This review charts the use of the concept of harvest index in crop improvement and physiology, concentrating on the literature from the last 20 years. Evidence from abstract journals indicates that the term has been applied most to small grain cereal crops and pulses, in India, Western Europe and the USA, and that it has been less useful for maize and tuber crops. Standard methods of measuring harvest index, the associated problems of measurement and interpretation, and representative values for a range of world species are reviewed. The values for modern varieties of most intensively-cultivated grain crops fall within the range 0.4 to 0.6. Variation between varieties of the same species is illustrated by trends in the harvest indices of old, outclassed and recent varieties of temperate and mediterranean wheat and barley (compared under uniform conditions); this shows a progressive increase throughout the present century, although improvement has been much slower in Australia and Canada than in the UK. In most cases, the improvement in harvest index has been a consequence of increased grain population density coupled with stable individual grain weight. The high heritability of harvest index is explored by examining its (rather weak) response to variation in environmental factors (fertilisation, population density, application of growth regulators) in the absence of severe stress. A fuller perspective is gained by reviewing aspects of the harvest index of rice, maize and tropical pulses. With rice, attention must be paid to the fact that the adhering lemma and palea (not primarily part of economic yield) can make up 20% of grain weight; and there are important interactions among biomass, grain yield and season length. Maize differs from most small grain crops in that harvest index (in N. American varieties) was already high at the start of this century, and increases in yield potential have been largely the consequence of increased biomass production. The harvest index of many pulse species and varieties tends to be low because selection has been for some yield in all seasons. Extension of the harvest index concept to express the partitioning of mineral nutrients as well as dry matter (e.g. the nitrogen harvest index) has provided a range of responses whose implications for production and breeding remain to be explored. It is concluded that even though the principal cereal crops appear to be approaching the upper limit of harvest index, and future yield gains will have to be sought by increased biomass production, there will still be a need for the concept of harvest index as a tool in interpreting crop response to different environments and climatic change.

summary Photoperiod can affect the growth and development of grasses and cereals in three distinct ways: by providing a cue for the start of the reproductive phase, by modifying the rate of reproductive development once established, and by causing changes in the rate of leaf area expansion and of dry‐matter production which are not necessarily related to reproduction. This review, which draws heavily on work with grasses from high latitudes in Scandinavia, deals mainly with this third influence of photoperiod, documenting the range of observed effects on plant development and physiology, and drawing results from other systems to help in exploring the underlying mechanisms. It is difficult to devise experimental treatments which differ only in daylength but involve realistic daily inputs of photosynthetically active radiation (PAR), to avoid possible interactions with shading responses. In the natural environment, photoperiod is confounded with the supply of radiant energy, and the spectral composition of solar radiation at the beginning and end of each day varies with latitude. It is concluded that the use of daylit phytotron chambers with daylength extension by low‐irradiance incandescent lamps is the most practical solution to this complex problem. A survey of experiments conducted under realistic irradiances showed that exposure of plants of a range of temperate and high‐latitude grasses and cereals to longer days, without increasing the supply of PAR, resulted in substantial increases (up to 200%) in dry‐matter production, and even greater increases in leaf area. These effects, which were common to vegetative and reproductive plants, tended to be most marked at lower temperatures. Growth analysis showed that, in general, this enhancement was a consequence of increases in leaf area ratio which, in turn, were caused by increases in specific leaf area rather than in leaf weight ratio. Higher rates of dry‐matter production were, therefore, a result of improved interception of PAR, although, in many experiments, net assimilation rates were lower. Photoperiodic stimulation of growth was generally associated with an unchanged or increased shoot: root ratio, reduction in the number of tillers per plant, unchanged numbers of leaves per tiller, but longer leaf sheaths and blades. Increases in blade length, in turn, were associated with increases in epidermal cell length, although there is also evidence of increased cell division, and in tissue succulence. It was concluded that photoperiodic stimulation of growth is a consequence of a positive feedback system, in which the additional photosynthate from the first leaves which develop under long days is invested in progressively larger leaves. Studies of CO2 exchange rates suggest that the observed decreases in net assimilation rate could be explained in terms of the number of cells or the amount of chlorophyll per unit of leaf area. The fact that increased growth results from increased interception of PAR indicates that, in swards, the effect will only be shown in spring...

Vegetative plants of Poa pratensis L. cv. Holt (origin 69°N) raised in short days gave large and significant increases in plant dry weight, plant height and leaf area upon exposure to continuous light, compared with 8‐h short days, at essentially identical daily inputs of radiant energy (8‐h summer daylight ± low intensity extension). For example, by the fourth harvest (after 26, 34 and 46 days at 21, 15 and 9°C, respectively), the dry weights of plants in long days were 81, 163 and 195% greater than those of the corresponding short‐day controls at the respective temperatures. Plant leaf areas in long days were between two and four times as large as control values by the end of the experiment. This was mainly due to increased leaf length caused by long‐day stimulation of cell extension and division. However, the photoperiod did not affect the partitioning of assimilates amongst leaves, culms and stolons. Most of these effects could also be brought about by exogenous gibberellin application to plants in short days. However, in contrast to the effect of long days, gibberellin treatment also induced stem internode elongation even in these vegetative plants. Examination by standard growth analysis procedures revealed that the observed increases in relative growth rate were due primarily to increased net assimilation rate followed, several days later, by increases in leaf area ratio when newly‐emerged leaves began to constitute a significant proportion of the leaf area. It is concluded that these reactions are of great adaptive significance for growth at the marginal temperatures prevailing at high latitudes.

The combined forces of developmental biologists, studying primordium initiation at the stem apex, and mathematical modellers, developing simulations of crop growth and development, have brought about considerable advances in the understanding of the control of flowering in wheat and barley. Nevertheless, there are still major gaps in this understanding including : what determines the basic rate of development (magnitude of the phyllochron or plastochron) ; how temperature and photoperiod interact to bring about the transition from vegetative to reproductive development ; and how flowering occurs eventually in the absence of inductive conditions. Although geneticists have tended to measure cereal flowering in terms of ' days from sowing or emergence to heading ', results of studies using aneuploids and molecular markers are compatible with the roles for photoperiod and lowtemperature vernalization established in purely-physiological or developmental investigations. They have also revealed the existence of ' earliness per se ' loci, whose detailed roles have yet to be established. Progress towards isolating and characterizing wheat and barley loci is hampered by the poor resolution of mapping (location to a precision of tens of thousands of base pairs). Neither of these broad approaches promises a rapid resolution of the factors controlling the induction of flowering. Two expanding areas of molecular genetics now provide potential for greater understanding of cereal flowering. First, the extensive homoeology among members of the Gramineae can be employed to establish the existence and location of genes or quantitative trait loci in rice which correspond to controlling loci in wheat or barley. Since the rice genome is 1\30th of the size of the wheat genome, the accuracy of mapping loci can be much higher, and there is greater potential for precise location of loci using techniques such as chromosome walking. With the ultimate cloning of individual genes, and the isolation of gene products, the relative roles of the 20 loci apparently involved in the induction of flowering of wheat could be explored. However, progress in the molecular genetics of Arabidopsis (the second area) may provide a more rapid route to understanding the control of flowering in cereals for several reasons : its small genome (1\4 that of rice) ; the likelihood of extensive homoeology with cereals, in spite of differences in codon usage between monocots and dicots ; the existence of a wide range of flowering-time mutants ; and the control of floral induction by a similar range of environmental factors including photoperiod and low temperature. It is likely that the MCDK (Martinez-Zapater, Coupland, Dean and Koornneef, 1994. In : Meyerowitz EM, Somerville CR. Arabidopsis. New York : Cold Spring Harbor Laboratory, 403-433) model, formulated to explain the genetic and environmental control of flowering in Arabidopsis, could be employed usefully in the formulation of experimental work on flowering in wheat and barley. This paper reviews these issues...