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a Hibridos Pioneer de Mexico, S.A. de C.V., Camino Viejo A. Valle de Banderas, km. 3, #19 Tapachula Nay. Mexico
b Hibridos Pioneer De Mexico, S.A. de C.V., Carr. Guadalajara, - Morelia km. 21 #8601, Nicolas R. Casillas - Mpio. Tlajomulco Jalisco, CP 45635 Mexico
c Pioneer Hi-Bred International, Inc., P.O. Box 552, Johnston, IA 50131-0552
d Pioneer Hi-Bred International, Inc., P.O. Box 1004, Johnston, IA 50131-1004
* Corresponding author (schoperj{at}phibred.com)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Those who conserve maize genetic resources are concerned that the introduction of improved varieties into these regions will result in the loss of genetic diversity. Iltis (1974) proposed the creation of in situ conservation of germplasm to complement ex situ conservation strategies. In situ conservation refers to the preservation of an entire agrosystem within a biological reserve. Controversy generated by such proposals led to research on the level of germplasm knowledge and its selection by farmers in the states of Chiapas and Jalisco (Bellon and Brush, 1994; Louette and Smale, 1996; Louette et al., 1997). This research indicated that present diversity in maize is the result of relatively controlled introductions of genetic material and not of geographical isolation. Farmers maintain cultivars through seed selection. Spatial and temporal separation are also used to maintain desired cultivars. When thought of in this manner, local cultivars constitute genetic systems that are open and evolving. The openness is a reflection of the curiosity and willingness of traditional farmers towards using new cultivars to solve their requirements for sustenance. Over the millennia, this approach has likely been a significant causal factor in the creation of modern maize (Louette et al., 1997).
More recently, concern has developed about the potential flow of transgenes from commercial cultivars into landraces and wild relatives of maize (Serratos et al., 1997). The two principal fears surrounding the introgression of transgenes into wild relatives and land races are possible genetic erosion and increased weediness of maize or teosinte plants containing a transgene (Rogers and Parkes, 1995). Two important vectors of introgression are seed and pollen. Farmer exchanges of commercial seed obtained from previous plantings can result in the transfer of transgenes into another area (Serratos et al., 1997). This process has been slowed by a moratorium on movement of maize seed containing transgenes into Mexico declared by regulatory officials.
Pollen management has begun to be investigated as a method for researchers to limit transgene flow in Mexico while allowing field research to continue. To date, research has focused on documenting the efficacy of detasseling as a method of controlling pollen shed as well as documenting cross pollination resulting from small scale plantings typical of research-scale plots (Garcia et al., 1998). Results indicated that routine breeding activities could be conducted with transgenic maize without danger of pollen dissemination and gene escape if transgenic plants were used as females and detasseled prior to anthesis. As might be expected, results also indicated that it is more difficult to control pollen dissemination if transgenic plants are used as pollinators. Precautions in addition to those described by Garcia et al. (1998) would be necessary to provide complete pollen control if 0.1% outcrossing to adjacent rows is deemed unacceptable for transgenic cross pollination.
Research on the use of spatial isolation to control maize hybridization can be difficult to evaluate. This difficulty is due to the variety of experimental conditions employed by researchers and, in some situations, the lack of essential information they provided in their reports. For example, Bateman (1947a)(b) performed several experiments but, because the small source of pollen was surrounded by a 3.3-m-high wall, his results were biased by reduced air circulation. Jones and Brooks (1950), in contrast, planted a very large source of pollen and likely had much greater wind speeds in their prairie locations in Oklahoma. As a result, the isolation distance required for purity was larger. In a thorough analysis of pollen dispersion and deposition, Raynor et al. (1972) measured maize pollen flow in terms of meteorological theory. They showed that maize pollen was not transported as far by the wind as pollen from several other wind pollinated species. Additionally, the authors documented dispersal both horizontally and vertically and noted that much of the pollen settled to earth within the source itself. Pollen concentrations 60 m from the source in the downwind direction averaged 1% of those at 1 m. Variables identified as important in deciding effective isolation distance were the size of the source planting and windspeed. Collectively results such as these, combined with experience have resulted in the seed industry practice of using 185 m to isolate adjacent plantings of maize.
Various models of pollen dispersal by wind pollinated conifers have been investigated and reviewed (Di-Giovanni and Kevan, 1991). Important physical factors found to influence dispersal were gravity, wind speed and direction, turbulence, air density, and air viscosity. Biological parameters that influence the effect of these factors include pollen density, pollen radius, and sedimentation velocity. Subsequent research (Di-Giovanni et al., 1995) that included measurements on maize pollen along with pollen from various wind pollinated trees, found maize pollen settled at approximately an order of magnitude faster rate than pollen from all of the other plant sources. Maize pollen was also found to settle at an approximately 30% faster rate than predicted by Stoke's Law. However, the combined understanding of the physical and biological aspects of maize pollen movement such that one can predict how it will move and how long it will retain viability is incomplete.
Knowledge of maize pollen biology is helpful in understanding various methods of managing pollen flow. Maize pollen is large, 90 to 100 µm in diameter, and spherical in shape (Wodehouse, 1935; Jones and Newell, 1948). It is among the largest particles that are commonly airborne (Raynor et al., 1972). At anthesis, water comprises about 60% of the fresh weight of maize pollen (Kerhoas et al., 1987). This is one of the highest water contents among angiosperm pollens (Hoekstra, 1986). Despite having a relatively low water potential at dehiscence (Schoper et al., 1987a; Westgate and Boyer, 1986a), maize pollen is generally considered desiccation intolerant relative to pollen of other species since it loses water rapidly and viability decreases sharply if grains are dried to water contents less than 0.4 g g-1 (Buitink et al., 1996). After anthesis, pollen dehydrates as it moves through the atmosphere until it lands on a receptive stigma. Because the atmosphere is quite dry, the pollen grains must land on a receptive silk shortly after being shed. Upon landing on a receptive stigma, the pollen absorbs water from the stigma and proceeds to germinate (Heslop-Harrison, 1979). Other factors such as high temperature can also negatively affect pollen viability (Roy et al., 1995; Schoper et al., 1987a,b). In commercial maize production fields, these risks are usually overcome by the copious quantities of pollen that are shed (Kiesselbach, 1999).
The objectives of our studies were to develop an improved understanding of the factors that influence pollen longevity in the field and to determine the required isolation distances for complete pollen control in a region of Mexico that is particularly suitable for maize research activities.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Procedures
Cultural Procedures
Experiments were conducted during 1997 to 1999 and all plants were grown using standard, optimal cultural practices for maize production. Fertilizer was applied at a rate of 220, 92, and 30 kg ha-1 of N, P, and K, respectively. To control weeds, the herbicides metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide, and atrazine, 6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine, were applied at rates of 1.4 and 1.3 kg a.i. ha-1, respectively. All plots were furrow irrigated 6 to 8 times following normal local practices. To control insects, the insecticides carbofuran, 2,3-dihydro-2,2-dimethyl-7-benzofuranylmethylcarbamate, and permethrin, (3-phenoxyphenyl)methyl (±)-cis,trans - 3 -(2,2-dichloroethenyl)-2,2-dimethylcyclopropane-carboxylate, were applied at a rate of 0.5 and 0.4 kg ha-1 at planting. Additional applications of foliar insecticides and fungicide were made as needed to maintain healthy plants and produce healthy seeds. All plots were grown at a density of 50 000 plants ha-1 and the spacing between rows was 0.8 m.
Pollen Longevity Experiments
The experimental design for measuring pollen viability as measured by seed set was a randomized complete block with four replications. The replications were blocked in time with one complete replication of treatments being completed per day. Blocking in time provided insurance against inclement weather. No precipitation occurred on any of the days when treatments were applied so all replications of data were used in the analysis of variance. Seven pollen treatments were common to both years and one additional treatment was added to the experiment that was planted in 1998. Four samples or plants were pollinated per treatment per day. The planting dates were 27 Nov. 1997 and 30 Dec. 1998 and the pollination dates were 27, 28, 29, and 30 Jan. 1998 and 10, 11, 12, and 13 March 1999. The single cross hybrid P3394 was used and plot row length was 5 m.
The seven pollen treatments that were used to generate seed set data in both years were: control (immediate crossing) and crosses made after 0.25, 0.5, 1, 2, 4, and 6 h of exposure to ambient atmospheric conditions. The additional treatment added to the experiment planted in 1998 was a control designed to test the level of contamination by inadvertent pollinations when the shoot bag was removed to allow pollination. In this treatment the bag was removed from the earshoot in the same manner as the other treatments but no pollination was made and a new bag was placed over the earshoot. Contamination levels were 2 to 3 kernels per ear in the experiment planted in 1997 and 0 in the experiment planted in 1998.
Additional visual scores of pollen condition for all treatments were also made immediately prior to pollination only in the 1998 experiment. These data were analyzed in the same manner as the seed set data except there was no year replication. The visual scores included the percentage of pollen grains that were spherical in shape and white in color when placed on a black background (the appearance of freshly dehisced pollen), the percentage that developed an intense yellow color, and the percentage of collapsed pollen. The scoring of pollen appearance was done at x25 magnification with a hand lens in the field.
At anthesis, approximately 10 mL of pollen was collected at the onset of dehiscence, which was between 1000 and 1100 h. All pollen was sieved to remove anthers and insects. The pollen was evenly distributed as a thin layer on a white plastic tray and allowed to dry in the open air and sunshine for the prescribed time for each treatment. Pollinations were made with a uniform volume of pollen (
2 µL) onto ears with similar silk lengths (8–9 cm). This volume of pollen is limiting to seed set on fully developed ears enabling us to measure fractional reductions in viability (Schoper et al., 1987a). Immediately before pollination the silks were trimmed to 2.5 cm of exposed silk tissue to facilitate more uniform pollen distribution over the silks.
Distance Isolation Experiments
The experimental design for all experiments involved planting a square, 4000-m2 block of an inbred pollen source that carried a genetic marker and surrounding it with 4 to 5 locations in each cardinal direction with the white seeded hybrid P3428 to assess the frequency of cross pollination. Each plot of the white seeded hybrid consisted of 4 rows, 4 m long. The distance of these plantings from the pollen source was 100, 200, 300, and 400 m in experiments with the exception of the addition of a 150-m plot in one of the experiments when it was repeated for the second year. This experiment therefore contained 20 locations of P3428.
Leaf Marker Experiments
Plants of a yellow seeded inbred line, AS44, carrying two dominant genetic markers (B, Pl) for intense purple leaf color were used as apollen source in an experiment planted on 27 Nov. 1997 and 30 Dec. 1998. A progeny test was performed to assess cross pollination with AS44 because other yellow seeded material was grown in the area and the purple leaf trait would be expressed in the F1 generation. The progeny tests for purple leaf phenotype involved planting all yellow seeds collected from the ears of P3428, seeds from AS44, and a sample of the white colored seed from the P3428 ears. The seeds of AS44 and P3428 ears were included as controls to ensure the correct identification of phenotype. The progeny tests were planted in May of each year. The number of purple leafed plants were then counted.
Seed Marker Experiment
Seeds of a second, non-Pioneer coded inbred line were planted in the same design on 30 Dec. 1998. This inbred line was homozygous for the genetic markers R-r, C1, C2, A1, A2, Bz1, Bz2, and Pr. Collectively, the phenotype for this genetic combination was observable directly on the P3428 ears as purple colored F1 seed.
Plots were monitored for synchrony to ensure that the pollen shedding of the inbreds containing the genetic markers was coincident with silk emergence of the white seeded hybrid. The 50% pollen shed and 50% silk emergence dates for the experiments involving AS44 were on the same day in the 1997 planting and 50% pollen shed of AS44 preceded 50% silk emergence of P3428 by 1 d in the 1998 experiment. In the experiment involving the purple seed genetic marker, 50% pollination preceded 50% silk emergence of P3428 by 2 d.
Atmospheric Water Status
Atmospheric water status was assessed by calculating atmospheric water potential (
atm) from temperature and relative humidity (Nobel, 1974). Temperature and relative humidity were measured by a hygrothermograph (Oakton, series 025028, Japan) located in the field where the experiment was conducted. Atmospheric water potential was calculated as follows:
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atm = atmospheric water potential (MPa), R = ideal gas constant = (0.0083 L MPa mol-1 deg-1), T = absolute temperature (K), V = molar volume of water = (0.018 L mol-1) and RH = percent relative humidity. |
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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atm in March 1999 relative to January 1998 by an average of 21 MPa from 1000 to 1700 h. These results contrasted with the typical 5 yr averages that show January as cooler and drier than March. The difference between the two months in atm on the basis of 5-yr average temperatures and relative humidities for 1000 to 1700 h was 53 MPa with January having a lower atm than March.
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atm. In the more humid year, pollen viability was reduced by 58% after 1 h of atmospheric exposure. No viable pollen was detected after 2 h of atmospheric exposure in either year. A slight increase in seed set occurred in both years 15 to 30 min after pollen collection.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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These results are consistent with the observations that maize pollen is desiccation intolerant and loses water and viability because of desiccation rapidly after dehiscence as is found in Gramineae generally (Buitink et al., 1996; Hoekstra, 1986; Kerhoas et al., 1987). The environmental conditions during our experiments were typical for the region and many areas where maize is adapted, i.e., mean daily high temperatures of 28 to 30°C and mean daily minimum relative humidities of 31 to 53%. Maize pollen has a uniquely low water potential (w) at dehiscence, about -2 to -3 MPa (Schoper et al., 1987a; Westgate and Boyer, 1986a) and seed has been produced when pollinations were made with pollen having a w as low as -12.5 MPa (Westgate and Boyer, 1986b). Our calculations of minimum daily atm during this experiment still exceed this very low pollen w by 7 to 9x. The 5-yr average minimum daily atm for January at -162 MPa is 13-fold lower than the lowest pollen w measured by Westgate and Boyer (1986b). Given the arid atmosphere, one would expect that pollen would be desiccating very rapidly during our experiments and this is supported by the collapse or loss of turgor of the pollen grains illustrated in Fig. 2.
Changes in the visual appearance of the pollen correlated well with losses in viability. Plump and spherical shaped pollen was viable. Maize pollen contains large amounts of starch and osmotically active solutes thus enabling turgor to be maintained at low ws, e.g., -3 MPa (Goss, 1968; Schoper et al., 1987a). Even so, pollen with some degree of collapse likely remains viable. Our estimate of percent collapse was conservative in that great deformation of the pollen wall was needed to affect the score. The correlation of reduced seed set with the development of intense yellow coloration was also high and provides another means of estimating viability. Collectively, our results document that assessing pollen appearance in the field with a relatively low magnification hand lens (x25) is a quick and useful method for monitoring pollen viability. Our results also indicate that pollen exhibits fractional loss of viability as it dries. These results suggest that the use of nonlimiting amounts of pollen by Westgate and Boyer (1986b) may have caused them not to detect a decline in viability as pollen approached very low w, e.g., –12.5 MPa.
The amount of outcrossing between the block of material carrying the genetic marker and the plots of the white seeded hybrid was very low. Only three cross pollinations were observed across all experiments, one in each of the 100-m South, 150-m South, and 200-m North plots. Pollen source strength should have been ample because the size of the pollinator blocks was large enough (4000 m2 or approximately 20 000 plants each) to generate considerable pollen, approximately 40 000 million pollen grains, if one assumes 20 million pollen grains are liberated per tassel as calculated by Kiesselbach (1999). Tassel size was large on this material so this may be a reasonable assumption. The theoretical number of receptive silks was also large for each plot of P3428. If the number of 1000 silks per developing ear is representative (Kiesselbach, 1999), there would have been 64 000 emerged silks available to intercept pollen per planting of P3428. Even though the numbers of pollen grains and silks were large it is important to note that these experiments were intended to investigate research scale plantings. Pollen source strength has been found to be an important variable in modeling pollen dispersal (Di-Giovanni and Kevan, 1991) and dispersal dynamics could be different on commercial scale plantings.
Our cross pollination data were consistent with the prevailing wind speeds and previous experiments related to isolation distance and settling rate of maize pollen. Our experimental area is relatively calm with an average maximum wind speed of <16 km h-1 during the afternoon and lower wind speeds during the morning hours when pollen is shed (Julio Nava, 1997, personal communication). Jones and Brooks (1950) measured outcrossing from maize at 503 m from the source pollen in Oklahoma, which is a relatively windy state. Raynor (1972) noted 60 m as the maximum distance for outcrossing in New York, which is less windy and cooler than Oklahoma. In Cuzalapa, Jalisco, Mexico and Iguala, Guerero, Mexico, Cervantes (1998) determined the maximum distance for outcrossing in maize to be 32 m. Both these Mexican locations typically have low wind speeds during the time of year when the experiments were conducted. In the experiments of Raynor (1972) and Cervantes (1998), 63 and 54% of the pollen shed was deposited within the source planting itself. The relatively high settling velocity of maize pollen (31 cm s-1, measured by Di-Gionvanni et al., 1995) combined with the range of wind speed experienced in these investigations confirms a relatively short dispersal distance for maize pollen. These results also illustrate the very low probability that viable pollen shed into the air will intercept a receptive silk in a non-target field at some distance.
Collectively, our results provided useful information for managing pollen flow via isolation distance. It was clear maize pollen does not remain viable in the atmosphere for longer than 2 h near San Jose del Valle, Nayarit, Mexico. Theoretically, viable pollen could move 32 km in this time if the wind speed during the morning pollen shed period was equivalent to the average maximum windspeed during the afternoon, pollen movement was linear, and the pollen did not settle. Actual measurements of pollen movement as measured by outcrossing from research scale plantings of material, indicated that viable pollen was barely detectable at 200 m and nonexistant at 300 m from the pollen source. The difference between the theoretical and actual measurements is likely a function of pollen settling and dessication combined with variations in the local physical factors controlling pollen movement, e.g., wind speed and air turbulence is typically lower during the morning than in the afternoon, etc.
Decisions regarding what might constitute adequate pollen control to protect landraces and teosinte in Mexico will depend on biological and nonbiological factors (Alvarez-Morales, 1999). Our research demonstrates and documents that distance isolation is an effective tool for managing pollen flow in research scale plantings. Its use in conjunction with other pollen management tools, e.g., detasseling (Garcia et al., 1998), temporal isolation, monitoring of known teosinte populations (Sanchez et al., 1998), and managing the scale of pollen release allows considerable management of gene flow. The extent to which precautions need to be applied to pollen flow depends ultimately on the implications of the flow of novel genes. If the consequences of novel gene flow are biologically significant, more precaution will need to be exercised than if experiments demonstrate no significant biological impact of the novel genes beyond that of traditional breeding activities.
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ACKNOWLEDGMENTS |
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Received for publication June 20, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Purple plants and purple seeds indicate cross pollinations in the leaf and seed marker experiments, respectively.