Ionic and pH signalling from roots to shoots of flooded tomato plants in relation to stomatal closure

Soil flooding damages shoot systems by inhibiting root functioning. An example is the inhibition of water uptake brought about by decreased root hydraulic conductance. The extent of any resulting foliar dehydration this causes is limited by partial stomatal closure that begins within 4 h and is maintained for several days. Root to shoot signals that promote closure in flooded tomato plants have remained elusive but may include changes in solute delivery to the shoot by transpiration. Accordingly, we examined total osmolites and selected mineral ions in samples of xylem sap flowing at rates approximating whole plant transpiration. After 2.5 h flooding,delivery of total osmolites and of PO4 3-SO4 2-Ca2+K+NO3 – and H+strongly decreased while Na+ remained excluded. Several hours later, deliveries of osmolites, PO4 3-, SO4 2-, Ca2+, and Na+ rose above control values, suggesting that, after approximately 10 h, root integrity became degraded and solute uptake deregulated. Deliveries of NO3 – remained below control values. Reducing or eliminating the supply of K+ to detached leaves to test the potential of decreased K+ delivery to close stomata proved negative. Decrease in H+ delivery was associated with sap alkalisation. However, raising the pH of buffer from 6.0 or 6.5 to 7.0 did not close stomata when tested in the presence of abscisic acid (ABA) at a concentration (10 (mu)mol m^–3) typical of the transpiration stream of flooded plants. It is concluded that despite their rapidity and scale, negative messages in the form of increased pH and decreased solute delivery from roots to shoots are, themselves, unlikely initiators of stomatal closure in flooded tomato plants.