Viewpoint: Consideration of apoplastic water in plant organs: a reminder.

The importance of apoplastic water was confirmed for the leaves of a range of species by a comparison of tissue solute concentrations determined by the extrapolation of water potential isotherms to 100% relative water content (symplastic solute concentration at full turgor) and concentrations derived more directly from frozen / thawed tissue, where there is dilution of the symplastic water fraction by the apoplastic water fraction. A thermocouple psychrometer was used for both water potential and solute potential measurements. Parallel measurements of the apoplastic water content, estimated by the extrapolation of pressure-volume curves to zero (1 / water potential) with a pressure chamber and measurements based on the dilution method, with a thermocouple psychrometer, showed that the two methods gave similar results. This lends support to the conclusion that water is lost from the symplast and not from the apoplast of leaves when these are subjected to increasing pressure in a pressure chamber. However, where tissues or organs are air-dried the loss of water occurs from both the symplast and apoplast.

Interaction between drought and chronic high temperature during kernel filling in wheat in a controlled environment.

Wheat plants (Triticum aestivum L. 'Lyallpur'), limited to a single culm, were grown at day/night temperatures of either 18/13 degrees C (moderate temperature), or 27/22 degrees C (chronic high temperature) from the time of anthesis. Plants were either non-droughted or subjected to two post-anthesis water stresses by withholding water from plants grown in different volumes of potting mix. In selected plants the demand for assimilates by the ear was reduced by removal of all but the five central spikelets. In non-droughted plants, it was confirmed that shading following anthesis (source limitation) reduced kernel dry weight at maturity, with a compensating increase in the dry weight of the remaining kernels when the total number of kernels was reduced (small sink). Reducing kernel number did not alter the effect of high temperature following anthesis on the dry weight of the remaining kernels at maturity, but reducing the number of kernels did result in a greater dry weight of the remaining kernels of droughted plants. However, the relationship between the response to drought and kernel number was confounded by a reduction in the extent of water stress associated with kernel removal. Data on the effect of water stress on kernel dry weight at maturity of plants with either the full complement or reduced numbers of kernels, and subjected to low and high temperatures following anthesis, indicate that the effect of drought on kernel dry weight may be reduced, in both absolute and relative terms, rather than enhanced, at high temperature. It is suggested that where high temperature and drought occur concurrently after anthesis there may be a degree of drought escape associated with chronic high temperature due to the reduction in the duration of kernel filling, even though the rate of water use may be enhanced by high temperature.

Contrasting effects of chronic heat stress and heat shock on kernel weight and flour quality in wheat.

Phytotron studies were conducted to compare the potential effects of chronic high-temperatures (daily maxima of over 20˚C) and heat-shock conditions (a few days of over 32˚C), on wheat (Triticum aestivum L.) yield and quality, to form a basis for the selection of improved high-temperature tolerance in wheat. The series of heat-shock treatments were designed to provide similar heat loads, by varying the duration (number of days) of each treatment. Studies involved two cultivars, Lyallpur and Trigo 1. Both showed a reduction in kernel weight in response to chronic high day temperatures (i.e. above 18˚C), with Trigo 1 more tolerant than Lyallpur. Kernel weight of both cultivars was also reduced by short periods of heat shock, and this was most evident at day/night temperatures above 30/25˚C. There was no reduction in the germination of the lighter weight kernels formed under either chronic high temperature or heat-shock conditions. Dough strength, as judged by mixing time, declined in both genotypes with prolonged chronic high temperature, and also following the most extreme of the heat-shock treatments - Trigo 1 showed an ability to resist these changes better than Lyallpur. The heat-related decreases in dough strength were associated with decreases in the proportion of the larger molecular size glutenin (most 'unextractable'). This change in quality was not however, associated with changes in flour protein content.

Tansley Review No. 27 The control of carbon partitioning in plants.

This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer). CONTENTS Summary 341 I. Introduction 342 II. General source-sink relationships 342 III. Control at the source 345 IV. The utilization of photosynthate: sink characteristics and limitations 353 V. Vascular constraints and temporary storage 360 VI. Concluding comments 366 Acknowledgements 366 References 367.