The basis of the decrease of plant growth at low temperatures is of interest both for its economic implications and for the opportunities it provides for the study of the biochemical and biophysical mechanism of cell expansion (29). Studies of the responses of plants to low temperature have tended to concentrate either on the mechanism of injury (either by chilling or by freezing [18]) or on the effect of temperature on cell division within the meristem (1, 7). As emphasized by Green (11), tissue extension is a function of cell expansion alone. Therefore, to understand growth limitation by temperature the phenomenon must be studied in the context of the single expanding cell. Recent technical developments, such as the simultaneous application of pressure probe (13) and convenient methods for analysis of cell wall properties (4), have begun to provide information about the biophysical properties of growing tissues at cellular resolution within the extension zone. Root tissue has proved to be ideal for this type of study. ' Financial support from the Science and Engineering Research Council (GR/E 36227) is gratefully acknowledged (J. P.). 222The expanding cells are amenable to pressure probe analysis and the growing zone can be accurately described (21,22).It is becoming increasingly apparent that the physical properties of the cell wall, rather than turgor pressure changes, are responsible for growth rate changes in many of these systems (8,28). This study extends this approach to the reduction in the growth of maize roots by low temperatures (2).The experimental design is similar to that of previous studies into the cause of altered elongation in cereal roots (21,23). If water transport into the expanding cells is not limiting, their expansion can be considered in the framework of the equation (19,24):where r = growth rate, X = wall extensibility, P = turgor pressure, and Y = yield stress threshold.A combination of pressure probe and growth rate measurement was used to examine the relative contributions of these parameters to the changes in root growth rate following a reduction in root temperature. Tissue plasticity was measured using the Instron/tensiometer as a further test and additionally to further examine the relationship between in vivo wall extensibility and in vitro plasticity.Since changes in cell wall properties and/or turgor pressure will be reflected in an altered longitudinal distribution of cell length (11,23) this was analyzed and provided additional information as to the microscopic location of the elemental growth points. The biophysical changes measured in this and similar studies have their bases in discrete biochemical events that are of considerable current interest. Detailed description ofthe time course and the precise location ofthese biophysical changes with temperature should provide clues as to which biochemical processes would reward further study. MATERIALS AND METHODS Plant MaterialMaize (Zea mays cv LG11) seedlings were grown under hydroponic conditions as previously described (...

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An experiment to measure the variation in the phenological and apical development of winter wheat (cv. Avalon) in England and Scotland is described. Ten sites which ranged from Aberdeen (57-2° N), the most northerly, to Newton Abbot (50-6° N), the most southerly, were included in the survey, and at each site seed was hand-sown in midSeptember, October and November 1983. Developmental stages and sampling procedures were precisely defined to ensure uniformity in scoring by the observers at each site. Temperatures during the growing season were in line with the long-term means, though spring was cooler at all sites and summer warmer at most. The range of monthly-mean temperatures between sites was about the same as the difference between consecutive months. The method of analysis of development rates and durations was in terms of thermal time, modified by sensitivity to photoperiod and a vernalization requirement that slowed early development until a number of days of low temperatures had been experienced.In general, crops at northern sites developed more slowly than those in the south and particularly the south-west of England. There was less variation in the timing of apical stages for later sowings. Developmental rates responded linearly to temperature and photoperiod, with the base temperature increasing for later phases of development. The effect of photoperiod in modifying the rate of development was apparent for all developmental phases from emergence to anthesis, longer days accelerating development, but there was no effect on the duration of the grain-filling period. Vernalization exerted its effect solely within the phase from emergence to double ridge, and had a major influence on the variation between sites only for the first sowing.

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The initiation of leaf and spikelet primordia was studied at sites ranging in latitude from Newton Abbot (506° N) to Aberdeen (57-2° N) in crops sown in the middle of September, October and November 1983. The rate of primordium initiation tended to decrease from south to north but there were also marked differences between quite close sites.The rate of leaf initiation increased with temperature but photoperiod had little effect; the rate of spikelet initiation was affected both by temperature and by photoperiod. There were differences in the total number of leaves initiated which were only partly explained by differences in vernalization.Expressing leaf and spikelet initiation rates in terms of thermal and photo-thermal time respectively showed a constant rate of leaf initiation and a constant and more rapid rate of spikelet initiation.