Gibberellic Acid Sensitivity among Common Bean Cultivars (Phaseolus vulgaris L.)

To lower seed yield loss from directly harvested common bean or dry bean, height of the lower pod-bearing nodes needs to be raised. The objective of this greenhouse study was to stimulate lower stem elongation by gibberellic acid (GA3) of dry bean cultivars. Seeds of cv. Matterhorn, erect indeterminate Type II, and cv. Poncho, prostate indeterminate Type III, were dipped in GA3 at 62.5 to 16,000 ppm and planted. After 14 d, the height of the unifoliate and first trifoliate nodes showed maximum stimulation of stem elongation by 1000 ppm GA3 for ‘Poncho’ and by 2000 ppm for ‘Matterhorn’. Application of 1 mL of GA3 at 0.031 to 2048 ppm to newly expanded unifoliate leaves showed cultivar differences. Whereas ‘Matterhorn’ was promoted at 64 ppm and reached a maximum height by 512 ppm GA3, ‘Poncho’ was promoted at 0.25 ppm and reached a maximum height by 8 ppm GA3. Flowering of ‘Matterhorn’ was unaffected by GA3; flowering of ‘Poncho’ was completely inhibited by 128 ppm. The sensitivity difference of cultivars was verified with other cultivars. Type I cultivars, which are all determinate, showed a full range of GA3 sensitivity. Dry bean cultivars may be regrouped based on the GA3 dose to which they respond. Individual response to GA3 rates of dry bean cultivars needs to be predetermined using a short-term, 2–3 weeks, greenhouse bioassay before field use of GA3. In dry bean (Phaseolus vulgaris L.), growth form, i.e., determinate vs. indeterminate, and growth habit, i.e., upright/erect/bushy vs. viny/ prostate, are among the most important characteristics for classifying cultivars from an agronomic viewpoint (Kelly, 2001; Laing et al., 1984; Singh, 1982). Dry bean is morphologically classified as determinate or indeterminate growth forms depending on whether the terminal meristem is reproductive (determinate) or vegetative (indeterminate) (Miklas and Singh, 2007). This characteristic is genetically controlled by the gene Finfin and unaffected by the environment (Koinange et al., 1996). Having a determinate terminal meristem was the result of FinFin or Finfin that is dominant over an indeterminate type (finfin) and this probably evolved through natural mutation of the wild-type Fin gene (Gepts, 1998). Indeterminate agronomic cultivars were classified into Type II and Type III based on vine growth extension and climbing ability. Determinate cultivars were classified as Type I and subdivided by their climbing ability. North American-grown commercial dry bean cultivars are described by Singh (1982) as: Type I = determinate, erect (bushy). Further classified into Ia (no climbing ability) and Ib (some climbing ability); Type II = indeterminate, erect (bushy). Further classified into IIa (no climbing ability) and IIb (some climbing ability = semiclimbing); and Type III = indeterminate, prostrate (viny). Further classified into IIIa (some climbing ability = semiclimbing) and IIIb (strong climbing ability = climbing). Vine length is highly affected by environment conditions, especially light (Kelly, 2001; Singh, 2001). The climbing phenotype of dry bean may be the result of a dominant gene, C1, whereas the nonclimbing types may be the result of a recessive gene, c1, that has evolved through natural mutation of C1 (Gepts, 1998; Kretchmer and Wallace, 1978). Type II and III dry bean cultivars are the most common ones grown in the U.S. High Plains. The lower pods of common dry bean grown in the field are very close to the ground. Because of this, the conventional practice in dry bean production in the U.S. High Plains is to harvest by first undercutting plants, conventional harvest, to minimize yield loss (Smith, 2004). The alternate method of harvest is direct harvesting but the yield loss in the Nebraska Panhandle may be greater than 10% even with the addition of lifters (Smith, 2004). In the Red River Valley, the mean of nine cultivars grown in four North Dakota locations over 2 years, seed yield was reduced from 2240 for conventional harvest to 1410 kg ha for direct harvest or 27% (Eckert et al., 2011). Most of the yield reduction was the result of seed loss during harvest, 4.5% by conventional harvest vs. 23.2% by direct harvest (Eckert et al., 2011). One possible method of reducing yield loss is to stimulate growth of lower internodes, those below the node with the first flower and pod, to raise lower pods higher off the ground and allow the cutting blades on a direct harvester to cut the stem below those pods. This may be accomplished by application of a growth-stimulating compound such as GA3. The ability of gibberellins to promote stem growth was known since the 1930s when a rice disease was identified to be the result of a pathogenic fungus Gibberella fujikuroi (Takahashi et al., 1991). Since then, there have been more than 130 gibberellins identified. Gibberellic acid, a key gibberellin, is highly active and well known to stimulate stem elongation (Davies, 2010; Marth et al., 1956). A greenhouse bioassay for GA3 applied to fully opened unifoliate leaves of snap bean cultivars (P. vulgaris) was developed showing a dose–response for stimulating stem elongation and exposure between 2 and 10 mg GA3/plant for maximum effect (Knoche et al., 1998, 2000). The objective of this study was to compare the GA3 dose–response of indeterminate dry bean cultivars with an erect, upright (Type II) growth or a prostate (Type III) growth habit and determinate cultivars (Type I). Materials and Methods Greenhouse conditions. Experiments were conducted in March and April of 2005, 2006, and 2007 in a greenhouse at the Panhandle Research & Extension Center of the University of Nebraska in Scottsbluff (lat. 41.9 N, long. 103.7 W, elevation 1208 m). The maximum daytime temperature was 35 C, and minimum nighttime temperature was 23 C. Metal halide lamps were used to supplement sunlight to maintain a 14-h photoperiod. Lamps were kept 1.5 m above plants. Pots were watered at planting and checked three times weekly and watered as needed to maintain a full water profile throughout the experiments. Plant material. Dry bean cultivars, i.e., common bean and dry edible bean, listed in Table 1, were obtained from various seed programs such as the University of Idaho, University of Saskatchewan, and Michigan State University and purchased through Kelley Bean Co., Scottsbluff, NE. Type II, indeterminate and upright, and Type III, indeterminate and prostate, cultivars are commonly grown in western Nebraska. The cv. Matterhorn and cv. Poncho were chosen as to initially represent Type II and Type III cultivars, respectively, because they are major cultivars grown in this area. Type I, determinate, cultivars are not commonly grown in this area, but as a result of the range of responses to GA3 application of Type II and III cultivars, Type I cultivars Received for publication 5 Dec. 2011. Accepted for publication 4 Apr. 2012. We thank Bob Hawley and Les Kampbell for their technical assistance and the financial support by the Nebraska Dry Bean Commission. Crop Physiologist. Alternate Crop Breeder. Extension Educator. Soil Scientist. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 47(5) MAY 2012 637 were also tested for their foliar response. Seeds were treated with streptomycin for pathogen suppression by Kelley Bean Co. using standard commercial practices. In the 2005 rate tests, seeds were planted in durable molded fiber pots that were 20 cm in diameter and 20 cm deep. In the tests conducted in 2006 and 2007, seeds were planted in plastic pots, 15 cm in diameter by 15 cm deep, because of acceptability, availability, and bench space limitation. Seeds were planted 2 cm deep in Fafard Superfine Germination Mix (American Clay Works, Denver, CO). In the seed application tests in 2005, emergence of all planted seeds and plant height of all emerged seedlings were determined. In all tests in which GA3 was applied to the foliage, four seeds were planted per pot and thinned after a few days to three plants before treatment for uniformity. On a per-cultivar basis, four pots were used for each GA3 treatment. Pots were arranged in an randomized complete block design based on GA3 treatment separated by cultivar on a greenhouse bench. The heights of nodes and apices were measured, and emergence and flowering were observed. Experiments were analyzed using SAS Proc analysis of variance and means were separated using least significant difference (SAS Institute, 2003) for each cultivar separately. Chemical preparation. Gibberellic acid was applied as Release LC, a 4% a.i. weight by weight, i.e., 1 g GA3/30 mL, formulation (Valent BioScience Corp., Long Grove, IL). Release LC was diluted serially with water from 16,000 to 0.031 ppm GA3. A stickerspreader-type surfactant, X77, at 0.125% was added to solutions applied to foliage until determined not to be needed in a greenhouse test in 2007 (unpublished data). Seed application experiments. Emergence tests were conducted in 2005 when 32 seeds of ‘Matterhorn’ and ‘Poncho’ were soaked in 15 mL GA3 solutions for 5 min. Seeds were removed and air-dried for 2 to 3 h at 20 C and then planted in pots placed in the greenhouse in 2005. Three seeds of each seed treatment of the two cultivars were planted together into each of four pots and were used as replicates. Therefore, the treatments were paired per pot while the pots were placed randomly by replication on a greenhouse bench. Emergence was determined at 7 d after planting (DAP) and height of unifoliate and trifoliate nodes were measured at 9 and 14 DAP. Foliar application method. Foliar applications were made to the unifoliate leaves at Stage V2 (Schwartz et al., 1993), which was reached between 10 and 14 DAP. Cosmetic squirt bottles (59 mL) were used to apply 1 mL (seven squirts) to the two unifoliate leaves (0.5 mL/leaf). The rates of GA3 in the initial dose test in 2005 ranged from 0.03125 to 2048 ppm increased by increments of 2·, i.e., 0.03125, 0.0625, 0.125, 0.25, etc. Cultivar sensitivity tests. In 2006, the doses were individualized to the cultivar types as a result of differential sensitivity between ‘Matterhorn’ (Type II) and ‘Poncho’ (Type III) observed in 2005 in the foliar application experiments. Nine cultivars were tested to determine whether they would show a wide range of GA3 sensitivity and could be grouped based on their sensitivity to GA3 in 2006. Cultivars CDC Pintium, Nordic, and Seafarer were classified as Type I; cultivars Ensign, Frigate, Matterhorn, and Vision were as Type II; and cultivars Marquis and Poncho were classified as Type III. The market class represented were Pinto (cvs. CDC Pintium, Poncho, and Vision), Great Northern (cvs. Marquis, Matterhorn, and Nordic), and Navy (cvs. Ensign, Frigate, and Seafarer) (Table 1). Cultivars Poncho, Marquis, and Frigate were treated with GA3 at 0, 0.125, 0.5, 2, 8, and 32 ppm applied to the unifoliate leaves 10 DAP (V2); and cultivars Matterhorn, Vision, Ensign, CDC Pintium, Nordic, and Seafarer were treated with GA3 at 0, 8, 32, 128, 512, and 2048 ppm. Height to the unifoliate node, i.e., plant height at time of treatment, was measured at treatment and measured again along with heights to the first and second trifoliate nodes and the plant apex at 7 DAT. New growth at 7 DAT was defined as the height of the plant apex at 7 DAT minus the plant height at the time of GA3 application (10 DAP), i.e., the height of the unifoliate node. As a result of the responsiveness of the Type I cultivars in 2006, 13 Type I cultivars were tested in 2007. These cultivars represented five market classes, ‘Pinto’, ‘Great Northern’, ‘Navy’, ‘Light Red Kidney’, and ‘Large White Kidney’ (Table 1). Unifoliate leaves were treated at 12 DAP with GA3 at 0, 0.125, 8, 512, and 2048 ppm. These rates were chosen based on the 2006 results. Apical height was measured at 7 DAT.