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Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in 1-m-long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20-d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as-yet unidentified inhibitor of stomatal opening. [ABSTRACT FROM AUTHOR]

Maize plants were grown in l-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than wellwatered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants. Water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0.25 g cm[SUP-3] (a bulk soil water potential of between -0.2 and -0.3MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying. [ABSTRACT FROM AUTHOR]

Potted seedlings of four lines of maize and Sorghum of differing drought tolerance were subjected to a single soil drying cycle and were only rewatered when the plants showed the first signs of wilting. Other plants remained well-watered throughout the experimental period. As plant water potentials decreased in the unwatered plants of three of the lines investigated (Sorghum Piper and M35-1, V-4146 and maize Farz 27), endogenous levels of farnesol-like antitranspirants increased. Closure of stomata correlated well with the increase in endogenous antitranspirant. In the fourth line (Sorghum M35-1, V-4184), stomata did not close as the level of plant water stress increased, although leaf diffusion resistance of even the well-watered plants of this line was quite high. In this line, there was no consistent relationship between plant water stress and antitranspirant level or between stomatal behavior and antitranspirant level. The involvement of farnesol-like antitranspirants in the control of stomatal behavior in water-stressed plants is discussed. [ABSTRACT FROM AUTHOR]

Stomatal conductance of individual leaves, was measured In a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration In the same leaves. Stomatal conductance showed a tight relationship With xylem ABA, but not with the current leaf water status or with concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non-compacted and irrigated soil, and to plant-to-plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions. [ABSTRACT FROM AUTHOR]

During two seasons, ABA concentrations were monitored in roots, leaves and xylem sap of field-grown maim The water status of soil and plant was also measured Plants were grown on plots with compacted or non-compacted soil, which were irrigated or remained unwatered, ABA concentration in the xylem sap before dawn and in the roots increases 25-fold and five-fold, respectively, as the soil dried, with a close correlation with the soil water status, but with no clear effect of the soil structures. In contrast to the results of several laboratory experiments, no appreciable increase in xylem [ABA] and reduction in stomatal conductance were observed with dehydration of the part of the root system located in soil upper layers. These responses only occurred when the water reserve of the whole soil profile was close to depletion and the transpiration declined. Xylem (ABA] measured during the day was appreciably higher in the compacted treatment than in non-compacted treatment, unlike that measured before dawn. Since a mechanical message is unlikely to undergo such day-night alterations, we suggest that this was due to a faster decrease in root water potential and water flux in the compacted treatment, linked to the root spatial arrangement These results raise the possibility that ABA concentration in the xylem sap could be controlled by two coexisting mechanisms: (1) the rate of ADA synthesis in the roots linked to the soil or root water status, as shown in laboratory experiments; (2) the dilution of ABA in the water flow from roots, which could be an overriding mechanism in field conditions. This second mechanism would allow the plant to sense the water flux through the root system. [ABSTRACT FROM AUTHOR]

Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots in unlikely. [ABSTRACT FROM AUTHOR]