CONTROL OF LEAF EXPANSION RATE OF DROUGHTED MAIZE PLANTS UNDER FLUCTUATING EVAPORATIVE DEMAND - A SUPERPOSITION OF HYDRAULIC AND CHEMICAL MESSAGES

We have analyzed the possibility that chemical signaling does not enti rely account for the effect of water deficit on the maize (Zea mays L. ) leaf elongation rate (LER) under high evaporative demand. We followe d time courses of LER (0.2-h interval) and spatial distribution of elo ngation rate in leaves of either water-deficient or abscisic acid (ABA )-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concent ration of ABA in the xylem sap and could be mimicked by feeding artifi cial ABA. Transpiring plants experienced a further reduction in LER, d irectly linked to the transpiration rate or leaf water status. Leaf zo nes located at more than 20 mm from the ligule stopped expanding durin g the day and renewed expansion during the night. Neither ABA concentr ation in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of t wo additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the t ranspiration rate and was observed even in well-watered plants. We sug gest that a hydraulic signal is the most likely candidate for this sec ond effect.