Introduction

Over many years of cultivar trials, it is evident that the potato (Solanum tuberosum L.) cultivar Russet Burbank does not grow well in Nebraska especially under the semi-arid conditions of the Panhandle and the High Plains. A key factor is the tendency of ‘Russet Burbank’ to produce misshaped tubers, i.e., tuber growth defects. Since ‘Russet Burbank’ is the primary cultivar used in French fry processing, growers in the High Plains cannot participate in this potato market. Stem tissue growth is promoted by internally-produced gibberellic acid (GA3), a natural plant hormone (Busov et al. 2008; Thomas and Sun 2004). Gibberellins are known to be involved in promoting stolon growth (Koda and Okazawa 1983) and play a role in tuber formation (Vreugdenhil and Sergeeva 1999). Working with gibberellin-deficient potato mutants (ga1), Bamberg and Hanneman (1993) showed a correlation between a greater number of gibberellin-producing (Ga1) alleles with a greater depression of tuber yield. Since potato tubers are stem tissue, it may be reasonable to predict that GA3, the most active of the over 100 compounds in this family, may play a role in the misshaping of tubers such as dumbbell, pointy end and knob formations. The application of GA3 to potato plant vines cv. Sebago (Myhre and Eddins 1957), repeated foliar application cv. Kufri Chandramukli (Sharma et al. 1998), or to roots cv. Green Mountain (MacLeod and Howatt 1958) dramatically increased the misshaping, including pointy ends and dumbbells, of harvested tubers. Therefore, an approach to reduce tuber misshaping may be to use a GA3-biosynthesis inhibitors such as prohexadione-Ca, a new compound, or chlormequat-Cl, a well-known compound; each block GA3 biosynthesis at different sites (Rademacher 2000). Although prohexadione and chlormequat are compounds inhibiting GA3 biosynthesis, their structures are dissimilar. Prohexadione-Ca may penetrate and translocate in the vine more readily than chlormequat-Cl (BASF, personal communication). Their charges are opposite with prohexadione as an anion and chlormequat as a cation explaining why one is salified with Ca and the other with Cl in their formulations.

Chlormequat-Cl (a.k.a. CCC) is a well-known GA3 biosynthesis inhibitor used as an anti-lodging agent on wheat (Espindula et al. 2009). The addition of chlormequat-Cl to potato cuttings lowered the endogenous biologically-active gibberellins in roots while promoting tuber formation (Abdala et al. 1995). Chlormequat-Cl when applied to foliage does penetrate the leaves and translocate to the potato tuber (Dekhuijzen and Bodlaender 1973). In field studies, foliar applications of GA3 caused deformation of tubers while reducing tuber yield and starch content (Sharma et al. 1998). Chlormequat-Cl, on the other hand, resulted in well-shaped tubers and increased tuber yield and starch content. Tuber yield increase by chlormequat-Cl recently has been attributed to an increase in the partitioning of photosynthate into tubers from leaves and promotion of certain antioxidant enzymes in the leaves; thereby increasing tuber growth (Wang et al. 2009, 2010).

There has been no previous studies reported on the application of prohexadione-Ca, another GA3- biosynthesis inhibitor currently used as an anti-lodging agent in wheat and rice, to potato. A recent greenhouse study on prohexadione-Ca applied to rhubarb (Rheum rhabarbarum L.) showed that prohexadione-Ca acted like chlormequat-Cl by promoting rhizome growth, i.e., larger and heavier (Rayirath et al. 2009). Application 12 weeks after emergence was more affected than at 8 weeks, and a higher dose of prohexadione-Ca was required to achieve the same response than chlormequat-Cl. Another GA3-biosynthesis inhibitor is daminozide (a.k.a. B9, B995) which has a similar mode of action as prohexadione-Ca (Brown et al. 1997). Daminozide applied near tuber initiation promoted tuber growth rate and caused knobbiness of tubers (Bodlaender and Algra 1966; Humphries and Dyson 1967).

The objective of this study was to determine whether GA3-biosynthesis inhibitors, growth retardants, when applied to potato foliage cv. Russet Burbank, could reduce the amount of tuber misshaping under the semi-arid conditions of the Nebraska Panhandle.

Materials and Methods

Field Conditions and Operations

Between 2001 and 2004, field trials were conducted on potato (Solanum tuberosum) cultivar Russet Burbank at the Univ. of Nebraska’s Panhandle Research & Extension Center, Scottsbluff (lat. 41.9 N, long. 103.7 W, elevation 3,963 ft). Soil was a Tripp fine sandy loam at pH 7.8–8.1 and organic matter content of 0.7–1.0 %. Planting was between 4 and 15 May depending on soil temperature, and plant emergence (50 %) was 22–27 days after planting. Production practices followed standard methods (Pavlista 1995). Rainfall and temperature were monitored by the High Plains Regional Climate Center (Changnon et al. 1990). Trials were conducted under an overhead, linear-move irrigation system throughout the season to apply 61 to 66 cm of water from late June to early Aug. Plots consisted of four rows spaced 91 cm apart and 9.1 m long. Plants were spaced 25 cm apart within the rows. Fertilizers were applied to achieve 235 kg N/ha, 123 kg P2O5/ha, and 62 kg S/ha; half was applied pre-plant and half applied pre-emergence. A mix of metribuzin and metolaclor was applied pre-emergence for weed control. Thiophanate-Me plus mancozeb dust was applied to seed pieces for fungal control except in 2003 when fludioxonil plus mancozeb was applied. Seed dusts were applied at 1 kg/100 kg seed-pieces. Early blight control was accomplished by sequential foliar applications of chlorothalonil Zn, mancozeb and azoxystrobin. Imidacloprid was applied in-furrow at planting for early-season insect control. Plots were flailed (vine killed) and harvested in September. The center two rows were harvested and plots were individually bagged. Harvested tubers were placed on a grader fitted with chains to separate tubers based on diameter: <4.8 (discarded), 4.8–5.7, 5.7–8.3, and >8.3 cm. Yield were of those potatoes with a diameter greater than 5.7 cm including misshaped tubers (culls).

Chemical Preparation and Application

Prohexadione-Ca and chlormequat-Cl were applied as the commercial products Apogee 27.5S and Cycocel 1 L, respectively. Doses are presented as grams active ingredient/ha. Application rates were 0, 70, 280, 1,120, and 4,480 g ha−1 containing X77 at 0.125 %, a non-ionic surfactant. Foliar applications were at 187 L ha−1 and sprayed with a CO2 sprayer set at 2.8 kg cm−2 using a 9501E nozzle atop each row and directed about 35 to 36 cm above the canopy. Prior to treatment applications, six representative plants scattered in the trial area were hand-dug and the weight of each tuber greater than 1 g was measured; the mean tuber weight was calculated, and the weight of the largest tuber represented the stage at application. In 2001, 2002 and 2003, treatments were applied on 30 Jul, 5 Aug, and 8 Aug or 8, 8, and 10 weeks after emergence (WAE), respectively. Weight of largest tubers at treatment was 60 g in 2001, 20 g in 2002 and 100 g in 2003. As a result, in 2004, 280 g prohexadione-Ca/ha was applied at different times, 4, 5.5, 8, 9.5, and 11.5 weeks after emergence, corresponding to weights of 3, 25, 100, 180, and 370 g for the larger tubers. Prohexadione-Ca and chlormequat-Cl were in separate adjacent trials and not directly compared.

Data Collection and Analysis

Harvested tubers were graded into groups based on diameter. Shape defects of dumbbells, pointy ends, folds, knobs, and growth cracks were determined on a random 50-tuber (>5.7 cm diameter) sample from each plot. Three 10-tuber random samples of tubers (>4.8 cm diameter) from each plot were placed end to end and side by side to measure tuber length and width, respectively (Pavlista 1997). Data from Prohexadione-Ca and chlormequat-Cl were analyzed separately as they were not randomized together. Trials were conducted as 5×5 Latin squares. Data from each trial were analyzed separately using Proc Anova in SAS 9.1.3 with means separated using least significant differences and significance was based on p < 0.05 (SAS Inst 2003). Although years were significantly different, there was no year by chemical rate interaction for yield or tuber shape defects. Interaction between year and tuber size at application cannot be determined.

Results

Prohexadione-Ca

Application of prohexadione-Ca at 280–4,480 g/ha when the larger tubers are entering the log phase of tuber growth, 20 g (early bulking), increased ‘Russet Burbank’ yield 14–17 % (Table 1). When the largest tubers were 60 g (early mid-bulking), prohexadione-Ca promoted yield 22 and 19 % at 70 and 280 g/ha, respectively, but not at 1,120 g/ha (Table 1). When prohexadione-Ca at 280 g/ha was applied to plants with their largest tubers weighing 100 g (mid-bulking) yield increased by 20 %, and when applied at 70 and 1,120 g/ha, prohexadione-Ca also increased yield but to a lesser extent, 11 and 9 % respectively (Table 1). Tuber misshaping, i.e., dumbbells, pointy ends, folds, knobs, and growth cracks, was not decreased by application of prohexadione-Ca (Table 1). Tuber length and width were measured and had no effect (Table 1). Prohexadione-Ca increased the length to width ratio, i.e., more elongated, only when the largest tubers were 100 g (2003), when applied at 280 and 1,120 g/ha.

Table 1 Comparing yield, tuber shape defects and tuber dimensions of potato cv. Russet Burbank grown in a semi-arid climate as influenced by foliar application of prohexadione-Ca at three tuber growth stages
Full size table

Because of the Prohexadione-Ca dose responses for the three tuber stages, each tested in different years, the most consistently effective dose of prohexadione-Ca, 280 g/ha, was applied in 2004 to five tuber growth stages when largest tubers were 3 g (4 WAE, late tuber initiation), 25 g (5.5 WAE, early bulking), 100 g (8 WAE, mid-bulking), 180 g (9.5 WAE, late mid-bulking), and 370 g (11.5 WAE, late bulking). Prohexadione-Ca at 280 g/ha significantly increased yield 14 % when applied to plants having tubers weighing 25 to 100 g (Fig. 1). However, concomitantly, the percent of tubers with shape defects increased from 48 % to 75 % (Fig. 1). Although a year by tuber size at application cannot be determined comparing 2001, 2002, and 2003, the trial in 2004 indicated that Apogee effects are significantly related to tuber size at application. There was no significant effect on tuber dimensions (data not shown).

Fig. 1
figure 1

Yield and tuber shape defects as affected by prohexadione-Ca applied at 280 g Al/ha at various tuber growth stages of Russet Burbank potato, 2004. Check represents data from untreated plants. Tuber stages correspond with treatment at 4, 5.5, 8, 9.5, and 11.5 weeks after emergence. Capital letters on yield curve indicate significant difference at p < 0.05 and ordinal letters on defect curve indicate significant difference p < 0.1

Full size image

Chlormequat-Cl

Application of chlormequat-Cl significantly increased yield 15–33 % when applied at early bulking at doses from 70 to 4,480 g/ha with a plateau at 1,120 and 4,480 g/ha (Table 2). At pre-bulking and early mid-bulking, chlormequat-Cl only at 4,480 g/ha was required to increase yield. When chlormequat-Cl was applied at pre-bulking and early bulking, there was no effect on tuber growth defects, but when applied at 280 g/ha or greater at early mid-bulking, growth defects were significantly fewer but remained above 50 % (Table 2). Tuber length and width, and the length to width ratio were not affected by chlormequat-Cl regardless of dose or tuber stage. Since chlormequat-Cl is no longer commercially available and has been replaced with prohexadione-Ca, it was not tested further.

Table 2 Comparing yield, tuber shape defects and tuber dimensions of potato cv. Russet Burbank grown in a semi-arid climate as influenced by foliar application of chlormequat-Cl at three tuber growth stages
Full size table

Discussion

In the Nebraska Panhandle, July temperatures were high from 2001 to 2004, averaging 33 °C day and 24 °C night (Table 3). These temperatures tend to favor dry weight accumulation in the vine as opposed to tubers and thereby delaying early tuber growth (Van Dam et al. 1996). Similarly, gibberellin-deficient (ga1) plants produce more yield than gibberellin-producing (Ga1) plants (Bamberg and Hanneman 1993). Menzel (1980) reported that foliar application of GA3 suppressed tuber production and was similar to the effect of high temperature. Chlormequat-Cl had the reverse effect and promoted tuber production. It was interpreted that temperature effects the endogenous hormone balance between gibberellins and inhibitors (Menzel 1980). GA3 not only suppresses tuber production but has been reported also to promote tuber misshaping (MacLeod and Howatt 1958; Myhre and Eddins 1957) and elongation, i.e., longer tubers (Lovell and Booth 1967). The high temperature and low rainfall climate of the Nebraska Panhandle may promote endogenous GA3 production which in turn could lower yield and raise the amount of tuber misshaping. In Ethiopia, paclobutrazol, another GA3-biosynthesis inhibitor (Rademacher 2000), applied to heat-stressed potato, overcame the effect of the high temperatures and promoted tuber production (Tekalign and Hammes 2005). Earlier, tuber promotion by paclobutrazol was reported on the potato cv. Russet Burbank (Balamani and Poovaiah 1985), the cultivar tested here.

Table 3 Rainfall and temperatures from May to Aug, 2001–2004 at the Univ. Nebraska Panhandle Research & Extension Center, Scottsbluff, NE
Full size table

In this field study on the potato cv. Russet Burbank, chlormequat-Cl did increase tuber yield when applied at 4,480 g ha−1 (4 lb AI/a) (Table 2). Yield promotion indicated a rate-response with maximum promotion between 280 and 1,120 g ha−1 when the largest tubers weighed 60 g (early mid-bulking) at application in 2001. Radwan et al. (1971) reported that chlormequat-Cl at 500 g ha−1 increased yield when applied about 6 weeks after emergence, but twice as much was required when applied at 4 weeks. Tuber misshaping was not affected by chlormequat-Cl except when the compound was applied when the largest tubers were 100 g (mid-bulking) and then misshaping was reduced (Table 2). Tuber length and width were not affected. Choudhri et al. (1976) reported that in greenhouse studies, chlormequat-Cl increased tuber length and width.

Next to the chlormequat-Cl trials, similar trials were conducted with prohexadione-Ca from 2001 to 2003. Prohexadione-Ca, like chlormequat-Cl, increased yield but at lower rates (Table 1). When applied to plants with their largest tubers at 20 g (early bulking), a maximum effect was achieved when prohexadione-Ca was applied at 280 g ha−1. Application of prohexadione-Ca to plants with their largest tubers at 60 g (early mid-bulking) had a maximum response only at 70 and 280 g ha−1. When applied to plants having 100 g tubers (mid-bulking), prohexadione-Ca increased yield from 70 to 1,120 g ha−1 with a maximum effect at 280 g ha−1 (Table 1). These results indicated a stage-related response to prohexadione-Ca, but since these trials were in separate years, a year effect would be possible. Differences between years is seen from the data obtained from untreated plots. However, there was no significant interaction between year and rate of compound for either material. Previous field studies on GA3-biosynthesis inhibitors were conducted based on chronological timing of application and therefore a clear relationship between tuber growth stage and effect was not reported. Therefore, in 2004, due to the 2001–2003 results, 280 g prohexadione-Ca ha−1 was applied throughout the season to plants at different stages of tuber growth. A yield increase occurred when the largest tubers were between 25 and 100 g, the first half of the bulking period, agreeing with the results of the previous years (Fig. 1). However, associated with the yield increases were increases in the percent of misshaped tubers, negating the yield advantage. In the 2001–2003 trials, a correlation between yield and tuber misshaping was not observed (Table 1) so other factors may play a key role. Using gibberellin-deficient (ga1) plants, Note that earlier studies on chlormequat-Cl showed that application at 4,480 g AI ha−1 shortly after emergence had no effect on yield (Gunasena and Harris 1969) and application shortly after tuber initiation was less effective than during early bulking (Radwan et al. 1971). Differences in response to prohexadione-Ca and chlormequat-Cl may also be due to differences in sites of inhibition in the gibberellin biosynthesis pathways and modifiers. Vega et al. (2006) reported differences in gibberellin-deficient (ga1) plants in their sensitivity to gibberellic acid. Cultivars may differ significantly to their sensitivity to different gibberellic acid biosynthesis inhibitors.

A last observational note is about the different tuber growth stages between the years 2001, 2002 and 2003 (Tables 1 and 2). This is clear when realizing the high percentage of misshaped tubers harvested. The year 2001 was unusually wet in June and July, emergence to early tuber bulking (Table 3) resulting in low yield and percent of misshaped tubers (Tables 1 and 2). On the other hand, 2002 was extremely dry, receiving about 1 cm rain in June and July (Table 3), resulting in a 2 to 3 week delay in tuber initiation and early tuber growth with little affect on tuber defects and less affect on yield than the wet spring on 2001. The differences in yield and tuber defects were significantly different between these 3 years although there was no significant year by application rate interaction for either compound. Temperature could be another factor affecting tuber development. There was no significant difference in the high/low temperature each month by year. Planting were in May, emergence occurred in June and tubers normally initiated in early July followed by early bulking toward the end of July, temperatures in July might be key. There was little difference in day and night (high/low) temperatures between 2001, 2002, and 2003, averaging 34/16 °C; 2004 was slightly cooler at 31/13 °C. In a greenhouse study, Menzel (1980) reported no significant difference in plant or tuber growth between 32/28 and 32/18 °C but growing plants under 22/18 °C induced several significant changes such as more tubers/plant and a higher harvest index. Since the day temperatures in 2001–2004 were around 32 °C, temperature did not probably play a major role. With a strong relationship between precipitation and tuber development, when plant growth regulators are applied, the tuber growth stage needs to be determined as well as monitoring early-season rain regardless of irrigation.

Conclusion

Prohexadione-Ca and chlormequat-Cl, known inhibitors of GA3 biosynthesis, can increase potato tuber yield of cv. Russet Burbank grown in the climate of the Nebraska Panhandle, but these compounds did not change the percent of tubers that were misshaped. The most effective stage for applying these compounds to promote yield is when the largest tubers are between 25 and 100 g. Prohexadione-Ca was more effective at lower rates than chlormequat-Cl.