Plant Growth Regulators

8 Plant Growth Regulator Potential on Sugarbeets

Downloaded by UNIV OF NEW HAMPSHIRE on May 25, 2014 | http://pubs.acs.org Publication Date: May 1, 1977 | doi: 10.1021/ba-1977-0159.ch008

E. F. SULLIVAN Agricultural Research Center, The Great Western Sugar Co., Longmont, Colo. 80501

Plant growth regulators used on sugarbeets should improve crop emergence rate, enhance production, and conserve sugar produced. Experimental results indicate that foliar applications at canopy closure are more likely to improve yield, while applications three to six weeks pre-harvest are more effective for promoting sugar content. Plant population and spacing affect sugar content, and excessive soil nitrogen causes low sugar and high root impurities at harvest. Three maturation points occur during later growth— namely, nitrogen depletion aging, temperature senescence, and finally, low temperature-induced growth cessation. Yield improvement by chemically promoting growth before August seems possible. Chemical regulation of root/top ratio later and adjustment of senescence to promote greater and earlier sugar buildup in the root are reasonable aims.

"seful plant growth regulator candidates on sugarbeets should ac^ complish one or more of the following objectives or benefits: ( l ) i m p r o v e seedling emergence rate and vigor to permit planting to final stand without supplemental adjustment; (2) increase root yield; (3) enhance root quality; and (4) conserve sugar produced during storage after harvest by chemical regulation of root respiration rate. The average emergence rate is 5 5 % from a monogerm seed with a germination potential of 9 5 % . Generally, sugarbeets are overplanted to offset stand loss caused by an adverse environment. Subsequently, the stand is mechanically or manually adjusted to the final or harvest stand. Supplemental stand adjustment is a costly practice. 68

In Plant Growth Regulators; Stutte, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.


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Essentially, experimental growth regulators are applied on sugarbeets to augment or impede natural growth processes and rates—e.g., to quicken seed germination, crop emergence, and seedling growth early and to retard top growth later on. Effective chemical systems are expected to quicken leaf canopy closure time by 10-14 days, which expands the growing season and yield. Sugarbeets have a relatively slow growth rate from soil emergence until the crop leaves cover the row. A better understanding of industrial methods, limits, and objectives might w e l l serve both the agricultural chemical supplier and researcher i n advancing their search for an effective and usable plant growth regulator on sugarbeets. Grower Considerations Growers receive direct benefit from producing the highest sugar yields possible per acre. H i g h sugar producers are capable of 10-12,000 lb per acre. This accomplishment requires expert management because root weight and sugar percentage are agronomically inversely related generally. Regulating the two factors beneficially and simultaneously from chemical application has not yet been demonstrated, although single factor adjustment has been attained. F o r example, gibberellic acid ( G A ) and 2-(chloroethyl)phosphonic acid (Ethrel) improve root yields but decrease sugar concentrations, whereas maleic hydrazide ( M H ) produces sugar increases which are inversely related to root weights. Today, field labor costs have little or no relationship to pricing crop protection and production chemicals. Current production is based on crop chemical technology rather than on labor use. Growers w i l l schedule costs for improving uniform crop emergence rate and sugar yield per acre because of direct participation i n benefits. The prices that growers and processors receive for sugar sets a practical upper limit on plant growth regulator cost and use. This is so because, as with a l l agricultural chemicals, the benefits derived from plant growth regulators must exceed the costs of application. 3

Growth Environment, Yield, and Sugar Content As i n other crops, response reliability among years and sites remains a major problem when sugarbeets are treated with plant growth r e g u lators. Application timing, variety selection, soil nitrogen fertility level, moisture and temperature regime, and other variables more than likely interact and regulate response magnitude. Nevertheless, experimental results indicate that topical applications made early i n the growing season (from the 12-leaf stage until closing of the rows) are more likely to improve root yield, while applications made three to six weeks before


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harvest are more effective for promoting sugar content. Plant growth regulator mixtures and split or sequence applications have been generally ineffective. Sugarcane ripeners, namely, N,N-bis(phosphonomethyl) glycine ( Polaris ), 3-trifluoromethylsulfonamido-p-acetotoluide ( Sustar ), and methyl-3,6-dichloro-o-anisate (Racuza), have been ineffective when applied topically on sugarbeets i n trials conducted by The Great Western Sugar Co. In the irrigated inter-mountain and the Great Plains regions, sugarbeets require approximately 180 days from planting to harvest for maximum sugar production. Post-thinning stands of 24-28,000 plants per acre usually diminish somewhat by harvest. After a week or so of recovery from mechanical thinning, seedling beets begin growing more rapidly since root systems are well developed, especially after row closure. Results from several proprietary compounds reveal that topically applied growth regulators for root weight stimulation should be applied when seedling beets have about 12-14 true leaves or when the canopy is about 8 0 % closed. Approximately 120 days are required to obtain the full effect from a treatment. Effective regulators applied at row closure are expected to increase the root weight by 1.5 ton/acre and to improve sugar yield by 6 - 8 % . In 1975, UC-51416 (confidential chemical structure) gave preliminary promise as a sugarbeet yield-enhancing chemical ( > 3 tons root weight per acre). Additional screening of new compounds and more rapid and reliable screening methods are needed to advance the use of plant growth regulators on sugarbeets. To date, no plant growth-modifying chemical has been released for commercial use on sugarbeets. Although plant population and spacing affect sugar content, an excessive soil nitrogen fertility level is the major cause of low sugar and high root impurities at harvest. A sidedressing of nitrogen in beet fields is discouraged after July 15 to ensure soil nitrogen depletion and plant uptake at a declining rate. Most beet sugar companies have effective nitrogen fertility monitoring programs based on a production ratio of 8-10 lb of available nitrogen per ton of beets. In practice, three maturation points occur i n the late growth pattern. Nitrogen depletion aging (physiological senesecence) is initiated between August 15 and September 1 (six weeks before harvest). A t this point petiole nitrate nitrogen levels should decline to 1000-1500 p p m for optimal sugar content at harvest. It is expected that a timely plant growth regulator application at this point w i l l further suppress top growth and nitrogen accumulation (amino nitrogen) i n the plant, especially if soil nitrogen is above critical levels, which usually occurs. Growth rate of the tops is naturally declining at this point and sugar buildup commences more rapidly. Growth regulators applied at tern-



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perature senescence or at the point where night temperatures significantly decline (first frost) and daytime growing conditions are warm and sunny may also be effective for sugar increase. Plant growth modifying chemicals are expected to lower plant nitrate levels which at high levels depress sugar content of roots. Effective chemicals applied at the late growth period should increase the sugar content of roots by 0.5-0.75%. Harvest roots usually contain 15-17.5% sugar. True senescence or plant growth cessation occurs on or about November 1 or when temperatures reach 26° F or lower for 6-8 hr. W h e n plant growth stops, sugar accumulation stops. Quality Considerations Substantial savings can be realized by root purity improvement from agronomic means such as variety improvement or from an effective chemical application. Harvest impurities consist primarily of amino nitrogen, betaine, and potassium, sodium, and chlorine. Direct delivery beets (sliced soon after delivery without piling) have the greatest amount of extractable sugar. Beet piles are usually covered with straw or plastic sheeting to protect against deep rim freeze. Storage impurities and root decay organisms which invade harvest wounds increase as the storage period lengthens. Sucrose losses throughout storage are estimated at an average of 0.5 lb refined sugar per day per ton of beets. M a i n storage impurities from prolonged root respiration periods causing sucrose dilution are non-sucrose sugars—namely, glucose, fructose, and raffinose. A plant growth regulator that conserves sucrose during storage by lowering respiration rate is a worthwhile objective but of more practical value if linked with field quality improvement. Risk-Benefit Evaluation Yield improvement beyond genetic and management inputs by chemically promoting top growth before August seems possible. Chemical regulation of root/tops ratio later in August and adjustment of senescence to encourage translocation of sugar to the root for earlier harvest are worthwhile objectives. A plant growth regulator that improves sugar yield seven years out of 10 would have sufficient field efficacy for chemical development and grower use. The risk-benefit margin may limit future cost of the chemical per acre to the average production cost per ton of beets since profitable farm production goals are primarily based on crop protection chemicals, nitrogen fertility, and adapted varieties. RECEIVED

September 21, 1976.


Plant Growth Regulators

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