Long-Distance Signal Transmission in Trees - Communication in Plants: Neuronal Aspects of Plant Life - page 335

in much of the root-signal research in trees has been the use of split-root
culture, where individual plants are grown with the root system divided
among two or more separated soil volumes with independent moisture
content. This situation shows that having a portion of the roots in dry soil
can trigger strong stomatal closure even when shoot water potential does
not decline, strongly supporting the hypothesis of a chemical signal derived
from roots triggering stomatal closure (Gowing et al. 1990; Stoll et al. 2000;
Augè and Moore 2002; Maurel et al. 2004).
23.2.1
From Where Does ABA Come?
It has been suggested that increased ABA delivery by the fraction of roots
growinginthedryingsoil,ratherthananincreaseinshootxylemsap
ABA concentration, is the signal for stomatal closure. Many studies have
shown that, with increasing water stress, ABA is released from root tips into
the transpiration stream and transported to the leaves, where it triggers
stomatal closure (Khalil and Grace 1993; Triboulot et al. 1996; Fort et al.
1998). However, Fort et al. (1997) reported no increase in ABA delivery to
the shoots of Quercus seedlings subjected to soil drying even though root
xylem sap ABA concentration was increased and stomatal conductance was
decreased. The importance of root-originated ABA in stomatal control, as
well as the apparent sensitivity of stomatal conductance to root-originated
ABA, may vary with genotype, within genotypes as a function of pheno-
typic plasticity, or with short-term changes in environmental parameters.
Furthermore, apparent sensitivity of stomatal conductance to xylem sap
ABA concentration decreases with time in water-stressed plants (Correia
et al. 1995).
23.2.2
How Much ABA Is Involved in the Response of Trees to Drought?
According to field studies, a threshold response of stomata to xylem sap
ABA concentration may exist, above which there are disproportionate in-
creases in stomatal closure with xylem sap ABA concentration. This xylem
sap ABA concentration was 500 nM in Pinus (Sturm et al. 1998) and Cean-
othus (Tenhunen et al. 1994) but 200 nM in Prunus (Wartinger et al. 1990),
indicating the existence of species differences in the stomatal control by
ABA. Maximum ABA concentrations were only on the order of 100−150 nM
in the study of Triboulot et al. (1996) and reached values of 300−350 nM
in Tilia and 200-220 nM in Populus (Niinemets et al. 1999). Augé et al.
(2000) found different values in xylem sap ABA concentration, from 600 to