The comparison of transgenic and non-transgenic sugarbeet cultivars
Stephen Kaffka, Gary Peterson, Dong Daxue
Introduction
The transgenic modification of sugarbeets is an active field of research and commercial development. Most of this work occurs in Europe and the starting point for transformation and subsequent cultivar development usually is elite sugarbeet germ plasm which grows best in Europe (Kaffka and Lemaux, 1996). Currently, two companies sell seed in California, Spreckels/Holly Hybrids and Betaseed. Of these, Betaseed has developed transformed herbicide tolerant sugarbeet lines related genetically to cultivars currently approved for sale in California. Tolerance is to glufosinate ammonium (G-A), a broad spectrum contact herbicide marketed as Liberty®. G-A is widely used by Betaseed as a marker gene in its transformation programs. Liberty® is not yet registered in California but efforts towards that end are underway. A broad spectrum, post-emergence herbicide would be a valuable tool in sugarbeet weed management, made more so by the possible loss of key sugarbeet herbicides due to regulatoy restrictions resulting from the 1997 Food Quality Protection Act.
There have been few direct comparisons between transgenic and commercial cultivars in California to date. There is no molecular breeding work being carried out on sugarbeet in California, and because of this, California, with its often unique requirements for sugarbeet cultivars, likely will remain the last sugarbeet growing region to benefit from recent developments in molecular biology. When advanced breeding lines become available, their performance under California conditions should be evaluated promptly.
Comparisons with commercially available cultivars are needed for several reasons. For example, Märlander (1996) has hypothesized that transgenic varieties should be lower yielding than the best commercial lines. He argues that the process of molecular transformation, followed by development of a commercial variety from transformed germplasm takes so long for sugarbeets that the resulting transformed variety will not be as high yielding as the best cultivars derived from classical breeding programs by the time they are released. The transgenics lower yield might offset any economic advantage resulting from the use of a transgenic trait. If the broad spectrum herbicide is expensive, further economic disadvantage occurs.
Other concerns include whether the process of transgenic modification
somehow interferes with plant metabolism and development. Lastly, modest
herbicide damage may still occur if tolerance is not absolute.
To evaluate the performance of a herbicide tolerant line at the request
of Betaseed, Inc., the yield of transgenic sugarbeets were compared to
two commercially available Betaseed cultivars in a trial carried out at
the agronomy farm at UC Davis in 1998. The objectives of this trial were:
2. to compare the growth of the herbicide tolerant lines with and without
G-A.
Betaseed cultivars 4035R, 4776R, and a closely related transgenic cultivar modified to be tolerant to the herbicide glufosinate ammonium were grown under field conditions on the UC Davis agronomy farm during the spring-fall period in 1998. The transgenic plants had the same maternal parent and similar pollinator and maintainer lines. Plots were ten rows (30 inches) wide and thirty feet long. The middle six rows were treated with the different herbicides used and the middle two rows were harvested for yield and quality determinations. Treatment combinations are listed in Table 1. Because of rain during the planting period, treatments 1, 2, and 4 were planted on May 11, while treatment 3 was planted on May 14, so comparisons with treatment 3 include this qualification. In accordance with the USDA/APHIS protocol, all transgenic beets were destroyed after harvest except for sub samples used for quality determinations. Betaseed analyzed roots for quality characteristics at their mobile facility in Tracey. No attempts to evaluate weed control efficiency were made.
Results
1.0 Yield
1. 1 Root yields. Root yields were significantly larger for cultivar 4035 than the other three treatments, which were not significantly different from each other (Table 2). All the treatments resulted in good yields for a late spring/fall harvested crop.
1.2 Sugar percent. Sugar percent was greatest for the two transgenic cultivars, compared to the two commercial cultivars (Table 2) and compared favorably with fall harvested crops generally in Yolo County.
1.3 Gross sugar yield. Gross sugar yield was similar among treatments 1, 2, and 4, but significantly lower for treatment three (Table 2).
2.0 Quality characteristics
There were significant differences among K, Na, and Amino N content of the four treatments (Table 3), but these differences were small, not consistent, and confounded by replication and replication by treatment interactions. The most interesting difference may be for Amino N. Cultivar 4035R had a significantly lower amino N content than the others. It was also observed to be noticeably greener and leafier than any of the other cultivars at harvest. It is possible that a longer green leaf area duration resulted in less N being translocated to crowns and roots. We do not believe that the yellowing viruses influenced yields in this trial, but no ELISA tests were carried out.
Specific single degree of freedom contrasts are a more appropriate technique for inferring differences among means when quantitative treatments are compared. In this study, the use or absence of Liberty® herbicide on the transgenic beets can be thought of as a quantitative treatment. Results from these contrasts for the two transgenic treatments are reported in Table 4. There were no significant differences among them in any yield or quality characteristic, except for the Na content of roots, so the use of Liberty® on these beets had little or no influence on beet growth or quality. Contrasts between the two transgenic treatments and 4035R are also reported in the table, and result in similar inferences to the use of the LSD tests in tables 2 and 3.
Discussion
Treatment 3 (Cv. 4776R) resulted in the lowest root and gross sugar yields. This treatment was planted three days later than the others. The relatively weaker performance of 4776R in this trial should not be regarded as typical. The most reliable comparisons are with the commercial cultivar (4035R).
The root yield of the transgenic line, both with and without Liberty® application, was less than 4035R (treatment 1), but sugar percents were greater, resulting in equivalent gross sugar yields. This appears to be a cultivar characteristic, and may reflect a greater similarity between the transgenic line and its European progenitors, than is true for 4035R. Sugar percent was higher despite larger amino N contents in the transgenic cultivars. An inverse relationship between yield and sugar percent is common in sugarbeet. Additional data are needed before Märlander’s concerns can be evaluated.
There were no important differences between the two transgenic treatments (Table 4). There seemed to be no adverse effects from the use of Liberty® at recommended rates on crop growth and development in this trial.
References
Kaffka, S.R. and Lemaux, P.G. (1996). The use of molecular breeding methods to advance the sugarbeet industry in California. DANR Spec. Pub. May, 1996. Department of Agronomy and Range Science, Universtiy of California, One Shields Avenue, Davis, CA 95616-8515. 48p
Märlander, B. (1996). Status and prospects for the use of genetically
altered (herbicide-tolerant) sugarbeets on German farms. [Gentechnisch
veränderte herbizid-resistente Zuckerübensorten. Stand und Aussichten
der Einführung in die landwirtschaftlichen Praxis in Deutschland.]
Die Zuckeruben Zeitiung, 5:14.
Table 1. Treatments;
1998 trial, Agronomy Farm, UC Davis
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(t/ac) |
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p= |
=0.013 |
=0.001 |
=0.16 |
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(ppm) |
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p= |
0.002 |
0.208 |
0.005 |
Table 4. Single degree of
freedom contrasts
| Contrast:
Dependent variable |
(+/-) Liberty |
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| Root yield |
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| Sugar % |
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| Gross sugar yield |
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| K |
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| Na |
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| Amino N |
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