Introduction

Chapter 1 described the general features of guttation and the structural biology of hydathodes, that is, mouths of guttation, in the light of recent information. Now, in the next few pages, the principles governing guttation and its quantification will be explained in detail as because knowledge and understanding flowing from an event, physical or biological, that cannot be measured and expressed in quantities is meagre. The quantification of guttation is essential to efficiently understand its physiological functions, the ecological relevance, and the issues related to the phytopathological and agricultural implications and for the production of pharmaceuticals by guttation.

Natural guttation

As stated in the previous chapter, guttation is a natural and spontaneous physiological phenomenon of sap exudation from the tips, margins, and adaxial and abaxial surfaces of leaves of a wide range of plant species. These exudations may occur either in intact plants in natural habitats in the field or polyhouses or may be induced in the laboratory or growth chambers or certain other enclosures. In this context, it is important to consider and describe the phenomenon of guttation, root exudation, and stump bleeding in an integrated manner, reflecting on root pressure as the driving force for these processes to occur.

Guttation is usually visible when the rate of transpiration is reduced due to increased humidity levels or reduced light intensity that leads to closure of stomata, or both. When the rate of transpiration is low and absorption of water is high, the phenomenon of guttation is visible. In conditions of optimum soil nutrients and water supply, the rate of guttation can be high under high humidity levels and reduced light intensity. The predominant factors that have a direct and positive influence on the guttation process are the absence of wind and clouds and the presence of negligible humidity in the upper strata of the atmosphere to reduce the greenhouse effect; however, humidity needs to be high near the ground (Luo and Goudriaan 2000; Singh et al. 2009a). It is not always true that guttation will occur if the moisture content of the soil is high because other factors also affect guttation, such as wind, which usually has an adverse effect.