Daylength (photoperiod) is an important external stimulus; most plants and animals have a means of determining the daylength and respond by what appear to be seasonal changes in growth and development. Changes in photoperiod provide a signal for predicting forthcoming conditions, providing an opportunity for organisms to co-ordinate their activity (especially reproduction) with the anticipated environmental conditions. This book aims to focus on how this is achieved by expounding current understanding of the mechanisms that operate in plants, insects and mammals to co-ordinate their physiology and development to seasonal changes.

The first six chapters deal with plants. Chapter 1 describes the theoretical models proposed for flowering in the long-day plant Arabidopsis thaliana, together with current understanding of the molecular mechanisms involved and the genetics of flowering time regulation. Chapter 2 deals with the response in the short-day species Oryza sativa, rice. Some of the genes here are homologous with those identified in A. thaliana and similar pathways have been elucidated. Chapter 3 details the measurement of the length of the dark period by plants of Pharbitis nil, morning glory. Chapter 4 outlines the progress made in studies of photoperiodic control of flowering using Lemna paucicostata. Chapter 5 addresses the regulation of flowering and the process of cold acclimation, as well as outlining the photoperiodic induction of dormancy and release from dormancy of buds. These temperature-mediated responses are different processes from photoperiod-mediated ones, and require meristematic cells to experience cold temperatures; unlike photoperiod-mediated responses, the chilling signal cannot be transported. Plants that grow vegetatively in year 1 and flower in year 2 in conditions that did not induce flowering in year 1 (winter annuals or biennials) provide the opportunity to combine studies on vernalization (a temperature response) and induction of flowering (a photoperiod response). A. thaliana has again been used to study vernalization requirements and these are described in chapter 6. Chapter 7 extends the broad coverage to fungi, with consideration of photoperiodism, light reception, daily rhythms and temperature cycle, with much evidence from studies on Neurospora crassa, which has been shown to measure the length of the dark period.

The second section of the book deals with photoperiodism in invertebrates. The chapters consider molluscs, copepods and insects, and deal variously with migration, diapause effects on morphology, polyphenism as well as perception of light, the role of clock genes and molecular mechanisms well outlined in a short introductory overview. Chapter 8 describes the cuing of animals for reproduction in longer days and some for better cold tolerance in short days. Elucidation of mechanisms appears to be very much in its infancy. Chapter 9 deals with copepods: vertical migration, feeding and reproduction are all affected by light, temperature and salinity. The authors also point out possible implications of light pollution from man. Given the importance of copepods in fish nutrition, an understanding of copepod behaviour has clear importance. In chapter 10, the behaviours of several insect species are examined focusing on migration, interactions between diapause (dormancy) and temperature, feeding and diapause and the effects of light stimulation. In chapter 11, the 19 insect species in which photoreceptors have been identified are listed. The photoreceptors are all in the head: the brain, compound eye or stemmata. Two photopigments have been identified and the role of clock gene expression is described. Chapter 12 shows how photoperiod provides the cue for diapause, with the brain initiating the control cascade(s). Known genes involved are listed in an excellent table. Chapter 13 examines seasonal changes in morphology such as colour, wing length and reproductive capacity with associated changes in the body structure. There are interactions of photoperiod with temperature, and the background colour of the habitat and food quality. Chapter 14 describes the seasonal changes in aphids from the early asexual reproduction to the later sexual stage leading to egg production with the production of female diapausing eggs, in the context of subsequent research and progress in understanding.

The third section of the book deals with photoperiodism in vertebrates, with chapters on fish, amphibians, reptiles, birds and mammals. Chapters 15–17 deal with photosensitivity, photoperiodism and photorefractoriness, which are discussed with examples, together with what is currently understood of the links between daylength detection and implementation of a response. Chapter 18 gives an outline of the current understanding of this field together with suggestions for the areas where modern techniques can be of most use to further understanding. Chapter 19 deals with photoperiodic regulation of energy balance, immune function and behaviour, although the considerable detail relates almost exclusively to studies on the Siberian hamster. Chapter 20 provides a useful general discussion, which ranges from small rodents to sheep, whose responses to season must vary due to differences in gestation time. Reference is also made to plants and non-mammalian animals in the search for overall coherence. Chapter 21 outlines the current understanding of signals and signal cascades, which respond to season and daylength.

The book concludes with a brief epilogue entitled ‘Future Directions’, which summarizes some of the foregoing detailed chapters. However, the observation that fungi, plants and animals all have photoperiodic responses, particularly of reproduction, surely prompts the question: what commonality is there in the mechanisms? The responses are often strikingly similar in the different groups of organisms, but are the molecular pathways the same? Is the genetic control system homologous and are the genes conserved? Unfortunately, this comparative approach is not made. We have a series of essays providing much detailed information, like we might have a list of ingredients for a cake, but no one has baked it. A tantalizing opportunity missed. Reductionism gets us so far, but we also desperately need ‘the big picture’. The lack of links with the practical commercial world is also disappointing. The chapter on rice and the development of varieties with different responses to photoperiodism is well covered, but modification of daylength for specific purposes has been common in commercial agriculture for over 50 years for both plants and animals. Comments in the Epilogue perhaps also emphasize the cost of large animal studies, and, worryingly, the dwindling number of laboratories where such studies might be undertaken. Notwithstanding these criticisms, the book does document a wealth of knowledge across a wide range of organisms. Given that we are entering a period of rapid environmental changes, a better understanding of the metabolism and physiology of the plants and animals upon which we are directly dependent, and of their pests, is critical. Unfortunately, most research workers will only be immediately interested in less than a third of the chapters in this book. However, if they at least dip into others, then with a better appreciation of the ubiquity of photoperiodism the bigger questions might be prompted and answers might emerge.