Summary 221

I. Introduction 221

1. The role of sucrose in plants 221

2. C4photosynthesis 222

II. Pathway of sucrose synthesis in C4plants 223

III. Control of sucrose synthesis in C4plants 225

1. The diurnal cycle in C3plants 225

2. The diurnal cycle in C4plants 227

3. Abiotic stress and sucrose synthesis 230

4. Integration of sucrose synthesis with other metabolic pathways 232

IV. Future prospects 233

Acknowledgements 234

References 234

Sucrose has a central role in the metabolism of all higher plants. Here we discuss the location of sucrose synthesis in the leaves of C4 plants, the control of flux through the pathway and its response to environmental conditions, and finally some of the prospects for the genetic manipulation of sucrose metabolism in C4 plants. Much of our knowledge about sucrose synthesis in plants comes from C3 species; it is evident, from those studies of C4 species that are available, that there are many similarities between sucrose synthesis in C3 and C4 plants. However, some of the basic regulatory mechanisms common to both C3 and C4 plants seem to have been adapted for the specialized photosynthetic metabolism of C4 plants. There are also some other important differences: for example, some C4 species possess photosynthetic cells with little or no capacity for sucrose synthesis. For these reasons it is not always appropriate to extrapolate directly from C3 to C4 plants. Even where data are available from one C4 species, usually maize, it should not be assumed that all C4 plants are necessarily the same in every respect. Although C4 plants constitute a small fraction of the plant kingdom as a whole, they include a disproportionate number of agriculturally important species. C4 plants also have some advantages over C3 plants for studies of the regulation of cell-specific gene expression and the control of metabolism. For these reasons we suggest that they are worthy of more attention by researchers.