Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0-25 cm depth in sand and 0-45 cm depth in loam) dropped to -60 or -70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03-0.04 vs. 0.06-0.08 mmol m(-3)) and midday leaf ABA concentration increased (250-300 vs. 400-450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75-0.88 vs. 0.64-0.70 mol m(-2) s(-1)). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in 'wet' soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4-5.2 vs. 0.6-4.0 mm/day of potential evapotranspiration).