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The idea of water transport pulsation in plants was put forward by a prominent Indian biologist D.Ch. Bos back in the beginning of the past century. According to his hypothesis, the ascending water flow in the plant is related to pulsation of parenchyma cells. As a result, a peristaltic wave arises, which pushes water upward. These pulsations resemble those of a contracting heart [1]. However, only decades later, these prophetic and very fruitful Bos's ideas obtained some confirmation. For example, rhythmic oscillations of exudation and water uptake by roots were observed; both processes exhibited self-oscillations with a period of 1 to 3 min [2][3][4][5][6]. Such microoscillations were observed not only in detached roots, but also in intact ones [7]. Shortperiod self-oscillations of the rates of transpiration and water uptake were detected in both detached and intact leaves [4,6,8]. Self-oscillations of the rate of ascending water flow were recorded for the stem [4,6]. Finally, phloem water transport displayed pulsations as well [6,9]. It is worth mentioning that self-oscillations of water uptake by the root or leaf and those of exudation and transpiration occur in a counter phase, i.e., the maximum of exudation or transpiration corresponds to the minimum of water uptake by the root or leaf and vice versa. This fact is very important. It indicates that water movement is not a passive process occurring along the continuous gradient of water potential. In contrast, water transport includes two successive and relatively independent (although interrelated) rhythmically alternating phases: (1) water uptake and (2) water release [2][3][4][5][6][7].It seems natural that such a complex dynamics of oscillations requires corresponding regulatory and signaling mechanisms [6]. The objective of this work was the detection of the mechanisms generating the pulsation of water transport in plant roots.Experiments were performed with detached roots of 20-to 40-day-old sunflower ( Helianthus annuus L., cv. Odesskii 63) plants grown on the half-strength Knop solution under controlled conditions (23 ° C, a relative humidity of 60%, and an illuminance of 10 klx with a 15-h photoperiod), as was described earlier [10]. Water uptake and exudation were measured simultaneously in the same root system. To this end, we used a special device shown in Fig. 1. This device is a hermetically sealed glass vessel. Its cover is supplied with (1) a curved tube, one end of which is attached to the stump of the detached root system placed in the vessel and the other end of which is attached to a graduated micropipette intended for measuring the exudation rate; (2) a curved tube directed oppositely and attached to another graduated micropipette intended for measuring the water uptake rate; and (3) the funnel for filling the device with water or tested solution. The device was hermetically sealed with vacuum putty and filled with water (control roots) or the solution of the tested agent. The rates of water uptake and exudation were measured every minute...

It is commonly known that roots not only absorb water required for plant life from ambient medium but also pump it into shoots. The pressure developed is called root pressure. In detached roots, this pressure is manifested in exudation. With accumulation of experimental results, it becomes increasingly clear that root pressure is composed of two constituents, osmotic and metabolic, differing in their nature. Osmotic constituent provides for root functioning like an osmometer (along the gradient of osmotic pressure between the xylem sap and external solution); the metabolic constituent functions with the involvement of the energydependent intracellular contractile systems that induce rhythmic microoscillations of hydrostatic pressure. The latter creates the gradient of water potential and directs water along it. Pulsation of exudation arises due to these microoscillations [1].It is commonly implied that water uptake from ambient medium is a prerequisite of its subsequent pumping to shoots. Meanwhile, an interesting phenomenon was observed recently: roots devoid of the possibility to absorb water from outside can also secrete the exudate [1][2][3][4]. In these experiments, detached sunflower roots were taken out of water, blotted thoroughly, and placed in airtight empty vessels. It seems likely that, according to the laws of physics, such roots can only evaporate water but not pump it upward. In fact, the roots evaporated water, as was evident from vessel perspiration and condensate formation on the vessel bottom, i.e., the conditions of humid chamber were produced. However, along with inevitable water evaporation, such roots, which we call conditionally as air roots (as distinct from water roots remained in the aqueous solution), started to exude, and this exudation was frequently more active and prolonged than exudation of water roots.It seems evident that exudation of air roots occurs at expense of endogenous liquid and the process of exudation results from a genetically programmed root capacity for translocation of water and solutes into shoots. In this case, water pumping resembles obviously secretion determined by a polarity of the root parenchymal cells. It is quite clear that elucidation of the mechanism of airroot exudation can near us considerably to the understanding of root-pressure mechanisms, and the air root is a suitable model for this task achievement. In fact, this model permitted us for the first time to separate absorbing and pumping root activity and to study the latter in the absence of water uptake from ambient medium, that is, to exclude root functioning like an osmometer. In this case, the osmotic constituent of the root pressure is eliminated, and exudation occurs only due to its metabolic constituent. Probably because of this reason, air-root exudation is much more sensitive than that of water roots to inhibitors of contractile systems and respiratory energy metabolism and to stimulators of metabolism: inhibitors suppress and stimulators accelerate exudation of air roots stronger than tha...

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.