ABSTRACT
Apart from the function of plant roots as organs for nutrient uptake, roots are also able to release a wide range of organic and inorganic compounds into the root environment. Soil-chemical changes related to the presence of these compounds and products of their microbial turnover are important factors affecting microbial populations, availability of nutrients, solubility of toxic elements in the rhizosphere, and thereby the ability of plants to cope with adverse soil-chemical conditions (1). Organic rhizodeposition includes lysates, liberated by autolysis of sloughed-off cells and tissues, as well as root exudates, released passively (diffusates) or actively (secretions) from intact root cells (Table 1; see also Chap. 2). In annual plant species, 30-60% of the photosynthetically fixed carbon is translocated to the roots, and a considerable proportion of this carbon (up to 70%) can be released into the rhizosphere (2,3), as pointed out in Chaps. 4, 6, and 12. This rhizodeposition is affected by multiple factors, such as light intensity, temperature, nutritional status of the plants, activity of retrieval mechanisms, various stress factors, mechanical impedance, sorption characteristics of the growth medium, and microbial activity in the rhizosphere. This chapter focuses on the release of water-soluble organic root exudates and highlights effects of the physiological status on root exudation and its significance for adaptations to adverse soil conditions and nutrient efficiency. Since the methods employed for collection and analysis of root exudates play an important role in the qualitative and quanti-
Table 1 Root Exudates Detected in Higher Plants
Class of compounds
Sugars
Amino acids and amides
Aliphatic acids
Aromatic acids
Miscellaneous phenolics Fatty acids Sterols Enzymes
Micellaneous
Single components
Arabinose, glucose, fructose, galactose, maltose, raffinose, rhamnose, ribose, sucrose, xylose
All 20 proteinogenic amino acids, aminobutyric acid, homoserine, cysrathionine, mugineic acid phytosiderophores (mugineic acid, deoxymugineic acid, hydroxymugineic acid, epi-hydroxymugineic acid, avenic acid, distichonic acid A)
Formic, acetic, butyric, popionic, malic, citric, isocitric, oxalic, fumaric, malonic, succinic, maleic, tartaric, oxaloacetic, pyruvic, oxoglutaric, maleic, glycolic, shikimic, cis-aconitic, trans-aconitic, valerie, gluconic
p-Hydroxybenzoic, caffeic, p-coumaric, ferulic, gallic, gentisic, protocatechuic, salicylic, sinapic, syringic
Flavonols, ftavones, ftavanones, anthocyanins, isoftavonoids Linoleic, linolenic, oleic, palmitic, stearic Campestrol, cholesterol, sitosterol, stigmasterol Amylase, invertase, cellobiase, desoxyribonuclease, ribonu-
clease, acid phosphatase, phytase, pyrophosphatase apyrase, peroxidase, protease
Vitamins, plant growth regulators (auxins, cytokinins, gibberellins), alkyl sulfides, ethanol, H+, K+ nitrate, phosphate, HC03-
tative interpretation of measured exudate data, methodological aspects are also discussed in an introductory section.
