Effects of Trinexapac-Ethyl on different wheat varieties under desert conditions of Mexico

A field study was conducted to determine the effect of Trinexapac-Ethyl (TE) on four wheat varieties (Rafi C97, Altar C84, Nacori C97 and Rayon F86) at three growth stages (first node, second node and flag leaf). TE application (150 g a.i. ha) reduced plant height and decreased grain yield at any growth stage. Yield decrease was associated to a significant reduction in the number of grains per spike and in grain weight. TE delayed flowering time and grain maturity, while stem diameter and spikes·m were not affected. Further studies are needed to clarify the effect of TE on wheat under desert conditions of Mexico.


INTRODUCTION
Wheat is annually grown in 825.1 thousand hectares in Mexico, with a production of 4.14 million tones [1].Profitability of this crop depends of yield, production cost and grain price.To obtain high yields it is necessary the use of new varieties resistant to pests and diseases, proper sowing date, good water management practices and proper nitrogen fertilization.However, yield may be affected by other factors such as frost damage and lodging [2].
Lodging in wheat is a serious problem under soils with high fertility (>300 kg•N•ha −1 ), irrigated condition and high planting density [3,4].Lodging can result in yield reduction up to 50%, reduced kernel weight, increase the N (protein) content of the grain, and reduce the milling quality [5][6][7].
Trinexapac-ethyl (TE) belongs to cyclohexandione group and inhibits gibberellins biosynthesis [9,10].TE prevents lodging not only by reducing the crop height, but also by strengthening the stem and crown root structures.In comparison to other growth retardants it is effective to increase yield grain and reduce plant height [7,[11][12][13][14][15][16][17][18].TE has other effects on wheat as increasing number of grains per spike, weight of 1000 grains, ear length and stem diameter [15,17].TE applications can delay grain maturity [7] but have little effect on photo-synthesis activity, root growth and development [19] and protein content in grain [17].
The objective of this experiment was to determine the effects of TE on the grain yield, growth traits, yield components and grain quality applied in different growth stages of four winter wheat varieties grown under desert conditions of Mexico.

Site and Wheat Management
The experiment was carried out in an Experimental Station of the National Research Institute for Forestry, Agricultural and Livestock (INIFAP) in Mexico (Latitude 30˚42'55''N, Longitude 112˚21'28''W and altitude 200 m) during 2005-2006.The soil was sandy loam with pH 7.96 and electrical conductivity of 1.22 dSm −1 .Wheat varieties were sown on December 15, 2005 at 150 kg•ha −1 .Size of experimental plot was of 5.0 m × 6.0 m (30.0 m 2 ).Sowing was done using furrow (1.0 m) distance with two rows for each furrow spacing of 0.35 m.Water was applied with drip irrigation and the rate of fertilization was 220-60-00 kg•NPK•ha −1 , applying 60 kg•ha −1 of N and P at sowing time.The remaining N was applied through the irrigation system during crop development.Other agricultural practices (weeds, pests and diseases control) were applied according to recommendations for this crop in this agricultural area [2].

TE Applications and Varieties
TE was applied at the dose of 150 g a.i.ha −1 on four wheat varieties: Rafi C97, Altar C84, Nacori C97 and Rayon F89.The characteristics of wheat varieties are described in Table 1 [2].TE was applied at three growth stages: First node, second node and flag leaf just visible 31, 32 and 37 according a Zadocks scale [20] and there was a check.Applications were made with a gasoline powered mist blower.

Measured Variables
Ten plants prior to harvest were randomly taken from the middle row of each plot to measure plant height, stem diameter, number of grains per spike and weight of 1000 grains.The number of spikes•m −2 was determined taking two samples in each plot.Plant lodging was measured in plot center using a lodging scale from 0 to 5, where 0 was wheat standing upright and 5 was wheat totally flat.Grain yield was obtained for the plot area and then converted to kg•ha −1 .

Statistical Analysis
Analysis of variance was done considering a split plot design with three replications.The varieties were placed in the main plots and TE applications in sub plots.Means were separated with the least significant difference test (LSD) at 0.05 probability level.

Grain Yield
TE at 150 g a.i.ha −1 decreased grain yield in the four wheat varieties at any growth stage.Grain yield reduction was 9.1%, 13.3% and 10.1% when TE was applied at first node, second node and flag leaf, respectively, with respect to the control.Altar C84 was less affected by TE application than Rafi C97 which had a yield reduction of 18.2% (Table 2).
Results from this work disagree to other researchers who report a grain yield increase when TE was applied at different rates and different growth stages in wheat [7,[11][12][13][14][15][16][17][18].An increase of 40% to 50% in grain yield was reported when TE was applied in wheat [11].Few studies have reported a decrease of grain yield in wheat due to TE.Other studies obtained significant yield loss on Dollinco wheat variety when TE was applied a low rates (75 g a.i.ha −1 ), showing that TE was phytotoxic to wheat due to high temperatures (23˚C) [16].Similar injuries were reported by other researchers [7,21] when TE was applied in early stages of wheat development and at higher rates (>125 g a.i.ha −1 ).In this work 150 g a.i.ha −1 was applied which might explain the reduction in grain yield.In addition, temperatures in the dessert are higher than in temperate zones.Both factors might have caused phytotoxicity of Trinexapac-ethyl to wheat.In a previous experiment conducted in Caborca, Sonora, Mexico TE was applied at 0.6 g a.i.ha −1 in two stages of wheat development and no damage to wheat was found [2].This finding supports the idea that in the actual experiment the rate of the 150 g a.i.ha −1 becomes dangerous to wheat.

Growth Traits
TE had a significant effect on plant height reduction according to the stage at which it was applied.At second node the treatment had the highest plant height reduction (26.1%), while at first node and flag leaf plant height reduction was 21.1% and 6.1%, respectively; this response was similar among wheat varieties (Table 2).Stem diameter was not affected by TE application but by the variety.Rafi C97 had the highest stem diameter (3.85 mm) although it was statistically similar to Nacori C97 (3.72 mm).Rafi C97 also had the lowest plant height (78.0 cm) (Table 2).Stem diameter and plant height are traits genetically controlled.Lodging was observed only in Rayon F89 in the untreated plots.Plants in that plots had the highest plant height and lower stem diameter, morphological traits associated with lodging problems [22,23].
A decrease of plant height from 5% to 27% due to TE has been reported [7,[11][12][13][14][15][16][17][18], values that are similar to this study.According to other researchers [7,11] the greatest response was obtained when TE was applied at the second node.Other studies indicated that early TE applica-  Means followed by the same letter in a column do not differ significantly (LSD 0.05); ** Significant at 0.01 probability level; N.S. = Non-significant.
tions decreased internodes length, while late applications shortened the peduncle of the plant [12].TE has increased stem diameter of wheat [14,16] results that contrast to this experiment.The absence lodging was also reported by other studies [12,17].In this experiment no lodging might be due to the absence of rain and strong winds during the crop development and due to an adequate water management through drip irrigation.TE delayed the time flowering (2 -5 days) and grain maturity (4 -7 days) with respect to untreated plots (data not shown).Similar results were reported in other experiment [7].

Yield Components
TE did not affect the number of spikes•m −2 .In contrast, there were differences among varieties for spikes•m −2 (Table 3).Rayon F86 had the highest number of spikes• m −2 (496.2),followed by Rafi C97 (423.2),Nacori C97 and Altar C84, with 358.2 and 359.7 spikes•m −2 , respectively.TE decreased the number of grains per spike at any growth stage.At second node had the highest percentage of reduction (27.6%), whereas at first node and flag leaf, percentage of reduction was 18.2% and 10.4%, respectively.Rafi C97 was more sensitive to TE since its reduction was 44.8% and Rayon F86 was the least sensitive (Table 3).The 1000 grain weight decreased with TE application at any growth stage.The highest reduction on 1000 grains weight was observed when TE was applied at flag leaf and second node growth stage (7.6% and 7.4%).Rafi C97 and Rayon F86 were more affected by TE applied at leaf flag stage, while Altar C84 and Nacori C97 when TE was applied at first node stage (Table 3).TE did not affect the number of spikes•m −2 , which agrees to result obtained by [16] and are different to results from [14,15] when they applied TE at a similar dose.In contrast, there were differences for spikes•m −2 among varieties (Table 3).Grain yield reduction due to TE in this experiment was due to reduction number of grains in the spikes and reduction in the weight of 1000 grains, results that are opposite to [14,17].The differ-ences in yield components among wheat varieties are due to their genetic traits as reported by other researchers [24,25].

CONCLUSION
Trinexapac-ethyl at 150 g a.i.ha −1 applied at different growth stages reduced significantly plant height and decreased average grain yield in different wheat varieties.The decrease of grain yield was associated to a significant reduction in the number of grains per spike and weight of 1000 grains.Further studies are needed to clarify the effect of TE on wheat under desert conditions of Mexico.

Table 1 .
Genetic and phenological characteristics of the four wheat varieties.

Table 2 .
Effect of TE applied in different growth stages on grain yield, plant height and stem diameter in four wheat varieties.

Table 3 .
Effect of TE applied in different growth stages on spikes per m 2 , grains per spike and weight of 1000 grains in four wheat varieties.