited on the stigma in excess of the number of viable ovules. Ter-Avanesian (1978) reported greatly reduced rates of pollen tube growth in cotton when there were few pollen grains on the stigma as compared with many pollen grains on the stigma. Under inclement weather conditions and/or in the absence of honeybees, the capacity for natural selfpollination in self-compatible almond cultivars could influence rates of pollen tube growth and subsequent fruit set. Our own data indicate that although an average of 7.6 pollen grains were deposited on the stigma (Table II), fruit-set was pollen-limited in 'Le Grand'. Rather than just a mass effect, the increased percentage fruit-set following self-pollination by hand as compared to natural self-pollination may reflect greater opportunity for gamete selection -- those 1 or 2 pollen grains in 300 that have a self-fertility allele or have lost a self-incompatibility allele. The relatively high p o l l e n : o v u l e ratio required for fruit-set in 'Le Grand' may reflect the residual self-incompatibility. Self-incompatibility is manifested by slower growth of pollen tubes in the style (Socias i Company et al., 1976) and ovule abortion before fertilization. Alternatively, the reduction in self-set may reflect a post-fertilization phenomenon associated with extreme inbreeding depression, which is expressed as very early abortion of embryos.
301
Anther--stigma contact apparently facilitated natural self-pollination and fruit set in 'Morello' cherry and certain cultivars of plum, pear and black currant (Hooper, 1939). A high degree of natural self-pollination in peach has also been implicit (if not discussed) in previous reports (Detjen, 1944; McGregor, 1976; Fogle, 1977; Weinbaum and Erez, 1983). As peach is a close botanical relative of the almond, it appears likely that the capacity for natural self-pollination was partially lost during the repeated backcrosses to the almond which resulted in 'Le Grand'. Retention of pollen grains following natural self-pollination was conspicuously greater in 'Le Grand' than in 'Texas', which has no peach parentage. My data indicate that the possibility of visually selecting the germplasm with increased capacities for natural self-pollination is a realistic expectation. The almond 'Truoito' has exhibited sufficient self-compatibility and natural self-pollination to produce a full crop with its own pollen in the absence of insect pollinators (Vasilakakis and Porlingis, 1984). However, no convincing documentation currently exists for other self-fruitful almond cultivars. More stringent testing of advanced selections is warranted prior to the future release of self-fruitful almonds. Cultivars which are biologically self-fertile may nevertheless be unsuited to commercial culture. The extent to which the potential for natural self-pollination is determined genetically and the significance of environmental influences has yet to be determined. The higher relative fruit-set in both cultivars in 1984 as compared to 1983 may represent the interaction between environment and genotype on pistil development. ACKNOWLEDGEMENTS
This research was supported by a grant from the United States--Israel (Bi-national) Agricultural Research and Development Fund (BARD). The author thanks Drs. D.E. Kester, P.E. Hansche, V.S. Polito, D.W. Ramming, W. Sherman, R. Socias i Company, P. Spiegel-Roy and M. Vasilakakis and Mr. L. Fenton for critical reviews of the manuscript, and Mr. T.T. Muraoka for invaluable technical assistance. REFERENCES
Detjen, L.R., 1944. Fruitfulness in peaches and its relationship to morphology and physiology of pollen grains. Univ. Del. Agric. Exp. Stn. Bull. No. 257, 24 pp. Fogle, H.W., 1977. Self-pollination and its implications in peach improvement. Fruit Var. J., 31: 74--75. Godini, A., Ferrara, E., Reina, S., Giorgio, V. and Guida, F., 1977. Contributo alia Conoscenza delle cultivar di mandorlo (P. amygdalus Batsch) della Puglia. III. Un triennio di prove d'impollinazione incrociata, 3rd Coll., Groupe Recherche et d'Etudes Mdditerran~en pour I' Amandier, Bari, Centre Internationale des Hautes Etudes d'Agronomie M~diterrandenes, MontPellier, pp. 194--206. Grasselly, Ch. and Olivier, G., 1976. Mise en ~vidence de quelques types autocompatibles parrni les cultivars d'amandier (P. amygdalus Batsch) de la population des Pouilles. Ann. Arnelior. Plant., 26: 107--113.
302 Griggs, W.H., 1953. Pollination requirements of fruits and nuts. Calif. Agric. Exp. Stn. Circ. 424, 35 pp. Hesse, C.O., 1975. Peaches. In: J. Janick and J.N. Moore (Editors), Advances in Fruit Breeding. Purdue University Press, West Lafayette, IN, pp. 285--335. Hooper, C.H., 1939. Hive bees in relation to commercial fruit production. J. Southeast. Agric. Coll., Wye, Engl., 44: 103--108. Kester, D.E. and Asay, R., 1975. Almonds. In: J. Janick and J.N. Moore (Editors), Advances in Fruit Breeding. Purdue University Press, West Lafayette, IN, pp. 387-419. Kester, D.E. and Griggs, W.H., 1959a. Fruit setting in the almond: The effect of crosspollinating various percentages of flowers. Proc. Am. Soc. Hortic. Sci., 74: 206-213. Kester, D.E. and Griggs, W.H., 1959b. Fruit setting in the almond: The pattern of flower and fruit drop. Proe. Am. Soc. Hortic. Sci., 74: 214--219. McGregor, S.E., 1976. Peach and nectarine. In: Insect Pollination of Cultivated Crop Plants. U.S. Dep. Agric. No. 496, Agric. Res. Ser. USDA, pp. 281--285. Pandey, K.K., 1959. Mutations of the self-incompatibility gene(s) and pseudocompatibility in angiosperms. Lloydia, 22: 222--234. Socias i Company, R., Kester, D.E. and Bradley, M.V., 1976. Effects of genotype a n d temperature on pollen tube growth in some self-incompatible and self-compatible almond cultivars. J. Am. Soc. Hortic. Sci., 101 : 490--493. Ter-Avanesian, D.V., 1978. The effect of varying the number of pollen grains used in fertilization. Theor. Appl. Genet., 52: 77--79. Thorp, R.W., 1978. Bee management for almond pollination. In: Almond Orchard Management. University of California Priced Publication 4092, pp. 57--61. Tufts, W.P., 1919. Almond pollination. Calif. Agric. Stn. Bull. 306, 32 pp. Vasilakakis, M. and Porlingis, I.C., 1984. Self-compatibility in 'Truoito' almond and the effect of temperature on selfed and crossed pollen tube growth. HortScience, 19: 659--661. Weinbaum, S.A. and Erez, A., 1983. Autogamy among selected peach and nectarine cultivars. Fruit Var. J., 37 : 113--114. Weinbaum, S.A. and Polito, V.S., 1985. Development of a convenient assay based on autofluorescence of the exine of the pollen grain wall. Mikroskopie, in press. Weinbaum, S.A., Polito, V.S. and Kester, D.E., 1985. Natural self-pollination in peach, almond, and peach x almond interspeclfic F, hybrids. Euphytica, in press.
ROLE OF NATURAL SELF-POLLINATION ~ OF ALMOND
295
SELF-FRU~FULNESS
STEVEN A. WEINBAUM Department of Pomology, University of California, Davis, CA 95616 (U.S.A.) (Accepted for publication 6 July 1985)
ABSTRACT
Weinbaum, S.A., 1985. Role of natural self-pollination in self-fruitfulness of almond. Scientia Hortic., 27: 295--302. The capacity for self-fruitfulness in the almond cultivar 'Le Grand' is limited by (a) self-incompatibility and (b) insufficient capacity for natural self-pollination. Expression of self-incompatibility following self-pollination by hand reduced fruit-set by 67 and 89% in 2 crop-years in comparison with fruit-set percentages achieved following cross-pollination by hand. Pollen retention following natural self-pollination equalled about 8 pollen grains/stigma and reduced fruit set by 50% when compared with branches receiving non-limiting (>300 grains)quantities of self-pollen. These data indicate that 'Le Grand' is (a) unsuited for planting in monoculture and (b) highly dependent on insect-mediated pollen transfer. Blossom observations made at anthesis indicated that the relative capacity for natural self-pollination among clones may be anticipated by the spatial relationship between dehiscing anthers and receptive stigmas. Keywords: almond; incompatibility; pollination; self-fertility.
INTRODUCTION All c o m m e r c i a l l y i m p o r t a n t a l m o n d cultivars in California e x h i b i t gam e t o p h y t i c self-incompatibility (Socias i C o m p a n y et al., 1976). Thus, fertilization o f ova p r o d u c e d b y a p l a n t o f given g e n o t y p e requires t h a t pollen o f an i n t e r c o m p a t i b l e g e n o t y p e be d e p o s i t e d on receptive stigmas. This is a c c o m p l i s h e d in c o m m e r c i a l o r c h a r d s b y (a) interplanting interc o m p a t i b l e g e n o t y p e s t h a t f l o w e r s i m u l t a n e o u s l y , and (b) bringing h o n e y bees into t h e o r c h a r d t o e f f e c t t h e intercultivar t r a n s f e r o f pollen (Tufts, 1919; Griggs, 1953). I n c o m p l e t e p o l l i n a t i o n is a principal barrier to maxim u m a l m o n d p r o d u c t i v i t y (Kester and Griggs, 1959a). Thus, t h e r e has been recurring interest in self-fertile a l m o n d cultivars t h a t , if available, c o u l d be p l a n t e d in solid blocks, t h e r e b y reducing certain m a n a g e m e n t p r o b l e m s (e.g. i n a d e q u a t e b l o o m overlap o f cross-compatible cultivars).
0304-4238/85/$03.30
© 1985 Elsevier Science Publishers B.V.
296 The efficiency of almond culture could be improved further if the dependence on honeybees for pollen transfer could be reduced, as is the case with peach (Weinbaum and Erez, 1983). This is important because the foraging of honeybees is limited during inclement weather (Thorp, 1978). Two sources of self-fertility (the ability of a cultivar to produce fruit with viable seed following self-pollination; Griggs, 1953) have been recognized within the cultivated almond. Self-fertility has been reported in a number of almond cultivars (e.g. 'Falsa Barese', 'Ferrante', 'Filippo Ceo' and 'Tuono'} grown in the region of Puglia in Italy (Grasselly and Olivier, 1976). The fruit-set percentages of these self-fertile cultivars, however, were often substantially lower following self-pollination than following cross-pollination (Godini et al., 1977). The second source of selffertility in almond has been derived on a limited scale from the peach via peach--almond interspecific hybridization. Presumed self-fertile almond cultivars such as 'Le Grand' have been recovered from consecutive backcross populations to almond (Kester and Asay, 1975}. 'Le Grand', released in 1972 by F.W. Anderson (plant patent 3130), is currently grown on about 400 ha in California, but the degree and consistency of its self-fertility have not been assessed critically. My objective was (a) to estimate the degree of self-fertility in 'Le Grand' and (b) to estimate the degree to which self-pollination, i.e. the transfer of pollen from anthers to stigmas, in this cultivar depends upon insects. Fruit-set resulting from natural selfpollination (i.e. pollen transfer without insect mediation) is apparently not limited by the amount of pollen in peach (Weinbaum and Erez, 1983). Natural self-pollination is presumably the result of anther--stigma contact, perhaps influenced by wind-buffeting of branches. MATERIALS AND METHODS Cultivars. - - Investigations were carried out at Manteca, California, over
a 2-year period using trees in their sixth and seventh leaf. 'Le Grand', a presumed self-fertile almond cultivar, was of primary interest. 'Texas' ('Mission'), a commercially important self-incompatible almond, whose bloom period is coincident with that of 'Le Grand', was also studied for comparative purposes. Assessment of self-compatibility
The degree of self-compatibility in 'Texas' and 'Le Grand' was assessed by comparison of fruit-set following pollination by hand of blossoms with either self-pollen or pollen of a cross-compatible cultivar 'Ne Plus Ultra'. In vitro germinability of freshly collected 'Texas', 'Le Grand' and 'Ne Plus Ultra' pollen on 10% sucrose was tested prior to application, and all pollen tested was >80% viable. Test branches were enclosed in fiberglass insect screen bags prior to anthesis to preclude honeybee-mediated pollination
297
throughout the period of receptivity. Each treatment was applied to 1 branch (about 200 flowers) on each of 10 trees per cultivar. A s s e s s m e n t o f natural self-pollination and r e t e n t i o n Q u a n t i t a t i o n o f p o l l e n retention. -- Bees were excluded as described above.
Samples consisting of stigmas and attached distal portions of styles of 20 individual flowers were collected from both 'Texas' and 'Le Grand'. Completion of stigmatic receptivity was indicated by the drying of stigmatic exudate and browning of the stigmatic surface, and samples were stored at room temperature in ethanol:glacial-acetic-acid ( 3 : 1 v/v) until processed. Only germinated pollen grains were retained on the stigma following processing (Weinbaum et al., 1985). Pollen grains were observed and counted in squashed preparations using epifluorescence microscopy (Weinbaum and Polito, 1985). R e l a t i v e fruit-set. -- Relative fruit set is an expression of the relative success
of natural self-pollination and is defined as the ratio of fruit-set percentages resulting from natural self-pollination divided by the fruit-set percentages following self-pollination by hand. Fruit-set data were obtained following the second drop (Kester and Griggs, 1959b), i.e. after abscission of nonfertilized ovaries. 'Texas' and 'Le Grand' blossoms were observed during anthesis to determine whether their comparative capacities for natural self-pollination could be anticipated by the proximity of anthers and stigma within the flower. RESULTS
Percentage fruit-set following hand-cross-pollination was distinctly higher in both cultivars in 1984 as compared with 1983, and apparently reflected more favorable temperatures at anthesis (Table I). 'Le Grand' and 'Texas' TABLE I E f f e c t o f p o l l e n source o n p e r c e n t a g e fruit-set o f 10 b r a n c h u n i t s a n d s e l f - c o m p a t i b i l i t y (see " M e t h o d s " ) o f 'Le G r a n d ' a n d ' T e x a s ' a l m o n d s following h a n d - p o l l i n a t i o n Year
Cultivar
F r u i t - s e t (% ± SE) Hand-self-pollination b y p o l l e n of:
1983 1983 1984 1984
Le G r a n d Texas Le G r a n d Texas
'Texas'
Hand-crosspollination b y p o l l e n of 'Le G r a n d ' ' N e Plus U l t r a '
-2.8±0.5 -3.4±0.8
12.6±2.3 -8.1±2.2 --
37.9±2.3 41.0±2.5 74.5±3.2 69.5~4.9
M e a n daily air temperatures during 5 days a f t e r d a t e of pollination
Selfcompatibility (%)
(°C) 11.2 11.2 13.4 13.4
_+ 0.3 -+ 0.3 -+ 0.1 _+ 0.1
32.2 6.8 10,9 4.9
--' --
7.6-+3.9 0.3 +_ 0.1
1.3+_0.4 4 . 4 - + 3 . 3 0.6 -+ 0.2 5.2 -+ 0.8
1984 ---
1983
>300 >300
1984
1983
1983
1984
Pollen r e t e n t i o n (grains/stigma)
F r u i t set (% -+ SE)
Pollen r e t e n t i o n (grains/stigma _+ SE)
12.6_+2.3 2.8 -+ 0.5
1983
8.1-+2.3 3.4 -+ 0.8
1984
F r u i t set (% -+ SE)
H a n d self-pollination
Natural self-pollination
1 - - i n d i c a t e s data n o t c o l l e c t e d .
LeGrand Texas
Cultivar
0.10 0.21
1983
0.54 1.53
1984
Relative fruit-set
Significance o f natural self-pollination a n d r e t e n t i o n in the s e l f - i n c o m p a t i b l e cultivar ' T e x a s ' and t h e selffertile cultivar 'Le G r a n d ' e x p r e s s e d as pollen r e t e n t i o n and relative fruit-set. Details in " M e t h o d s "
T A B L E II
c£) O0
299
exhibited equivalent capacities for fruit set following hand-cross-pollination in both years (Table I). Fruit-set of 'Le Grand' was 2.4 (1984) to 4.5 (1983) times greater than fruit-set of 'Texas' (Table I). These data confirm that 'Le Grand' exhibits greater self-compatibility than 'Texas', a standard self-incompatible commercial cultivar. In 1983 and 1984, the percentage self-compatibility of 'Le Grand' was only 32.2 and 10.9%, respectively (Table I). As more than 300 pollen grains were deposited by hand, fruit set by self-pollination in 'Le Grand' was apparently limited by factors other than the availability of self-pollen. The percentage self-compatibility of 'Texas' was much lower. Hand-self-pollination of 'Le Grand' which deposited >300 grains/stigma also resulted in 10 and 2 times more fruit-set in 1983 and 1984, respectively, than did natural self-pollination (Table II). Thus, the 7.6 grains retained per 'Le Grand' stigma following natural self-pollination (Table II) were insufficient to support maximum fertilization. The higher relative fruit-set of both cultivars in 1984 (Table II) may reflect higher temperatures during bloom in 1984 as compared with 1983 (see Table I). A relative fruit-set value >1 in 'Texas', in 1984 only, indicated that the amount of self-pollen deposited on the stigma was not the primary factor limiting fruit-set in that cultivar. The greater proximity of anthers and stigma within blossoms of 'Le Grand' allowed us to anticipate the greater capacity of this cultivar for natural self-pollination as compared with 'Texas'. 'Le Grand' blossoms exhibited frequent contact between receptive stigmas and dehiscing anthers. The blossom phenotype of 'Texas' differed greatly, i.e. stigmas were positioned above all anthers prior to anther dehiscence, and elongation of the style persisted following anthesis. There was no apparent possibility for natural self-pollination in 'Texas'. DISCUSSION
Self-fertility, the ability of a cultivar to produce fruit with viable seed following self-pollination (Griggs, 1953), is desirable but perhaps insufficient to meet the demands of commercial almond culture. Commercial cultivars, even if self-fertile, should exhibit fruit-set percentages which approximate 25--30% or higher (Kester and Griggs, 1959a). Over a 2-year period, 'Le Grand' was incapable of approaching this level of set with its own pollen even following self-pollination by hand (Table I), and appears unsuited for monoculture. That 'Le Grand' is only partially self-compatible is supported by the reduced fruit-set percentage following self-pollination by hand as compared with cross-pollination by hand. The inhibition appeared greater in 1984 than in 1983 (Table II), and is consistent with a reduction of stylar penetration by incompatible pollen tubes with increasing temperatures (Socias i Company et al., 1976). The percentage fruit
300 set of 'Le Grand' following self-pollination by hand was distinctly greater than that of 'Texas', but does not appear commercially adequate. Self-set of both cultivars following natural self-pollination was greater in 1984 (a warmer year) than it was in 1983 (Table II). The fruit-set percentage achieved by 'Texas' following natural self-pollination in 1984 was equivalent to that achieved in 'Le Grand'. This small (5+%) self-set may be attributed to the phenomenon termed pseudo-self-compatibility (Pandey, 1959). This phenomenon reflects both environmental and genetic influences (Pandey, 1959) and is widespread within the species (R. Socias i Company, personal communication, 1984). Self-compatibility has been the only trait associated with self-fertility in the past (Socias i Company et al., 1976). The desirability of transferring "the self-fertility gene of the peach into the almond genome ..... " has been discussed by Hesse (1975). It is not clear whether Hesse intended a literal interpretation and referred only to self-compatibility, or whether he referred to a complex of undefined traits which contributed to self-fruitfulness and were presumably controlled by a single gene. I propose that the capacity for natural self-pollination may be integrally related to the level of selffruitfulness in these cultivars--particularly under inclement weather conditions when honeybee activity is severely reduced. The possibility of eliminating the dependence on insect vectors for pollination is intriguing, as California almond growers spend an estimated $14 million a year to rent honeybees (E.C. Mussen, Extension Apiculturist, University of California, Davis, CA, U.S.A., personal communication, 1984). Although only one pollen grain is theoretically necessary to set an almond fruit, there is increasing evidence that quantities of viable pollen must be deposited on the stigma in excess of the number of viable ovules. Ter-Avanesian (1978) reported greatly reduced rates of pollen tube growth in cotton when there were few pollen grains on the stigma as compared with many pollen grains on the stigma. Under inclement weather conditions and/or in the absence of honeybees, the capacity for natural selfpollination in self-compatible almond cultivars could influence rates of pollen tube growth and subsequent fruit set. Our own data indicate that although an average of 7.6 pollen grains were deposited on the stigma (Table II), fruit-set was pollen-limited in 'Le Grand'. Rather than just a mass effect, the increased percentage fruit-set following self-pollination by hand as compared to natural self-pollination may reflect greater opportunity for gamete selection -- those 1 or 2 pollen grains in 300 that have a self-fertility allele or have lost a self-incompatibility allele. The relatively high p o l l e n : o v u l e ratio required for fruit-set in 'Le Grand' may reflect the residual self-incompatibility. Self-incompatibility is manifested by slower growth of pollen tubes in the style (Socias i Company et al., 1976) and ovule abortion before fertilization. Alternatively, the reduction in self-set may reflect a post-fertilization phenomenon associated with extreme inbreeding depression, which is expressed as very early abortion of embryos.
301
Anther--stigma contact apparently facilitated natural self-pollination and fruit set in 'Morello' cherry and certain cultivars of plum, pear and black currant (Hooper, 1939). A high degree of natural self-pollination in peach has also been implicit (if not discussed) in previous reports (Detjen, 1944; McGregor, 1976; Fogle, 1977; Weinbaum and Erez, 1983). As peach is a close botanical relative of the almond, it appears likely that the capacity for natural self-pollination was partially lost during the repeated backcrosses to the almond which resulted in 'Le Grand'. Retention of pollen grains following natural self-pollination was conspicuously greater in 'Le Grand' than in 'Texas', which has no peach parentage. My data indicate that the possibility of visually selecting the germplasm with increased capacities for natural self-pollination is a realistic expectation. The almond 'Truoito' has exhibited sufficient self-compatibility and natural self-pollination to produce a full crop with its own pollen in the absence of insect pollinators (Vasilakakis and Porlingis, 1984). However, no convincing documentation currently exists for other self-fruitful almond cultivars. More stringent testing of advanced selections is warranted prior to the future release of self-fruitful almonds. Cultivars which are biologically self-fertile may nevertheless be unsuited to commercial culture. The extent to which the potential for natural self-pollination is determined genetically and the significance of environmental influences has yet to be determined. The higher relative fruit-set in both cultivars in 1984 as compared to 1983 may represent the interaction between environment and genotype on pistil development. ACKNOWLEDGEMENTS
This research was supported by a grant from the United States--Israel (Bi-national) Agricultural Research and Development Fund (BARD). The author thanks Drs. D.E. Kester, P.E. Hansche, V.S. Polito, D.W. Ramming, W. Sherman, R. Socias i Company, P. Spiegel-Roy and M. Vasilakakis and Mr. L. Fenton for critical reviews of the manuscript, and Mr. T.T. Muraoka for invaluable technical assistance. REFERENCES
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