Bioassay of Winter Wheat for Gibberellic Acid Sensitivity

Increasing winter wheat seedling growth would make it a better winter cover crop. Gibberellic acid (GA3) seed treatment may accomplish this by stimulating stem growth. A bioassay, mimicking field conditions, could determine the relative sensitivity of conventional and semi-dwarf cultivars. In growth chambers set for cool (10℃/4℃) and warm (21℃/4℃) conditions, wheat seeds were treated with 0 and 125 to 16,000 ppm GA3. The cultivars Goodstreak (tall or conventional) and Wesley (semi-dwarf) were compared as standards. Emergence and plant height were measured. “Goodstreak” showed a significant growth promotion at 500 ppm GA3 when seeds were dipped and 2000 ppm when GA3 was applied in-furrow under both temperature regimes. “Wesley” in general required the same or a higher dose of GA3. Separately, the seeds of nine other cultivars were treated with GA3 as the standards. Based on maximum height promotion, the most sensitive cultivars under cool conditions were Goodstreak, Harry, Millenium, and Wahoo;under warm conditions, the most sensitive cultivars were Alliance, Goodstreak, Jagalene, and Millenium. In general, the least GA3 sensitive cultivars were Arrowsmith, Scout66, and Wesley. “Buckskin” and “InfinityCL” were intermediate. The rye cultivar Rymin also was tested and showed less sensitivity to GA3 than “Goodstreak”. When 6 benzyladenine (6BA) with GA3 was applied to “Goodstreak” and “Wesley” seed, emergence, plant height and weight, and tiller formation were reduced. Wheat cultivars will respond to GA3 and differ in the amount of GA3 needed. The results of this growth chamber study will guide subsequent field trials.

Gibberellic Acid Promotes Early Growth of Winter Wheat and Rye

Winter wheat (Triticum aestivum) planting in Nebraska is recommended for mid Sep but summer crops are often harvested around Oct 1. Also, weather may delay planting. Could gibberellic acid (GA3), a growth stimulant, overcome the delayed seedling growth from late planting? Irrigated field trials were planted from 2005 to 2010 applying GA3 to wheat seed of cvs. Goodstreak and Wesley. In 2005, dip, spray and furrow GA3 applications to seed were tested. Dip and spray gave similar results. Furrow application was calculated too costly. Further tests used seed dips. In 2006 to 2008 planting, wheat was planted about 15 Sep, 1 Oct and 15 Oct. Trials planted in 2007 and 2008 included winter rye (Secale cereale) cv. Rymin. Heights in mid Nov showed a reduction with later planting. When planted about 1 Oct and 15 Oct, the delayed growth of Goodstreak, compared to planting two weeks earlier, was fully overcome by GA3 at 250 ppm. For Wesley, 1000 ppm GA3 was needed to nearly overcome later planting. Rymin rye gave an intermediate response between the two wheat cultivars. Spring and summer heights were not affected by GA3. Spring biomass, yield and harvested seed germination showed no GA3 effect. In later trials (planted in 2009 and 2010), the cytokinin 6-benzyl adenine (6BA) was added to GA3 to stimulate tiller formation. In the 1st year, 6BA at 2000 ppm depressed height, weight and yield without tiller promotion. Repeating the trial with lower rates (31 - 125 ppm 6BA) did not offer an additional advantage to GA3. The results indicate that seed application of GA3 can overcome delayed growth resulting from delayed planting of winter wheat and rye under irrigation.