The relationship between carbon income and expenditure over the life of a leaf is described and related to the productivity benefits of altering the timing of senescence initiation. In genetic variants with delayed leaf senescence (‘stay‐greens’) deconstruction of the photosynthetic apparatus during leaf senescence is partially or completely prevented. Although the stay‐green phenotype is superficially similar in all species and genotypes, the genetic and physiological routes to the trait are diverse. In one type of stay‐green, chlorophyll catabolism is disabled. Legumes and monocots with pigment breakdown lesions are discussed. Sorghum is presented as an example of another kind of stay‐green in which perennial tendencies have been bred into a monocarpic annual crop species. Transgenic approaches are briefly discussed (enhanced endogenous cytokinins, reduced ethylene production or perception). An alternative route towards making a stay‐green phenotype is through quantitative trait mapping and marker‐assisted selection. Loci for greenness in pearl millet have been identified, some of which are associated with drought responses or flowering time. Finally the question of the limits on stay‐green as a productivity‐enhancing character is addressed.

Using cereal crops as examples, we review the breeding for tolerance to the abiotic stresses of low nitrogen, drought, salinity and aluminium toxicity. All are already important abiotic stress factors that cause large and widespread yield reductions. Drought will increase in importance with climate change, the area of irrigated land that is salinized continues to increase, and the cost of inorganic N is set to rise. There is good potential for directly breeding for adaptation to low N while retaining an ability to respond to high N conditions. Breeding for drought and salinity tolerance have proven to be difficult, and the complex mechanisms of tolerance are reviewed. Marker-assisted selection for component traits of drought in rice and pearl millet and salinity tolerance in wheat has produced some positive results and the pyramiding of stable quantitative trait locuses controlling component traits may provide a solution. New genomic technologies promise to make progress for breeding tolerance to these two stresses through a more fundamental understanding of underlying processes and identification of the genes responsible. In wheat, there is a great potential of breeding genetic resistance for salinity and aluminium tolerance through the contributions of wild relatives.

Genomic discovery in oat and its application to oat improvement have been hindered by a lack of genetic markers common to different genetic maps, and by the difficulty of conducting whole-genome analysis using high-throughput markers. This study was intended to develop, characterize, and apply a large set of oat genetic markers based on Diversity Array Technology (DArT).

SUMMARY: In this review, changes in plant gene expression in response to environmental stresses are discussed using the examples of high and low temperature treatments. While some changes may contribute to acclimatory processes which improve plant survival or performance under stress, others may be ‘shock’ responses indicative of sensitivity. The heat‐shock response, which is almost ubiquitous among eukaryotic organisms, is characterized by repression of normal cellular protein synthesis mediated at both the transcriptional and the translational level, and induction of heat‐shock protein (HSP) synthesis. There is a correlation between HSP synthesis and induced thermotolerance in plants, but the evidence for a causal relationship is not conclusive. The possible biochemical functions of some of the HSPs are now becoming apparent; they are believed to play an important role in preventing accumulation of damaged proteins in the cell during heat shock. Although no other environmental stress elicits the full heat‐shock response, certain treatments do induce synthesis of subsets of the HSPs, and the reasons for this are considered. Alterations in gene expression in response to low temperatures are more diverse and usually less dramatic than the heat‐shock response, with which they share little, if any, homology. Biochemical adjustments during cold treatment are discussed, with particular reference to those which contribute to acclimation. Several genes whose expression is induced by cold have been cloned and characterized, and in some cases it is possible to attributein vivofunctions to them; they include enzymes of lipid, carbohydrate and protein metabolism, structural proteins and putative cryoprotectants. The use of transgenic plants is further facilitating an investigation of the biochemical factors which are important in cold acclimation. Drought, osmotic stress and abscisic acid induce expression of many of the same genes as does cold treatment; it seems likely that some of the products of these genes contribute to increased freezing tolerance by protecting against intracellular dehydration. Contents Summary 1 I. Introduction 1 II. High temperature stress 3 III. Low temperature stress 10 IV. Concluding remarks 20 Acknowledgements 21 References 21