Moisture Deficits and Grain Sorghum Performance: Drought Stress Conditioning1

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Abstract
The exploitation of drought stress conditioning responses in field crops may enable substantial water savings in limited irrigation. We analyzed the effectiveness of stressing grain sorghum [Sorghum bicolor (L.) Moench] to varying degrees prior to the grain‐fill period in inducing a drought stress conditioning response during grain fill. A line source sprinkler system was used to apply two sets of treatments on the three sorghum hybrids in a field study conducted on a sandy soil (Typic Ustipsamment): Gradient irrigation (G) applied throughout all growth stages (GGG), or full irrigation (I) during the vegetative and reproductive periods, followed by gradient irrigation during the grain‐fill period (IIG). Large differences in total dry matter (DM) production and grain yields were observed due to different timings or magnitudes of water stress during grain fill (IIG) or throughout the growing season (GGG). However, the harvest index remained the same regardless of stress treatment. A substantial mobilization of stored assimilates from the stem to the grain was observed (up to 73% of grain weight) on the IIG treatment. The stress adjustment of hybrid RS 626 was significant enough to be of practical importance. Grain yields for the GGG and IIG were the same. Much greater seasonal soil water depletion occurred in the GGG than in the IIG treatment, while soil water extraction during grain fill was similar between the two treatments, although the IIG began this stage with a full profile. Dry matter and grain weight were much less sensitive to seasonal evapotranspiration (ET) deficits in the GGG treatment than in the IIG treatment, but seasonal water‐use efficiency in the GGG treatment was substantially higher. All hybrids had the same water‐use efficiency (WUE) in DM and grain production during the grain fill period. Hybrids, NB 505 and NC + 55X tended to escape the effects of grain fill stress by earlier maturity. Leaf area and canopy temperature comparisons suggest that they were not as physiologically sensitive to drought stress as hybrid RS 626 when stress was imposed with a prior conditioning stress. The results indicate that the magnitude of a drought stress conditioning response may be substantial, but will depend on the genotype used, the phenological timing of the treatment, and the irrigation regimen employed.