Crop water deficits and winter wheat plant height: Integration of field experiments and modeling. G.S. McMaster 1, G.W. Buchleiter 1, M. Moragues 2, S.D. Haley 2, P.F. Byrne 2, T.J. Trout 1, and W.C. Bausch 1 USDA-Agricultural Research Service, Fort Collins, Colorado and Colorado State University, Fort Collins, Colorado GENERIC RESPONSES (Pooling 24 varieties) Since near-isolines were not used in this experiment, and there were only 3 genotypes without the semidwarfing gene (Rht-B1b or Rht-D1b), caution is needed in interpreting the results: Final plant height of Semidwarf genotypes were ~84% of Tall genotypes for all irrigation treatments except DRYLAND, where final heights of Semidwarf genotypes were 92% of Tall genotypes (Fig. 6). Similar results were noted when subdividing the Semidwarf class into Tall-, Medium-, and Short-Semidwarf groups (Fig. 7). Slightly different results were noted when pooling plant height among treatments over time between height classes (Fig. 8). Here the Tall height class differed from the 3 Semidwarf height classes in having increasingly greater plant height over time. Plant height is important in water and soil conservation, canopy energy balance, as a general indicator of plant health, and harvesting practices (McMaster et al., 2000). Genetic controls of plant height are generally well understood, and this trait can be used to demonstrate approaches in integrating genetic knowledge and crop modeling (Weiss et al., 2009). However, gene expression for different genotypes across a significant range of water deficits is still uncertain. This poster presents the first year of a field experiment exploring how different drip irrigation treatments controlling the timing and amount of soil water influence plant height of 24 winter wheat genotypes. INTRODUCTION METHODS Plant material: 24 winter wheat genotypes were grown in at the USDA-ARS Limited Irrigation Research Farm in eastern CO (Greeley). Irrigation treatments: Drip irrigation was used for 5 water treatments controlling the amount and timing of soil water. The DRYLAND treatment received no irrigation, while the FULL treatment maintained soil water content above about 80% of field capacity. Three other treatments controlled water availability from about 30-50% of field capacity, except for periods at certain developmental stages (Jointing and Anthesis) where soil water was maintained at about 80% of field capacity. These treatments were denoted as JOINTING (J), ANTHESIS (A), and J+A. Height measurements: Repeated plant height measurements were taken from early spring until final height after anthesis from the soil surface to the top of the canopy (prior to heading) or top of the terminal spikelet glumes (after heading) for between 4 and 9 places within a plot. Acknowledgments This work is part of a collaborative effort between the USDA-ARS Agricultural Systems Research Unit, USDA-ARS Water Management Research Unit, and Colorado State University Dept of Soil & Crop Sciences. Partial support was also provided by the Colorado Agricultural Experiment Station and Colorado Wheat Research Foundation. Particular gratitude goes to Debbie Edmunds and Emily Heaton. References Flintham, J. E., A. Börner, A. J. Worland, and M. D. Gale Optimizing wheat grain yield: effects of Rht (gibberellin-insensitive) dwarfing genes. J. Agric. Sci., Cambridge 128: McMaster, G.S Phenology, development, and growth of the wheat (Triticum aestivum L.) shoot apex: A review. Adv. in Agron. 59: McMaster, G.S., R.M. Aiken, and D.C. Nielsen Optimizing wheat harvest cutting height for harvest efficiency and soil and water conservation. Agron. J. 92: Weiss, A., P. S. Baenziger, G. S. McMaster, W. W. Wilhelm, and Z. I. Al Ajlouni. In press. Quantifying phenotypic plasticity using genetic information for simulating plant height in winter wheat. NJAS-Wageningen Journal Life Sciences. Web site for PhenologyMMS program describing wheat developmental stages: Contact Information USDA-ARS Agricultural Systems Research Unit 2150-D Centre Avenue, Suite 200 Fort Collins, CO USA Phone: Fax: In modeling wheat plant height based on presence/absence of Rht-B1b or Rht-D1b genes, for all genotypes both the relative growth rate and duration of shoot growth should be reduced as water deficits increase, and it is likely that this is a curvilinear response function. Preliminary data suggest that plant height of the Tall height class is reduced more under severe water deficits (DRYLAND) than Semidwarf height classes, although this could be an artifact in that very little internode elongation occurred in all genotypes under DRYLAND conditions and the spikes were nearer the top of the canopy than in the other treatments. INTEGRATION WITH MODELING HEIGHT CLASS RESPONSE Figure 3. Percent of FULL treatment plant height at final height. Figure 6. Comparing final plant height for Tall and Semidwarf height classes by irrigation treatment. Figure 4. Relative height growth rates by treatments. Figure 5. Duration of main phase of plant height growth. For the first year of data, when pooling all 24 variables, increasing water deficits reduced plant height from the Full treatment. The 3 irrigation treatments of J, A, and J+A were not significantly different and only slightly shorter than the Full treatment. Similar patterns were observed when comparing treatment height as a percent of the Full treatment. Water deficits reduced both relative growth rate and duration of growth. BACKGROUND Under minimal water deficits, the presence of semidwarfing genes (Rht-B1b or Rht-D1b) have been shown to reduce the height of tall parents by 22% via gibberellic acid insensitivity (Flintham et al., 1997). Variation in plant height among semidwarf genotypes is due to other minor genes influencing plant height inherited from tall parent genotypes. Therefore, the semidwarf class can be subdivided into Tall-, Medium-, and Short- Semidwarf subclasses. Whether height reduction of Tall and semidwarf genotypes (or classes) remains constant across environments differing in water deficits is unclear. Figure 8. Comparing plant height for Tall and 3 Semidwarf height classes over time and combining irrigation treatments. Figure 7. Comparing final plant height for Tall and 3 Semidwarf height classes by irrigation treatment. Figure 1. Plant height over time. Figure 2. Percent of FULL treatment plant height over time.