The Problem of Water Outflow from Roots

Abstract

In general, it is assumed that plant roots are merely organs of water absorption and that water is conducted from root to shoot. It is not taken into consideration that roots may be able to exude water and then water streams in the reverse direction from the leaves to the roots. Recently, it has been stated, though not without opposition, that roots of spermatophytes should be able to effect active water secretion excellently, provided the leaves absorb water from a moist atmosphere. This would be a process analogous to guttation, but in the opposite direction. In several papers Breazeale et al 19611953. (1950) described experiments made at the Arizona Agricultural Experimental Station which in their opinion demonstrated active water secretion from the roots. They assert to have demonstrated that liquid water vigorously leaks out from roots if the shoots are kept in an artificial fog atmosphere, and even, though less strongly, if the shoots are located in air of high water vapour saturation. They claim to have observed this water outflow from root system hanging freely in an empty glass flask as well as from roots dipping into liquid, and also, from plants rooted in soil. In the latter case soil moisture in the rhizosphere is said to have increased due to active water secretion up to field capacity and even above it (Breazeale et al. 1950). If plants with their roots were sealed in flasks, the exuded water gathered within the containers, or increased the volume of the solution into which the roots were dipping. The authors believed they had proved that the pressure induced within the plants owing to water uptake into leaves was greater than that after water absorption by roots. The leaf was said to be a much more efficient organ for water absorption from the liquid as well as from the vapour phase. It was reported that tomato shoots absorbed water from the vapour phase down to 75—85% relative humidity (Breazeale et al. 1955). The pressure supposedly exerted by tomato roots secreting water was measured by means of manometer tubes inserted into the bottom of flasks in which plants were sealed with roots dipping into solution. After 3 to 4 days the water column in the manometers had reached 100 cm., after 14 days 140 cm. (Breazeale 1953 and McGeorge 1953).