| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a HortResearch, Private Bag 3123, Hamilton, New Zealand
b Plant Breeding Genetics & Biochemistry Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines
* Corresponding author (esacks{at}hortresearch.co.nz)
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Interest in converting annual grain crops to perennials has a long and somewhat dubious history. As early as the 1920s and 1930s, work was initiated to develop perennial cultivars of wheat (Triticum aestivum L.) and rye (Secale cereale L.) for grain production, via introgression of perenniality genes from wild relatives (Reimann-Philipp, 1986; Tsitsin, 1965; Wagoner, 1990). More recently, the Land Institute of Salina, KS, has made progress in developing perennial tetraploid grain sorghum [Sorghum bicolor (L.) Moench, Piper and Kulakow, 1994]. Developing perennial forms of staple grains would have several benefits: lower production costs and simplified farm management by harvesting multiple times from a single planting; improved soil conservation because of less frequent plowing and more consistent ground-cover; and more efficient nutrient cycling and water use from increased soil organic matter (Wagoner, 1990).
Though the early perennialization efforts produced some useful new forage cultivars, breeding barriers resulting from the combination of different genomes found in the annual cultivars and the perennial wild relatives hampered development of perennial grain crops (Jones et al., 1999; Reimann-Philipp, 1986; Wagoner, 1990). Additionally, introgressing the many genes that contribute to perenniality traits (e.g., morphology such as rhizomes or stolons, tolerance to off-season stresses such as cold and drought, and tolerance to long-term pressures from pests and pathogens) is necessarily a long-term endeavor. A final hurdle for developing a perennial grain crop is that its usefulness and acceptance will depend on the economic balance of reduced production costs versus any reduction in yield potential that may be associated with perennial growth.
Of all the annual staple grains, rice may be the easiest to perennialize and to introduce successfully to farmers. Genotypes of cultivated Asian rice (O. sativa) are annual or weakly perennial. Under irrigated conditions, some cultivars of rice can produce a ratoon crop (Chauhan et al., 1985) but strongly perennial genotypes that can survive for multiple years are not currently available. Oryza sativa is derived from O. rufipogon (Khush, 1997), a wild species that includes annual (= O. nivara Sharma et Shastry), perennial, and annual-perennial intermediate forms (Morishima et al., 1984). Perennial forms may have stolons and are typically found in sites with permanently standing water (Morishima et al., 1984). Annual forms are typically found in seasonally dry sites and produce more seeds than perennial forms (Morishima et al., 1984). It is not known which form or forms were the starting points for the multiple domestications that led to modern cultivated rice. Though minor breeding barriers are sometimes observed, O. sativa and O. rufipogon have a common diploid genome (AA) and typically yield fertile interspecific progeny.
Recently, work to perennialize cultivated rice has been initiated in at least three laboratories (Mohamed El-Defrawy, Assiut University, Assiut, Egypt, 2000, personal communication; Li, 1998; Tao et al., 2003, in press) in addition to the International Rice Research Institute (IRRI) project. Work in China has focused on perennial growth as a means to fix heterosis. At IRRI, we are working to develop perennial rice primarily for the hilly upland production systems of Southeast Asia because current benchmark yields of annual upland rice are low (approximately 1–2 Mg/ha), and the natural resource costs of growing annual crops in these locations are high.
The distinguishing feature of upland rice is that it is grown without standing water (unpuddled) in aerobic soils. Annually, upland rice is grown on approximately 19 million ha throughout the world, with approximately 12 million ha in Asia (IRRI, 1997). Much of Indonesia, Malaysia, and the southern Philippines would be especially well suited for perennial upland rice because rainfall is plentiful nearly all year (Huke, 1982) and approximately 1.7 million ha of annual upland rice are currently grown (IRRI, 1997).
Though high-yielding (approximately 3–5 Mg/ha) upland rice is grown in Brazil and northern China with the aid of purchased inputs, most upland rice is grown as a subsistence crop with few or no purchased inputs. In Southeast Asia, upland rice has traditionally been grown in a slash and burn system, which is sustainable with 10- to 20-yr rotations (Brady, 1994; Kleinman et al., 1996). In recent years, population pressures have resulted in shorter rotations and more land being used for agriculture (Brady, 1994). Excessive burning to clear land has destroyed vast areas of rainforest, endangered species, and contributed to global climate change (Brady, 1994). Erosion of soil from newly cleared, plowed, or fallowed land reduces potential farm productivity, and fills downstream reservoirs with silt (Pimentel et al., 1987; Sarma et al., 1995). Reduced reservoir capacity limits the availability of water for productive irrigated rice during the dry season (approximately 5–8 Mg/ha) and increases the potential for destructive flooding during the wet season (Pimentel et al., 1987). Thus, agricultural practices in the uplands have broad regional consequences.
Given the special cultural significance of rice in Southeast Asia, we believe that if perennial upland rice is developed, it can be packaged as part of an integrated multiple-crop rotational system that would improve the sustainability of food production in the hilly uplands and downstream. The main objective of this study was to assess the potential of O. rufipogon as a donor of perenniality traits for upland rice, by evaluating interspecific progeny in nonpuddled field conditions. In addition, we investigated the potential for successfully selecting perennial lines, after rapidly advancing generations without direct selection pressure for perennial growth.
|
MATERIALS AND METHODS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
At IRRI, the wet season is from June through December and the dry season is from January through May. During the dry season, the field was separated into a wet side and a dry side by an 18-m -wide strip of nonirrigated land. The wet side was sprinkler irrigated for approximately 5 h once per week. The soil on the dry side was saturated by 7 to 16 h of sprinkler irrigation over 1 to 2 d starting on 29 Feb., 30 Mar., and 2 May 2000. Natural precipitation and evaporation during the experiment is presented in Fig. 1.
|
Seeds of the cultivar controls and F1 families were sown in flats on 5 July 1999 and grown in a glasshouse. Seedlings were transplanted to the field on 27 July 1999. Within each plot, plants were spaced 25 cm apart, for a total of 12 seedlings. All plots in the F1 trial were on the dry side of the field.
An 
lattice (0,1) design (Patterson and Williams, 1976) with three replications and eight blocks per replication was used. We compared the 47 entries (cultivars and F1 families) and the two entry types (cultivar or F1 progeny). The efficiency of parental selection for perennial growth was predicted by regressions of the F1 progenies' 1-yr survival data from this study on the O. rufipogon parents' 9 mo and 20 mo vigor data from an earlier study (IRRI, 1998).
Rapidly Advanced F4s and BC1F4s Trial
To determine if perennial lines could be efficiently selected from rapidly advanced progeny, the families studied in the F4 and BC1F4 trial were derived without prior selection for perennial growth. The F1, F2, and F3 generations had been selected for cultivated upland plant type when the plants were ready for their first harvest. We evaluated 46 rapidly advanced F3–derived F4 and BC1F4 lines. F4 and BC1F4 seed were sown directly in field plots on 16 July 1999. Within plots, three to five seeds were planted every 10 cm. Four weeks after planting, seedlings were thinned to one plant every 10 cm for a total of 30 plants per plot. An lattice (0,1) design with two locations (wet and dry sides), three replications per location, and nine blocks per replication was used.
Data Collection
For the F1 trial, individual plants in plots were measured. For the F4 and BC1F4 trial, data were taken on whole plots. IRRI's 1 to 9 scale for vegetative vigor (IRRI, 1996) was modified to include a value for dead plants: 1 = extra vigorous, 3 = vigorous, 5 = normal, 7 = weak, 9 = very weak, 10 = dead. Initial vigor was recorded on 8 Sept. 1999, before the first harvest. Approximately one year after planting, vigor and survival were recorded on 11 July 2000 for the F1 trial, and on 24 July 2000 for the F4 and BC1F4 trial. For the F4 and BC1F4 trial, the percentage of each plot that was covered by the plants was visually estimated on 24 July 2000. To avoid loss of seed from shattering and to comply with Philippine plant quarantine regulations, panicles of the interspecific progeny were enclosed in nylon net bags (approximately 15 x 30 cm). Panicles of the cultivar controls were also enclosed in net bags but not before bird damage was observed on these earlier flowering entries. Harvested seed was weighed. Plant height, basal diameter, and culm number were recorded between 5 and 15 Sept. 1999 for the F1 trial. For the F4 and BC1F4 trial, plant height was measured on 25 Aug. 1999; basal diameter and culm number were measured between 14 and 16 Sept. 1999. Panicle threshability was determined after harvest and drying by means of IRRI's standard 1 to 9 scale (IRRI, 1996): 1 = difficult (< 1% shattering), 9 = easy (51–100% shattering).
Statistical Methods
Analyses of variance were conducted with SAS procedure MIXED, using type III sums of squares (Littell et al., 1996). Entries, entry type (cultivar or progeny), and location (wet side or dry side) were considered fixed effects. Replications (nested in locations for the F4 and BC1F4 trial) and blocks nested in replications were considered random effects. Interactions between fixed effects and random effects were considered random. Degrees of freedom were calculated via the Satterthwaite option. Means were obtained with the lsmeans statement. Regressions, correlations, and tests of their significance were obtained with SAS procedure REG (SAS Institute Inc., 1990).
|
RESULTS AND DISCUSSION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
|
Though we lacked a complete factorial crossing design to estimate effectively combining abilities, it is worth noting that many of the best F1 families were derived from Azucena (Table 2). Azucena produces thick, deep roots that enable it to avoid drought stress (Courtois et al., 2000; Yadav et al., 1997). Genes from Azucena that confer drought avoidance may have helped Azucena's interspecific progeny to survive the dry season, though drought avoidance alone was insufficient to confer perenniality to this cultivar. Some F1 families that survived 1 yr had poor vigor during the dry season but recovered quickly at the beginning of the rainy season. In contrast, some survivors, including the most perennial F1 family (Azucena/106144-18), remained vigorous throughout the dry season, even when the surface of the soil was dry and cracked (Fig. 2B).
The percentage of surviving individuals in F1 families at 1 yr was moderately correlated (r = 0.55***) with the vigor of the O. rufipogon parent in an earlier study (IRRI, 1998) taken 20 mo after transplanting. In contrast, F1 survival at 1 yr was not significantly correlated (P = 0.39) with parental vigor at 9 mo. These observations were consistent with results from an earlier observational study of O. sativa/O. rufipogon F1s (Sacks et al., 2003, in press) and an earlier study of O. rufipogon accessions (IRRI, 1998). Taken together, these studies strongly suggest that phenotypic selection for perennial growth should be done no earlier than 1 yr after planting.
The 1999 wet season harvest started in October and ended in December. Some plants were harvested again during the middle of the dry season, in March 2000. Wet season yields of the cultivars ranged from 1.2 to 11.6 g/plant and were lower than expected because of bird damage. On the basis of our previous observations of these cultivars in the field and greenhouse, we would have expected wet season yields of the cultivars to range from approximately 10 to approximately 35 g plant-1. On average, wet season yield was higher for F1s (Table 2) than for cultivars, but this higher F1 yield was likely an artifact because only the cultivars were not bagged early enough. Though cultivar yields were lower than expected, the regression (b = 0.75** ± 0.24) of F1 family means on parental cultivar means indicated that the highest yielding progenies were generally derived from the highest yielding cultivars. Although negative associations between seed production and perennial growth are typically observed in wild populations of plants (Gadgil and Solbrig, 1972; Morishima et al., 1984), no significant associations between wet season yield and 1-yr survival or vigor were observed for the interspecific progeny in this study. In fact, some of the most perennial F1 families were among the best for yield (Table 2). The highest yielding perennial plant was from the highly perennial family, Azucena/106114-11 (Table 2); the individual produced 52.7 g during the wet season and 12.6 g during the dry season, and had a 1-yr vigor score of 3. Some of the F1s produced an additional crop at the end of the second wet season in 2000 (Fig. 2), but these yields were not quantified. Though large environmental errors and differences in plant size limit inferences from single plant yields, observations of the F1s suggest that some perennial genotypes may be able to reach or exceed benchmark yields of 1 to 2 Mg/ha for upland rice in Southeast Asia.
Dry season yields were significantly higher for F1s than for cultivars (Table 2). Since panicles of all plants were equally bagged and protected from birds in the dry season, the comparison between cultivars and progenies was unbiased. As expected, cultivars produced few or no seed during the dry season because they were either dead or dying (Table 2). To our knowledge, ratooning or dry season cultivation is not currently practiced with upland rice. These data demonstrate a newly discovered opportunity to develop upland cultivars that can produce a dry season ratoon crop even under water stress. If there were little or no first-season yield penalty for perennial growth, a ratoon crop could help increase food security for subsistence farmers, especially in favorably wet locations or years.
Plant height, basal diameter, and culm number were typically greater for F1s than cultivars (Table 2). Height for most F1s was at the upper end of the desired range of approximately 80 to 100 cm. Plants that are too tall are at risk for lodging. Plants that are too short are poor competitors with weeds, which upland rice farmers usually list as their primary biotic constraint. For height, the regression (b = 0.37* ± 0.16) of F1 family means on parental cultivar means suggests that short O. sativa parents could be crossed with tall F1s to obtain acceptable BC1s.
As expected, japonica cultivars produced fewer culms than indicas (Tables 1 and 2). The F1s produced about twice as many culms as the parental cultivars (Table 2). A non-significant regression for culm number of progeny on cultivar parents suggests that the F1s' genes for high culm number originated primarily from the O. rufipogon parent. The linear correlation between 1 yr survival and culm number was weak (r = 0.46**). We would expect perennial plants to put more resources into vegetative growth than would annual plants. However, the high tillering ability observed for some cultivars (e.g., IR64 and UPL-Ri-5) and progenies was not sufficient to confer perenniality (Table 2). Further investigations are needed to clarify if the number of culms per plant before the first harvest plays an essential role in perenniality.
Awning and shattering are undesirable traits that were generally greater in F1 progenies than cultivars (Table 2). All of the O. rufipogon parents, which we maintained in pots in a screenhouse, produced awns and shattered easily. Observations of the F1s were mostly consistent with the dominant inheritance that is typically observed for awning and shattering (Jennings et al., 1979). However, some F1 individuals and families had little awning and shattering (Table 2). Limited awning and shattering in some F1s may be the result of natural genetic variation in O. rufipogon for recessive alleles, or natural introgression of O. sativa genes in some of the O. rufipogon accessions. Gene exchange between cultivated O. sativa and wild populations of O. rufipogon has been documented previously (Majumder et al., 1997; Oka and Chang, 1961). In this study, there were correlations between 1-yr survival and awn length (r = 0.62***), and 1-yr survival and shattering (r = 0.44**). A causal relationship between perennial growth and awning or shattering is improbable. More likely explanations are linkage, and/or introgression of O. sativa genes in some of the O. rufipogon accessions. Backcrossing perennial progenies to O. sativa or selecting against awning and shattering in early selfing generations should allow recovery of the cultivated type, but concurrent selection for perennial growth would be advantageous.
Rapidly Advanced F4s and BC1F4s Trial
Highly significant differences (P < 0.001) were observed among entries for all traits measured. Location and entry x location effects were nonsignificant for all traits. Phenotypic appearance of the progenies was generally similar to that of the cultivars. No stoloniferous progenies were observed.
After 1 yr in the field, none of approximately 3060 control cultivar plants had survived (Table 3). In contrast, 10.9% of approximately 8280 progeny plants survived 1 yr. Thus, overall survival for F4 and BC1F4 progenies was lower than for F1 progenies. On the wet side of the field, 13.8% of the progeny survived 1 yr, but on the dry side only 8.1% survived. Though the differences in location were marginally nonsignificant (P = 0.06), low potential for perennial growth, as seen on the wet side, may have obscured the real effect of differences in drought tolerance on survival. Survival among progeny families ranged from 1.8 to 32.6% (Table 3). Thus, the best surviving inbred progeny families were less perennial than the best F1 families (Tables 2 and 3). Some of the better-surviving inbred progenies were BC1F4s, suggesting that selection for perenniality in the BC1 generation could be effective.
|
Because real plant breeding populations are limited in size by availability of resources, attempts to introgress many genes simultaneously in an initial selfing series is unlikely to produce a line that has all (or even most) of the desired wild-parent genes and meet a minimum threshold for yield and agronomic traits. Thus, further intermating and backcrossing with the best initial lines would be required to obtain an acceptable cultivar. Given that we observed large and significant differences among noninbred families (Table 2), it may be possible to speed up the breeding process by crossing the best noninbred individuals from the best families.
Consistent with the F1 trial, some of the most strongly perennial F4 and BC1F4 progenies yielded best (Table 3). Xiao et al. (1998) reported that two QTLs from a single O. rufipogon accession were associated with yield increases of 17 and 18% when introgressed into an O. sativa background. The potential of O. rufipogon as a source of yield-improving genes has barely been explored, but it appears promising.
Many inbred progenies also produced a dry season ratoon crop, whereas cultivars produced little or no seed during the dry season (Table 3). Some of the highest yielding entries in the wet season produced a dry season crop equivalent to approximately 17% of their wet season yield (Table 3). Considering the lack of selection for dry season yield in developing this material, these data support the conclusion from the F1 trial that it will be possible to develop cultivars that can produce a ratoon crop during the dry season.
Height, basal diameter, and culm number were taken at an earlier developmental stage in the F4 and BC1F4 trial than in the F1 trial, so direct comparisons between trials for these traits are not possible. In the BC1F4 trial, ranges of height, basal diameter, and culm number were similar for parents and progenies (Table 3). Thus, selection for cultivated plant type was effective.
Reduced awning in the F4 and BC1F4 entries relative to the F1 entries indicates that selection for this trait was also effective. However, many inbred progenies were highly shattering relative to the controls (Table 3), indicating that selection against this trait may be more difficult than for reduced awning and plant morphology characters.
|
CONCLUSIONS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Received for publication August 23, 2001.
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  V. D. Picasso, E. C. Brummer, M. Liebman, P. M. Dixon, and B. J. Wilsey Crop Species Diversity Affects Productivity and Weed Suppression in Perennial Polycultures under Two Management Strategies Crop Sci., January 16, 2008; 48(1): 331 - 342. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. J. Sacks, J. P. Roxas, and M. T. Sta. Cruz Developing Perennial Upland Rice II: Field Performance of S1 Families from an Intermated Oryza sativa/O. longistaminata Population Crop Sci., January 1, 2003; 43(1): 129 - 134. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Vadose Zone Journal | |||
| Journal of Plant Registrations | Soil Science Society of America Journal | ||||
| Journal of Natural Resources and Life Sciences Education |
Journal of Environmental Quality |
||||
