pH, abscisic acid and the integration of metabolism in plants under stressed and non-stressed conditions: cellular responses to stress and their implication for plant water relations

A paradigm for the response of plants to stress is presented which suggests that plants move towards a state of minimal metabolic activity as a stress intensifies and remain in that state until that stress is relieved. The pa radigm is based on the proposition that cells that interface with the trans piration stream employ variations on the following theme to move towards th at state. Tension on the apoplastic water opens a mechanosensitive Ca2+ cha nnel, a response that is augmented by apoplastic ABA. The resulting elevate d cytoplasmic Ca2+ deactivates a plasmalemma H+/ATPase and also activates a K+-H+ symport. The inflow of K+ and H+ depolarizes the membrane and render s the apoplast less acidic, the protons being removed to the vacuole and th e K+ ions being re-exported via the K+ outward rectifying channel. The onse t of darkness in guard and mesophyll cells deactivates the plasmalemma H+/A TPase and then the events outlined above ensue except that these cells do n ot appear to utilize either Ca2+ or ABA during these changes, In stressed c ells it is proposed that elevated cytoplasmic Ca2+ activates the release of an ABA precursor from a stored form. ABA is then released in the apoplast after export of the precursor if the activity of the K+-H+ symport has brou ght the apoplastic pH close to 7.0. It is proposed that aquaporins in the x ylem parenchyma and mesophyll cells are opened by elevated cytoplasmic Ca2 when the water potential of the transpiration stream is high so that water can be stored in the 'xylem parenchyma reservoir'. The water in this reser voir is then used to increase the water potential in the transpiration stre am when the water column is under tension and to help repair embolisms by a mechanism that resembles stomatal closure.