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Reproductive Biology |
2Department of Biology, 3Department of Horticulture, 4the Intercollege Graduate Degree Program in Ecology, The Pennsylvania State University, University Park, Pennsylvania 16802 USA
Received for publication December 5, 2000. Accepted for publication March 27, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Lycopersicon esculentum • male function • microgametophyte • mycorrhiza • pollen tube growth • sex allocation • soil fertility • Solanaceae • tomato
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Mengel and Kirkby, 1982
; Coltman et al., 1987
; Vose, 1987
). Under these conditions, infection of plant roots by vesicular-arbuscular mycorrhizal (VAM) fungi is especially beneficial. Mycorrhizal fungi form mutualistic relationships with 
85% of terrestrial angiosperm species (Law, 1988
). Mycorrhizal infection enhances P uptake from the soil, primarily by increasing the absorptive surface area in contact with the soil solution (Hayman, 1983
). Thus, mycorrhizal plants generally have a higher P status than nonmycorrhizal plants (Koide, 1998
). When soil P is limiting, mycorrhizal infection often increases vegetative growth in the host plant (Smith and Read, 1997
). Despite the large body of research on mycorrhizal effects on vegetative growth, relatively little attention has been given to its effects on plant reproduction, especially the male function. However, it is reasonable to expect that mycorrhizal infection and high soil P conditions will have beneficial effects on pollen performance because of increased resource availability. It is also reasonable to predict that mycorrhizal infection will improve other factors (e.g., flower production, pollen production, and pollinator visitation) that influence fitness through the male function under natural field conditions.
During pollen development, the tapetal layer of the anther wall endows pollen grains with storage products (or their precursors) that are metabolized upon germination and initial tube growth (e.g., Stanley and Linskens, 1974
; Baker and Baker, 1979
; Jackson, Jones, and Linskens, 1982
; Wetzel and Jensen, 1992
; Clément, Burrus, and Audran, 1996
). For example, stored phytate is hydrolyzed into phosphate and myoinositol, which are used by the pollen tube for cell wall and membrane synthesis (Jackson and Linskens, 1982
; Dickenson and Lin, 1986
). The quantity and quality of these storage products can affect pollen performance, as measured by percentage of germination, pollen tube growth rates, and the ability to sire seeds in competition with pollen from other plants (reviewed in Stephenson et al., 1994
; Delph, Jóhannsson, and Stephenson, 1997
). Thus, any environmental conditions that affect resource availability to the sporophyte, and therefore provisioning of storage products during pollen development, can potentially influence pollen performance.
Recent studies have shown that pollen performance is affected by environmental conditions during pollen development, such as leaf herbivory, temperature, and soil nutrient availability. Simulated leaf herbivory resulted in reduced in vitro pollen tube growth rates in Silene vulgaris (Delph, Jóhannsson, and Stephenson, 1997
), reduced in vivo pollen tube growth rates in Lobelia siphilitica (Mutikainen and Delph, 1996
), and reduced ability to achieve fertilization under competitive conditions in Cucurbita texana and S. vulgaris (Quesada, Bollman, and Stephenson, 1995
; Delph, Jóhannsson, and Stephenson, 1997
). In Trifolium repens, pollen developed under cool temperatures had higher percentage of germination and grew longer pollen tubes in vitro than pollen developed under warm temperatures (Jakobsen and Martens, 1994
). Similarly, pollen developed under cool temperatures from wild and cultivated Cucurbita pepo plants grew longer pollen tubes in vitro and sired more seeds in pollen mixtures than pollen developed under warm temperatures (Jóhannsson and Stephenson, 1998
). In Raphanus raphanistrum, pollen from plants grown under low nutrient conditions sired fewer seeds in competition than pollen from plants grown under better nutrient conditions (Young and Stanton, 1990
). Similarly, when soil nutrient levels were varied independently in C. pepo, pollen from plants grown under low nitrogen and low phosphorus conditions sired fewer seeds in competition than pollen from plants grown under high nitrogen and high phosphorus conditions (Lau and Stephenson, 1993, 1994
).
In a preliminary study, mycorrhizal infection increased in vitro pollen tube growth rates of C. pepo grown in the field (Stephenson et al., 1998
). If there is a correlation between in vitro and in vivo measures of pollen performance, then mycorrhizal infection should also improve in vivo competitive ability. In the study reported here, the effects of mycorrhizal infection and soil P availability on pollen performance (i.e., pollen germination, pollen tube growth, and the ability to achieve fertilization in pollen mixtures) were examined in a cultivated variety of tomato (Lycopersicon esculentum Mill.). In a related study, the effects of mycorrhizal infection on fitness through the male function (i.e., siring success in open-pollinated experimental arrays) were examined under field conditions. Only a few other studies have measured the effects of soil P availability on in vitro and in vivo pollen performance. Furthermore, this research is the first to consider mycorrhizal effects on pollen performance.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In the Ailsa Craig cultivar, the stigmas are exerted, allowing cross-pollination. Both cultivars were obtained from the Tomato Genetics Resources Center at the University of California, Davis, USA. The VFNT Cherry produces six to eight flowers per inflorescence and small fruit (mean fruit mass = 0.7 g); Ailsa Craig produces two to four flowers per inflorescence and large fruit (mean fruit mass = 5.7 g). Both cultivars are self-compatible and show indeterminate growth and reproduction.
In each cultivar, two phenotypes, wild type (wt; has a purple-green stem) and anthocyanin deficient (ae; has a bright green stem), were used. A different single-locus mutation in each cultivar resulted in the anthocyanin-deficient phenotype (Tomato Genetics Cooperative, 1996
). Selfing produced these true-breeding lines. Wild-type plants were homozygous dominant; anthocyanin-deficient plants were homozygous recessive. When anthocyanin-deficient mother plants were used in hand and open pollinations, paternity, and thus siring success of pollen developed under different treatments, was determined easily by scoring seedlings for stem color. Ailsa Craig mother plants were homozygous for an additional recessive mutation (bu; has a bushy morphology; Tomato Genetics Cooperative, 1996
) in order to differentiate "self" from outcross progeny. (Father plants were homozygous for the dominant tall allele.) For more details about the tomato cultivars, see Poulton (2000)
and Tomato Genetics Cooperative (1996)
.
Greenhouse study with the VFNT Cherry cultivar
Soil was collected from a low-P (14 µg/g Olsen extractable P) field at the Pennsylvania State University Agricultural Experiment Station at Rock Springs, Pennsylvania, USA. Indigenous mycorrhizal fungi were destroyed by autoclaving the air-dried soil at 105°C for 90 min. Then the soil was stored for 2 wk to avoid the potentially phytotoxic effects of autoclaving (Rovira and Bowen, 1966
). In order to improve drainage, the field soil was mixed in a 1:3 ratio with sterile medium-grade sand.
On 4 June 1997, VFNT Cherry tomato seeds were planted in trays containing 50% PGX growing mix (Premier, Riviere-du-Loup, Quebec, Canada), 40% perlite, and 10% whole-soil inoculum (75 spores of Glomus etunicatum Becker and Gerd. per milliliter of inoculum) for mycorrhizal seedlings or 10% autoclaved low-P field soil for nonmycorrhizal seedlings. On 23 June, two seedlings were transplanted into each 20 cm round x 16 cm deep "Azalea" pot (Kord, Bramalea, Ontario, Canada), containing the soil mixture. Mycorrhizal seedlings were inoculated again with 60 mL of whole-soil inoculum directly around their roots; nonmycorrhizal seedlings received 60 mL of autoclaved low-P field soil directly around their roots with 5 mL of spore washings to ensure comparable nonmycorrhizal microbial inputs (Koide and Li, 1989
). Plants were arranged in a randomized block design across three benches. (Two benches had three blocks each; one bench had two blocks.) Soil P treatment was established by watering plants once per week with 500 mL of one-third strength Hoagland's nutrient solution without P for "low-P" plants and with 1 mmol/L KH2PO4 (full strength) for "high-P" plants (Machlis and Torrey, 1956
). A preliminary P response study had determined the concentration of KH2PO4 required for nonmycorrhizal plants to have the vegetative growth of mycorrhizal plants. This resulted in three treatment combinations: nonmycorrhizal, low P (NMPO); nonmycorrhizal, high P (NMP3); and mycorrhizal, low P (MPO). A drip irrigation system initially supplied plants with additional water as needed. At 9 wk after transplanting, 400 mL of water was supplied to each plant daily (except on the days when Hoagland's solution was provided). Thus, there were 48 pots, with eight replicates per treatment–phenotype combination.
One randomly selected plant from each pot was removed at 3 wk after transplanting (leaving one plant per pot). At 5 wk after transplanting, a root sample was taken midway between the stem and pot edge in each pot with a no. 15 cork bore. The hole was filled with the autoclaved soil mixture. Roots were rinsed out of the soil samples and initially stored in formaldehyde-acetic acid-ethanol (FAA) solution. Then the roots were cleared and stained with trypan blue to determine level of mycorrhizal infection (NMPO, NMP3 
0%; MPO = 70%) using a grid intercept technique (Koide and Mooney, 1987
).
In vitro pollen performance
The first plants began to flower 4 wk after transplanting. To maintain high levels of pollen production, all flowers were removed twice per week (after pollen collection) to prevent fruit production. Starting at 7 wk after transplanting, pollen was collected on six dates (about every 2 wk) to assess in vitro germination and tube growth. Although flowers can last up to 3 d, pollen was only collected from newly opened flowers. In the field, tomato flowers release pollen when moved by the wind or vibrated by bees; in the greenhouse, manual vibration is required to achieve pollination (Cribb, Hand, and Edmondson, 1993
). In this study, a modified electric toothbrush vibrated the flowers at a high frequency, releasing pollen from the anther cones. On each plant, all newly opened flowers were vibrated, and pollen from that plant was collected in a single gelatin caplet. Then the pollen was germinated in a modified Brewbaker and Kwack (1963)
liquid medium (14% sucrose) using a hanging drop technique (Kearns and Inouye, 1993
). In multiwell culture dishes (24 wells per dish), one pollen load (mean = 1600 pollen grains per load) was transferred on the flat end of a wire into a 10-µL drop of the liquid medium in each well. The dishes were inverted in humid conditions for germination and tube growth. Pollen tube growth was stopped after 4 h by adding a 10-µL drop of ethanol-acetocarmine-glycerin to each well.
Percentage of germination was determined by counting the first 200 pollen grains encountered on a grid across the well under a light microscope. A pollen tube had to be longer than the diameter of the pollen grain for it to be considered germinated. Pollen tube length was determined by measuring the first 30 pollen tubes encountered on a grid across the well using a computerized image analysis system (Rich, Ranken, and George, 1989
).
For both measures of in vitro pollen performance, fixed-effects analyses of variance (General Linear Model; Minitab, 1997
) were performed with three factors and their interactions: date (three levels), treatment (three levels), and phenotype (two levels). Pollen was collected over 3 mo during the growing season (August, September, and October). Block had no effect and was dropped from the analyses. Mean pollen tube length was calculated for each well (i.e., each pollen sample). These means were used in the corresponding analysis of variance. Least square means were calculated because sample size varied across treatment–phenotype combinations.
In vivo siring success
Starting at 8 wk after transplanting, hand pollinations were performed on four dates (about every 3 wk) with pollen mixtures in order to determine the effects of mycorrhizal infection and soil P availability on in vivo competitive ability. Pollen was collected with a modified electric toothbrush as previously described, pooling pollen by treatment–phenotype combination across all plants (i.e., six bulk samples per day). The pollen samples were weighed, and equal masses of pollen were combined in a single gelatin caplet. Preliminary pollen counts performed on microscope slides showed a strong correlation (R2 = 0.97, n = 36) with mass. All possible 50 : 50 pollen mixtures of different treatments were created (i.e., NMPO vs. NMP3, NMPO vs. MPO, and NMP3 vs. MPO). For each pollen mixture, reciprocal crosses (e.g., NMPO wt vs. NMP3 ae and NMPO ae vs. NMP3 wt) were performed in order to eliminate any effects due to phenotype during the statistical analyses. Pollen mixtures within each gelatin caplet were thoroughly mixed using a test tube agitator. Preliminary hand pollinations with 50 : 50 pollen mixtures of the same treatment (e.g., NMPO wt vs. NMPO ae) were performed the previous year to test the effectiveness of the mixing technique (i.e., paternity ratios were not significantly different among fruit from the same pollen mixture). Forty nonmycorrhizal mother plants were watered with standard one-third strength Hoagland's solution (i.e., NMP1 ae) as previously described. All hand pollinations were performed in the morning. Mature flower buds on mother plants were emasculated in order to prevent selfing. Then the stigmas were saturated with pollen by dipping them into the gelatin caplets. Preliminary slides (created by dipping saturated test stigmas into a drop of acetocarmine) confirmed that more than enough pollen was present on each test stigma to ensure full fertilization of ovules and, therefore, pollen competition (mean = 2400 pollen grains per stigma). In addition, these slides showed that treatment and phenotype did not affect pollen grain size (Poulton, 2000
). On each mother plant, hand pollinations were performed with up to three different pollen mixtures on each date (maximum 12 total pollinated flowers per plant). At least six replicate hand pollinations were performed for each reciprocal cross on each date (minimum 36 total hand pollinations per date). Emasculated flowers were labeled by date and pollen mixture for future harvest.
Mature fruit from hand pollinations were harvested and their seeds recovered. Then the seeds were dried and stored temporarily until enough greenhouse space was available for screening. All seeds were planted in 10-cm2 pots (maximum 25 seeds per pot) containing PGX growing mix. Some fruit contained enough seeds that they required more than one pot. At 2 wk after emergence, seedlings were scored for stem color to determine paternity.
Log-linear analysis of seedling paternity data (log-linear analysis; StatSoft, 1997
) was performed because seedling counts represent discrete data. The interpretation of log-linear analysis is similar to analysis of variance. For each hand pollination, chi-square values indicate the significance of the main effects (treatment and phenotype) and the two-way interaction (treatment by phenotype).
Field study with the Ailsa Craig cultivar
In the summer of 1998, Ailsa Craig tomato plants were germinated and grown in a greenhouse under two treatment combinations, NMPO and MPO. Seeds were planted on 8 July, using the same procedures for germination, transplanting, and establishment of treatments as in 1997. On 7 September, after all plants began to flower, they were transported to the field. Their pots were sunk into the ground in two 4 x 4 m experimental arrays. Array A included 12 NMPO wt tall fathers, 12 MPO ae tall fathers, and 12 NMP1 ae bu mothers; Array B included 12 NMPO ae tall fathers, 12 MPO wt tall fathers, and 12 NMP1 ae bu mothers. In both arrays, each row and column contained 2 NMPO fathers, 2 MPO fathers, and 2 NMP1 mothers. Experimental arrays were separated by enough distance (750 m) to minimize pollen transport between them.
Open pollination in the field was allowed for 3 wk. During this time, the number of pollinator visits to each father plant was observed for 2 h on each of four dates. Several species of bee (including Bombus spp. and Apis mellifera) were the primary pollinators. Number of open flowers per plant was also recorded on these dates, and total flower production per plant was recorded for the 3 wk. At the end of the 3 wk, the mother plants were returned to the greenhouse to provide optimal conditions for fruit maturation. The father plants were harvested to determine final leaf biomass. As in 1997, seeds from the fruit were recovered and later planted in the greenhouse. At 2 wk after emergence, seedlings were scored by stem color to determine paternity. All anthocyanin-deficient seedlings were allowed to grow for four more weeks to differentiate "self" (includes other NMP1 mothers) from outcross progeny (NMPO or MPO fathers) based on the bushy mutation.
For total flower production per plant, a fixed-effects analysis of variance (General Linear Model; Minitab, 1997
) was performed with two factors: treatment (two levels) and phenotype (two levels). For pollinator visitation (number of pollinator visits per plant per hour), a fixed-effects analysis of variance was performed with three factors: date (four levels), treatment (two levels), and phenotype (two levels). Date represented the four different dates (9, 14, 18, and 23 September) on which pollinator visitation was observed. Log-linear analysis of seedling paternity data (log-linear analysis; StatSoft, 1997
) was performed as previously described. Chi-square values indicate the significance of the main effects (treatment and phenotype) and the two-way interaction (treatment by phenotype) when expected counts are based on the number of plants of each treatment and the number of flowers produced by each treatment.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In vivo siring success
Of the 3003 seeds planted to evaluate in vivo siring success, 2030 seedlings grew large enough to determine paternity based on stem color. There was no indication that phenotype affected seedling germination. Treatment significantly affected seedling paternity in two of the three pollen mixtures (Table 2). Pollen from NMP3 plants sired 7.6% more seeds than pollen from NMPO plants (Fig. 2). Pollen from MPO plants sired 7% more seeds than pollen from NMPO plants. However, there was no significant difference in siring success in pollen from NMP3 and MPO plants. Phenotype significantly affected seedling paternity in all pollen mixtures. Pollen from wt plants sired more seeds than pollen from ae plants. This phenotype bias was expected based on preliminary hand pollinations with 50 : 50 pollen mixtures of the same treatment (e.g., NMPO wt vs. NMPO ae) performed the previous year (Poulton, 2000
). Despite the phenotype bias, the reciprocal crosses showed the same trends in siring success. In all pollen mixtures, the treatment by phenotype interaction was significant. The wt phenotype amplified the existing differences in siring success among the treatments.
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). Despite the phenotype bias, the arrays showed the same trends in siring success (although only significant in Array B). There was also a significant treatment by phenotype interaction. The wt phenotype seemed to amplify the existing differences in siring success between the treatments. However, the treatment by phenotype interaction again cannot be separated from location because each treatment–phenotype combination appeared in only one array.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Lau and Stephenson, 1993, 1994
; Jakobsen and Martens, 1994
; Quesada, Bollman, and Stephenson, 1995
; Mutikainen and Delph, 1996
; Delph, Jóhannsson, and Stephenson, 1997
; Jóhannsson and Stephenson, 1998
; Stephenson et al., 1998
). For example, high soil P conditions in the field improved siring success in pollen mixture studies with C. pepo (Lau and Stephenson, 1994
). In another study with C. pepo grown in the field, pollen from mycorrhizal plants showed faster in vitro pollen tube growth rates than pollen from nonmycorrhizal plants (Stephenson et al., 1998
). Thus, it is not surprising that mycorrhizal infection and high soil P conditions have beneficial effects on pollen performance in tomato. This study also revealed that, under field conditions in low P soils, mycorrhizal plants produced significantly more flowers (and thus more pollen) and sired significantly more seeds than nonmycorrhizal plants. Consequently, mycorrhizal infection translates into increased reproductive output through the male function (male fitness) under field conditions. This large increase in the number of seeds sired appears to be due primarily to the increase in flower production (pollen production), but there was also an increase in the number of seeds sired (although not significant) that was independent of flower number (i.e., more seeds were sired due to the greater in vivo performance of pollen from mycorrhizal plants).
In this study, there is a strong correlation between in vitro and in vivo pollen performance for each treatment in the VFNT Cherry cultivar. Pollen from NMP3 and MPO plants grew longer pollen tubes in vitro and sired more seeds in vivo than NMPO plants. (Differences among the treatments in siring ability were not due to differences in percentage germination, at least under in vitro conditions.) Similarly, other studies have shown a strong correlation between in vitro and in vivo measures of pollen performance (e.g., Delph, Jóhannsson, and Stephenson, 1997
; Jóhannsson and Stephenson, 1998
). Moreover, pollen tube length in vitro did not differ significantly between NMP3 and MPO plants, nor did they differ in their in vivo ability to sire seeds when their pollen was deposited simultaneously onto stigmas. Thus, mycorrhizal effects on pollen performance are presumably related to improved P acquisition.
In contrast to treatment, phenotype had different effects on in vitro and in vivo pollen performance. Pollen from ae plants had significantly higher percentage germination and slightly longer pollen tubes than pollen from wt plants under in vitro conditions. However, in controlled pollen mixture pollinations on ae mother plants, pollen from wt plants always sired more seeds than pollen from ae plants. Several possible explanations exist for these different phenotype effects on in vitro and in vivo pollen performance. Because pollen germination and tube growth occurred over a much longer time period in the hand pollinations than in the in vitro studies (stopped after 4 h), duration of pollen tube growth may have played a major role in the different phenotype effects. In addition, there are significant chemical differences between the in vitro and in vivo growing environments. No liquid medium exists that supports pollen germination and tube growth as long or as well as stylar tissue. It is possible that the mutation that causes the sporophyte to be ae also reduces flavonoid production in pollen grains. Flavonoids, which are used to produce the pigment anthocyanin, are also essential for pollen tube growth (reviewed in Taylor and Hepler, 1997
). The effect of reduced flavonoid production in pollen grains, as seen in tomato, forsythia, Brassica oleracea, wheat, and several species of Nicotiana (Sedgley, 1975
; Ylstra, 1995
; and references therein), may only be evident in ae stylar tissue. Similarly, other differences between the phenotypes may exist due to the mutation itself or other closely linked traits. The ae mutation was introduced into the wt line by chemical induction, not transformation (Tomato Genetics Cooperative, 1996
). Thus, it is possible that the two inbred lines differ by more than just the mutation.
In pollen competition studies with the VFNT Cherry cultivar, pollen from NMP3 and MPO plants sired more seeds than pollen from NMPO plants presumably because their pollen tubes grew faster, as indicated by the in vitro studies. However, in natural populations, mycorrhizal infection may have a much greater effect on fitness through the male function than in pollen competition studies. In experimental arrays of the Ailsa Craig cultivar, the difference in siring success between pollen from NMPO and MPO plants was even greater than in the VFNT Cherry cultivar. Other studies have found that different tomato cultivars show varying levels of response to mycorrhizal infection (see Bryla and Koide, 1990a, b
). However, other aspects of the design of the experimental array probably affected siring success as well. Although each array contained the same number of NMPO and MPO plants, MPO plants produced more flowers in the field than NMPO plants. Thus, more pollen was available for export from MPO plants than NMPO plants. Other studies have found that the number of seeds sired in a population is positively correlated with flower and pollen production (e.g., Schoen and Steward, 1986
; Devlin, Clegg, and Ellstrand, 1992
). In addition, MPO plants received more pollinator visits per hour than NMPO plants. As seen in many species, pollinator visitation often increases with plant size and floral display (e.g., Schaffer and Schaffer, 1979
; Stephenson, 1979
; Willson, Miller, and Rathcke, 1979
; Schemske, 1980a, b
; Davis, 1981
; Paton and Ford, 1983
). Thus, in natural populations, mycorrhizal infection may affect siring success through both the quantity and the quality of pollen grains dispersed.
Two recent studies have also shown that mycorrhizal infection increases pollen production. In a preliminary greenhouse study with C. pepo, mycorrhizal plants produced marginally more pollen grains per staminate flower and significantly larger pollen grains than nonmycorrhizal plants, with these differences increasing over an 8-wk period (Lau et al., 1995
). Similarly, in Cucurbita foetidissima, mycorrhizal infection increased staminate flower production, and thus total pollen production, under field conditions (Pendleton, 2000
). Thus, mycorrhizal infection can affect total pollen production at two levels, flower production and pollen production per flower.
Mycorrhizal fungi are extremely common, occurring in a wide variety of biomes (Read, 1991
). In many environments, mycorrhizal infection improves plant growth and P status when soil P is limiting (Smith and Read, 1997
). Because levels of mycorrhizal infection and soil P can show spatial and temporal heterogeneity (e.g., Hammer, O'Brien, and Lewis, 1987
; Koide and Mooney, 1987
; Moore, 1992
), plants growing very close together can experience different environmental conditions (Stephenson et al., 1998
). The quantity and quality of pollen produced by these plants depend to a large extent on resource availability during pollen development. Thus, mycorrhizal infection and high soil P conditions can affect fitness through the male function by increasing the number of pollen grains available for pollination and improving their performance in competition with conspecifics. This study is the first to demonstrate the beneficial effects of mycorrhizal infection on both in vitro pollen tube growth and in vivo siring success. Furthermore, mycorrhizal infection improved in vivo siring success both under conditions of pollen competition and in experimental arrays growing under field conditions.
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FOOTNOTES |
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5 Author for reprint requests (FAX: 814-865-9131; e-mail: as4{at}psu.edu
).
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