Effects of preliminary heat-shock treatment on accumulation of osmolytes and drought resistance in cotton plants during water deficiency

To elucidate how plants adapt to overheating followed by water deficiency, experiments with two cotton ( Gossypium hirsutum L.) cultivars (Ok-oltin and INEBR-85) were performed. Preliminary heat-shock (HS) treatment (45°C for 1.5 h) increased resistance of both cultivars to subsequent progressive soil drought [40 days without watering, with soil moisture gradually decreasing from 70 to 20% of field moisture capacity (FMC)]. HS induced accumulation of amino acids and amides and increased their contribution to the osmotic pressure (OP) of the leaf cell sap. HS also enhanced resistance to water deficiency and to overheating of the leaves, especially in cv. INEBR-85, the more drought resistant of the two cultivars. The results suggest the existence of common resistance systems to both stress factors, in particular, accumulation of amino acids and amides (mainly arginine, proline and asparagine) – their concentration in the cell sap increased up to 240-, 160- and 150-fold, respectively.     

Leaf Growth,Photosynthetic Rate,and Phytohormone Contents in Cucumis sativus Plants under Progressive Soil Drought

Effects of progressive soil drought on leaf growth, the rate of photosynthesis, and phytohormone contents were followed in the experiments with cucumber (Cucumis sativus L.) plants. Suppression of photosynthesis by drought did not immediately cause growth retardation, because the latter was observed one day earlier than the inhibition of photosynthesis. In the meantime, growth retardation could be caused by a decline in IAA and cytokinin contents, rather than ABA accumulation, because ABA accumulated when the growth has been already suppressed.     

Changes in the Levels of IAA and ABA in Cucumber Leaves under Progressive Soil Drought

Changes in the IAA and ABA contents in cucumber (Cucumis sativus L.) leaves during adaptation to drought were studied. An increase in the water-retaining capacity and heat resistance of leaves indicating the onset of adaptation occurred when the leaf growth has been already suppressed. There was a transient increase in the ABA content during the initial stage of adaptation. An increased IAA content was maintained for a longer period, throughout about two-third of the adaptation period. A second increase in the ABA content was observed before the onset of leaf permanent wilting, when IAA content already decreased. Our data suggest that not only ABA, but also IAA are involved in the development of defense responses during the adaptation to drought.     

Stimulatory effects of adrenalin and noradrenalin on root water-pumping activity and the involvement of G-proteins

To evaluate the involvement of G-proteins in the signal transduction during stimulatory action of neurotransmitters, adrenalin and noradrenalin, on root exudation and the ivolvement of G-proteins in water transport in the root and creation of the root pressure, we tested the effects of guanosinethiodiphosphate, an inhibitor of GTP-binding capacity of G-proteins, and guanosinethiotriphosphate, a stimulator of this capacity. Experiments were performed with detached roots of 5–7-day-old maize (Zea mays L.) seedlings and the mittens produced from them due to the removal of the vascular cylinder. The latter are a convenient model to study the nature of the root pressure due to its strongly limited possibility to function as an osmometer during the early step of exudation. In the “mittens,” adrenalin and noradrenalin enhanced exudation, increased its temperature coefficient (Q₁₀), root pressure, and its metabolic component much stronger than in detached roots with the vascular cylinder retained (conventionally named as “intact” roots). In control treatment (with water), guanosinethiodiphosphate retarded exudation on the average by 30% in intact roots and by 50% in mittens, simultaneously reducing Q₁₀ from 3.0 to 1.7 in intact roots and from 4.0 to 1.3 in mittens. Guanosinethiotriphosphate exerted an opposite action: it stimulated exudation on the average by 30% in intact roots and by 60% in mittens; the Q₁₀ value increased from 3.0 to 3.6 in intact roots and from 4.0 to 5.8 in mittens. These data indicate that G-proteins are involved in the control of water transport and creation of the root pressure (without any other treatments). Guanosinethiodiphosphate neutralized completely adrenalin-and noradrenalin-induced stimulation of exudation, resulting in the level substantially below the control one, especially in mittens. Guanosinethiotriphosphate enhanced stimulatory effects of both neurotransmitters, mainly in mittens, whereas its effect on intact roots was relatively weak, especially in experiments with noradrenalin. It should be emphasized that the mittens responded to both neurotransmitters and the regulators of G-protein activity much stronger than intact roots. The data obtained argue for the G-protein involvement in (1) transduction of adrenalin and noradrenalin signals stimulating root water-pumping activity and (2) the control of water transport and creation of the root pressure under normal conditions. Experiments with mittens indicate that this G-protein involvement could by mainly related to the functioning of the root cortex parenchymal cells and the formation of the metabolic component of the root pressure.