Plant roots. Growth, activity and interaction with soils

It is (or should be) self-evident that life on Earth depends mainly on life in earth, and in this respect soil–plant interactions are of key importance. This book brings together areas that are still often compartmented into fields such as chemical and physical aspects of soil science (where plants are still sometimes regarded as a ‘black box’ of uncertain relevance), plant physiology (now sometimes re-badged as plant functional biology), and soil microbial ecology. Agricultural scientists have, of course, rarely been guilty of ignoring soil factors in relation to plant growth and productivity. Plant ecologists sometimes have, and to some of them it's the soil that is the ‘black box’ when it comes to understanding plant population and community ecology. Models of the impacts of global climate change are increasingly including soil-based factors, for example carbon sequestration below-ground, and there is increasing awareness that as world climates change nutrient availability is likely to influence changes in plant populations and communities: plants don't just rely on water, light and CO2. This all makes Peter Gregory's book very useful at the present time. In his short Preface, Gregory relates how the book came about. Influences included a ‘career change’ in Australia, and there is certainly a strong Australian flavour in the research that is cited. He gives very sensible caveats. Thus, it is very true that roots of few plant species (mostly cereals and legumes) have been studied in detail and that many studies in the literature have been done with young plants grown in solution or non-soil media. The roots are often considered unrealistically as uniform absorbing surfaces, and bacterial and fungal associations found under field conditions–whether in agro/forest or natural plant ecosystems–are still often ignored in laboratory-based work. However, as Gregory says, this situation is changing. Diversity below-ground in terms of root structure and function, and interactions with bacterial and fungal populations at the soil–plant interface are now receiving increasing attention, with increasing contributions from study of molecular biology, both of plants and microbes. The brief introductory chapter touches on evolution of roots and sets the scene for the substantive chapters that follow. Types of roots, their structure and the architecture of root systems are dealt with first, followed by a chapter on development, growth and turnover that has a good section on methods for measuring root systems. Attention is next turned to root functions, with sections on anchorage (and anchorage failure), followed by material dear to the hearts of plant physiologists, namely uptake of water and nutrients. The chapter on the physicochemical environment is wide-ranging, focusing on temperature, gravity and other tropic responses, aeration and water, and nutrients and nutrient-related stresses; it ends on the highly topical subject of atmospheric CO2 concentrations. There is next a very thorough chapter on the biological environment in which roots grow, and which they can influence. It covers general interactions with bacteria, fungi, protozoa, nematodes and mesofauna, before turning to the major symbiotic associations with N2-fixing rhizobia and mycorrhizal fungi. Root parasites and pathogens have a separate section, as do the parasitic weeds Orobanche and Striga, and root herbivory. Although the rhizosphere is mentioned throughout, it receives a chapter to itself. This focuses mainly on chemistry–some, of course influenced by the plant (e.g. pH and release of organic acids and other root material). Given the preceding chapter on soil biology, it would have been worth making the point that for the large majority of land plants (and excluding notably the Brassicaceae and Chenopodiaceae) the natural rhizosphere is actually a much more extensive mycorrhizosphere. Therefore many experimental studies with artificial growth media may have very limited relevance to the real world of soil. When Sally Smith made this provocative comment at the international conference ‘Rhizosphere 2004’ held in Munich it did not go down well with some in the audience. Nevertheless it is true irrespective of whether or not the plant itself shows identifiable benefits directly from the symbiosis. Finally, there are two quite short chapters, one on genetic control of root system properties–an area receiving increasing attention in agricultural science–and one on ‘root systems as management tools’. This rather clever title encompasses a range of topics, including optimal root systems for nutrient acquisition and competition for resources, as in intercropping and agroforestry (and hence relevant to natural plant ecosystems), biological drilling, allelopathy, biofumigation and phytoremediation. It serves a good purpose in extending and integrating the topics covered earlier but it is rather an abrupt way to end the book. I would have liked to see a ‘Conclusions’ chapter with suggestions about possible future advances and even encouragement to those whose sights are set mainly above-ground to help open up the ‘black box’ of soil. The book is commendably free of typographical mistakes – given the nature of root anatomy I quite liked the use of ‘stellar’ for ‘stelar’ (tissue) on page 118. On the whole the illustrations are well selected and there is a selection of colour plates grouped together in the middle of the book. I am not convinced that a whole page (Fig. 6·10) had to be devoted to a quite complex data set to reinforce the point made in the text that there is no simple relationship between the extent of arbuscular mycorrhizal fungal in soil and host-plant responses. I also some had concerns about Fig. 8·5, which showed ‘yield advantage of wheat lines selected for narrow xylem vessels’, using results not published when the book was written. This had seven points from separate field experiments, averaged over two genetic backgrounds, and would have been strengthened by indication of variance about means to suggest which of the points actually gave a significant positive percentage yield advantage (mean values ranged from − 1 % to 8 %). The text (p. 262) says the project (by R.A. Richards and J.B. Passioura) ended because one of the varieties had disease problems and other traits were perceived as more important to yield improvement. This sadly goes to show that no matter how good the ‘science’, improving plant productivity is never easy. I would have preferred references to be grouped together at the end of the book, rather than at the ends of individual chapters. However, this is another minor quibble. Unfortunately, the index is poor. First, whoever did it obviously had problems in deciding whether or not include plant names. Thus, lucerne is included but not lupin, though this plant is mentioned repeatedly. Rice is there but not wheat. As another example, many references to mycorrhizas are not in the index–giving a false impression that this topic is mainly compartmented in Chapter 6, although this is not the case. This poor indexing may mean that those who first turn to the index for information about some topics will miss it–unless they decide to read the whole book. These points aside, this book is very well designed for its target readership in advanced university or college courses that cover (or should cover) soil–plant relationships, as well as soil, plant, agricultural and environmental scientists who are seeking to widen their knowledge of root growth and functions. Like this reviewer, they will find many useful references for further reading. Last, and by no means least, the very clear style of writing means that the book should be attractive to readers whose native language is not English.