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Roots of wheat seedlings (Triticum aestivum L. cv. Weibulls Starke) were cooled (+ 1 °C) for 24 h while the shoots were kept at 25°C. The treatment induced an increased water deficit in the leaves. Fresh weight, dry weight, and the uptake and distribution of potassium and calcium were measured before and after cooling. Growth, measured both as fresh weight and dry weight increase, was reduced during the cold treatment. Afterwards (at 20° C), growth recovered to nearly pre-stress rates. Analysis of the potassium fluxes in and out of the roots by "Rb- techniques showed that influx, and to a lesser extent efflux, were inhibited at low temperature. The result was a net potassium uptake raw of one-third that of unstressed plants. After the cooling period the potassium influx increased to the rate of control plants. The potassium efflux increased to one and one-half times the rate of unstressed wheat so that net uptake was negative. The increase in potassium efflux was explained by a higher permeability of the root cell membranes after cooling. The net uptake of calcium was reduced to one-third by root cooling. Contrary to potassium up- take, calcium uptake increased under post-stress conditions, partly due to a low efflux rate. During root cooling there was a redistribution of dry matter from the leaves down towards the lower part of the shoot. Afterwards the original distribution of dry matter was reestablished. The net flow of potassium and calcium followed a similar pattern as dry matter, suggesting a growth-regulated flow. [ABSTRACT FROM AUTHOR]

The transpiration response to recurrent light periods was studied in water-stressed wheat seedlings. Seedlings were stressed by three methods: addition of mannitol to the root medium, root cooling and drying of the roots in air. All three methods induced almost equal effects on transpiration regulation during alternating dark and light intervals. Exogenous abscisic acid supplied to the shoots of excised plants had qualitatively the same effect as water stress. Water stress and ABA increased the time lapse between light-on and the onset of transpiration increase and lowered the amplitude of transpiration increase and lowered the amplitude of transpiration increase in light. Weak light introduced before strong light shortened the delay times. [ABSTRACT FROM AUTHOR]

Transpiration and water uptake were measured simultaneously using a micro-wave hygrometer and a recording potometer. Wheat seedlings (Triticum aestivum L. cv. Weibulls Starke) were kept with the roots at +1°C in nutrient solution and with the shoots in air at 20°C for 24 h, a treatment which induced a decrease of about 2% in relative water content of the leaves. The induced deficit rapidly disappeared after returning to normal root temperatures (20°C.). The treatment produced after-effects on steady rates of water absorption and transpiration in light, whereas the times taken for transient changes in transpiration brought about by changes in light conditions were not affected. The after-effects were not completely eliminated 24 h after removal of stress. The magnitude of the after-effects varied with the leaf water content, though some contribution of root cooling to the after-effects cannot be excluded. The decreased water flow through the plant after removal of stress can be attributed mainly to increased stomatal resistance and not to effects on root conductivity. The increased stomatal resistance appears linked to increased membrane permeability rather than to a decrease in the active mechanism for ion uptake. [ABSTRACT FROM AUTHOR]