The effect of a drying top soil and a possible non-hydraulic root signal on wheat growth and yield

Research has shown that plants are adversely affected by a drying top soil and that this effect may be caused, at least partly, by a non-hydraulic (hormonal) root signal in response to soil water status and/or soil strength. Most studies done to elucidate root signal effects were limited to the short-term, typically 2 to 3 weeks of plant growth. This study was done to detect a possible non-hydraulic root signal in response to a drying top soil and to measure its effect on plant growth and production from emergence to maturity. Wheat (Triticum aestivum L.) plants were established in the growth chamber in soil-filled polyvinyl chloride tubes, 120 cm long and of an internal diameter of 10.2 cm. Soil was well fertilized and wet to field capacity at emergence when three treatments were imposed: (1) tubes were watered from the top as needed to eliminate stress (IR); (2) tubes had a constant water table at a soil depth of 100 to 120 cm, with no applied water (WT); and (3) plants grew on stored soil moisture and were rewatered when water stress developed (SW). Soil moisture profiles, soil strength, shoot water status and different shoot developmental traits as well as yield were measured. The upper part of the soil column in WT became dry and hard while roots reached the water table in less than 30 d. There was no significant difference in midday leaf water potential (LWP) and relative water content (RWC) between IR and WT, while SW plants developed comparatively lower LWP and RWC. WT as compared with IR, resulted in earlier heading, smaller flag leaf area, reduced shoot weight, reduced plant height and greater leaf epicuticular wax content. The earliest effect was seen at 20 d after emergence. Tillers developed after this date had less dry weight in WT than in IR. WT also resulted in smaller biomass but a greater harvest index than IR, so that both treatments had the same grain yield per plant. Since IR and WT treatments did not differ in leaf water status, the effect of WT on the plant was intepreted to result from a non-hydraulic root signal in response to the dry and hard top soil.