Guttation: path, principles and functions

Abstract

Guttation is a process of natural secretion of fluid from leaves via specialised structures called ‘hydathodes’, which are located at the tips, margins, and adaxial and abaxial surfaces of leaves. Hydathodes form natural openings but, unlike stomata, are open permanently and offer little resistance to the flow of fluid out of leaves. Each hydathode is formed of colourless cells, and appears as stomata-like pores in the epidermis or epithem, also known as ‘transfer tissue’. The cells of epithem are soft and made of loosely arranged thin-walled parenchyma cells and without chloroplast, and are involved in absorption and secretion. Internally, they are connected by tracheary endings to a large chamber with masses of thin-walled parenchymatous tissue surrounded by a sheath layer. Ultrastructurally, the epithem cells have a dense cytoplasm, numerous mitochondria, an extensive endoplasmic reticulum system, many small Golgi-derived vesicles, numerous peroxisomes, and are interconnected by abundant plasmodesmata. Functionally, there are two types of hydathodes, namely, epidermal ones that actively exude fluid, and epithemal hydathodes that passively exude fluid. Natural guttation is often observed during early morning or late hours of the day. However, it can also be induced as desired in intact or excised plants under pneumatic pressure. Earlier notions regarding harmful effects on plants of guttation have now been addressed by botanical and physiological research discoveries regarding the basic and practical utility of guttation. This knowledge could lead to new health care applications on the one hand and ease global food-security concerns on the other.