Introduction

As explained in Chapter 1, guttation is the process of exudation of liquid through permanently open pores called hydathodes located at the tips, edges, and surfaces of uninjured leaves in a wide range of plant species (Singh and Singh 2013; Singh 2013, 2014a,b, 2016a,b). This phenomenon is now known to play a significant role in the soil–plant–animal–environment systems (Chapter 7) and in the production of a number of recombinant proteins and pharmaceuticals, that is, drugs and vaccines, for animal and human use (Chapter 8). Despite these advances, the mechanism of this event is, however, not fully understood. What exactly triggers guttation when the environmental conditions favor low transpiration, particularly during night, is unknown. This chapter attempts to present the ‘cause and effect’ elements of events such as bleeding, oozing, exudation, and root pressure in an integrated manner to represent the whole guttation process.

Mode of guttation

The mode of guttation constitutes a sequence of events supposedly originating in the roots, travelling through the shoots, and finally culminating in exudation as guttation from the hydathodes of leaves in a liquid form. For instance, transpiration from a vigorously growing tomato plant will cease under a bell jar following saturation of the atmosphere in the jar, but continued absorption of water through the roots and its ascent under pressure from below would result in a slow exudation of water from the tips and edges of the leaves. At night or briefly even before sunset, transpiration usually does not occur because the stomata of leaves close, but guttation appears as water drops. Guttation is mostly noticed in plants growing on well-irrigated land or in humid habitat. Water enters plant roots because their water potential is lower than that of the soil, and the continuance of this process leads to its accumulation in the plant, which creates hydrostatic pressure. This pressure forces some waterto exude through the hydathodes as guttation drops. This pressure, which is essentially a ‘pushed up’ rather than a ‘pulled up’ phenomenon, provides the basis for the exudation force, giving rise to upward sap flow and finally its oozing out of leaves as guttation (Canny 1995, 2001; Singh and Singh 1989; Singh et al. 2009b).