The influence of photoperiod on the dry matter production of grasses and cereals

Abstract

summaryPhotoperiod 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 or under regular defoliation.Some experiments have shown that the full range of developmental effects induced by long days can equally be induced by the exogenous application of gibberellins. These findings are consistent with observed effects of gibberellins at the cellular level (loosening of cell walls, etc.) but it is too early to conclude that endogenous gibberellins play a part in the chain of events from the perception of the photoperiodic signal to the cellular changes leading to larger leaves.The fact that photoperiodic stimulation of dry‐matter production is shown by grasses originating from a range of maritime environments from 52° N northwards suggests that this response is not a specific adaptation to the cool long days of the high latitude summer. It is more likely that the related photoperiodic (short‐day) depression of growth, which facilitates the cold‐hardening of these grasses in autumn, is more important. Grasses and cereals from lower latitudes are equipped to grow well during the growing season at high latitudes, but because they are not programmed to recognize the correct photoperiodic cue in autumn, they will be subject to winterkill. CONTENTS Summary 233 I. Introduction 234 II. Experimental approaches 235 III. Stimulation of dry‐matter production 236 IV. Growth analysis 238 V. The role of gibberellins in the photoperiodic stimulation of growth 247 VI. Adaptive significance of photoperiodic stimulation of growth 248 Acknowledgements 251