Genotypic variation in ‘early warning signals’ from roots in drying soil: intrinsic differences in ABA synthesising capacity rather than root density determines total ABA ‘message’ in cowpea ( Vigna unguiculata L.)

Summary In a comparison of six cowpea cultivars, we determined the variation in abscisic acid (ABA) production as an ‘early warning signal’ produced in response to drought stress. By imposing drought only to the upper 20 cm rooting zone, we compared the rates of ABA synthesis relative to (i) total root mass and (ii) inherent variation per unit root mass. We were able to relate the intensity of the stress response to these two factors, and determine which is quantitatively more important as the primary signal indicating responsiveness to drought stress. Plants were grown in 1.2 m long columns and a soil drying treatment imposed in such a way that that upper roots were in dry soil and deep roots in soil at field capacity. Relative water contents (RWC) of stressed plants were similar and not significantly different from those of well watered controls. However, roots accumulated ABA in the dehydrated zone, where root water content ranged from 10–12 g g −1 DW. The soil moisture contents and root ‐water contents in the dry zone were similar for each of the different varieties. However, the ABA contents were significantly different in drought‐stressed (upper) roots and ranged from 7.82 nmol g −1 DW in cv . APC 689 to 16.02 nmol g −1 DW in cv . APC 370, such that for varieties with similar overall root weights (e.g. APC 580 and APC 540) the different ABA contents were related to the capacity for ABA synthesis. The relationship between stomatal conductance and total root ABA was assessed, with a negative relation ( r = 0.90, n = 24, P = 0.05) suggesting that the intrinsic capacity of cowpea varieties for ABA synthesis could play an important role in regulating stomatal conductance in a drying soil and provide useful selection criteria for tolerance to drought stress.