The relationship between stomatal aperture (a) and guard cell pressure (P-g) was measured directly in four different species (Vicia faba, Tradescantia virginiana, Ginkgo biloba and Nephrolepis exaltata) using a special cell pressure probe technique, The effect of epidermal turgor (P-ep) on this relationship was also measured in T. virginiana. The relationship was sigmoidal for V. faba and T. virginiana, but entirely convex for G, biloba and N. exaltata, Epidermal turgor,vas found to have a pronounced closing effect on stomata of T. virginiana. Maximum aperture with full epidermal turgor (0.92 MPa) was about half that with zero epidermal turgor, Also, with full epidermal turgor stomata of I: virginiana did not begin to open until P-g was more than 1.25 MPa. These characteristics were used to develop an expression for a as a function of P-g and P-ep. Results for the different species are compared and discussed in terms of possible advantages and limitations of water economy.

A decrease in steady-state leaf transpiration rate with increased vapour pressure difference between leaf and air, which is reversible and independent of leaf water status, is evidence for feedforward control of stomatal aperture (Cowan 1977), A recent survey of gas exchange data by Monteith (1995), covering 52 sets of measurements on 16 species, reported that evidence for feedforward control was rare and usually reliant on a single point. We conducted gas exchange experiments on an additional 13 species and observed an apparent feedforward response in only two. However, the response was not reversible and depended upon experimental procedure, In view of this we discuss the appropriate use of the term 'feedforward'.

ABSTRACTPressure within guard cells in strips of intact epidermis of Tradescantia virginiana was controlled with a pressure probe apparatus after the guard cells had been filled with silicone oil. Pressure was increased and decreased incrementally between 0.0 and 4.1 MPa to cause inflation and deflation of the guard cells. At steady‐state guard cell pressures, the width of the stomatal pore was recorded and plotted against pressure. The pressure required for near‐maximum aperture was 4.1 MPa. Aperture as a function of pressure was sigmoidal.

Measurements were made of net rates of CO2 assimilation in lichens at various ambient concentrations of CO2 in air and in helox (79% He, 21% O2). Because of the faster rate of CO2 diffusion in the pores of lichen thalli when filled with helox than when filled with air, a given net rate of assimilation was achieved at a lower ambient concentration of CO2 in helox. The differences were used to estimate resistances to diffusion through the gas-filled pore systems in lichens. The technique was first tested with five lichen species, and then applied in a detailed study with Ramalina maciformis, in which gas-phase resistances were determined in samples at four different states of hydration and with two irradiances. By assuming, on the basis of previous evidence, that the phycobiont in R. maciformis is fully turgid and photosynthetically competent at the smallest hydration imposed (equilibration with vapour at 97% relative humidity), and that, with this state of hydration, diffusion of CO2 to the phycobiont takes place through continuously gas-filled pores, it was possible also to determine both the dependence of net rate of assimilation in the phycobiont on local concentration of CO2 in the algal layer, and, with the wetter samples, the extents to which diffusion of CO2 to the phycobiont was impeded by water films. In equilibrium with air of 97% relative humidity, the thallus water content being 0.5 g per g dry weight, the resistance to CO2 diffusion through the thallus was about twice as large as the resistance to CO2 uptake within the phycobiont. Total resistance to diffusion increased rapidly with increase in hydration. At a water content of 2 g per g it was about 50 times as great as the resistance to uptake within the phycobiont and more than two-thirds of it was attributable to impedance of transfer by water. The influences of water content on rate of assimilation at various irradiances are discussed. The analysis shows that the local CO2 compensation concentration of the phycobiont in R. maciformis is close to zero, indicating that photorespiratory release of CO2 does not take place in the alga, Trebouxia sp., under the conditions of these experiments.

Properties and display of foliage were studied in relation to gas exchange characteristics of tropical mangrove species. Rates of assimilation of CO2 were maximal at leaf temperatures of approximately 30°C. The species operated with higher water use efficiencies than do most C3 species, and water-use characteristics became increasingly conservative with increase in the salinity tolerance of the species. Changes in three properties of leaves, i.e. inclination, area, and succulence, contributed to maintenance of leaf temperatures near air temperatures with minimal evaporative cooling. Interspecific differences in water-use characteristics, and their relationship to the maintenance of favourable leaf temperatures, could affect the competitive abilities of mangroves and the structure of mangrove forests.

The sensitivities of rate of assimilation of CO2 and rate of transpiration of water vapour to change in stomatal aperture are examined. We suggest that stomatal aperture is adapted to equalise the marginal benefit of assimilation and the marginal cost of transpiration. We show how the temporal and spatial variation of stomatal aperture is determined by the variation in external environment and leaf internal physiological properties. We indicate how the increase in cost of transpiration relative to the benefit of assimilation may determine the decline in stomatal aperture with decrease in the amount of water available to the roots of a plant, even though the plant may suffer no immediate lesion due to shortage of water.

Most current photosynthesis models, and interpretations of many wholeleaf CO2 gas exchange measurements, are based on the often unstated assumption that the partial pressure of CO2 is nearly uniform throughout the airspaces of the leaf mesophyll. Here we present measurements of CO2 gradients across amphistomatous leaves allowed to assimilate CO2 through only one surface, thus simulating hypostomatous leaves. We studied five species: Eucalyptus pauciflora Sieb. ex Spreng., Brassica chinensis L., Gossypium hirsutum L., Phaseolus vulgaris L., and Spinacia oleracea L. For Eucalyptus, maximum CO2 pressure differences across the leaf mesophyll were 73 and 160 microbar when the pressures outside the lower leaf surface were 310 and 590 microbar, respectively. Using an approximate theoretical calculation, we infer that if the CO2 had been supplied equally at both surfaces then the respective mean intercellular CO2 pressures would have been roughly 12 and 27 microbar less than the pressures in the substomatal cavities in these cases. For ambient CO2 pressures near 320 microbar, the average and minimum pressure differences across the mesophyll were 45 and 13 microbar. The corresponding mean intercellular CO2 pressures would then be roughly 8 and 2 microbar less than those in the substomatal cavities. Pressure differences were generally smaller for the four agricultural species than for Eucalyptus, but they were nevertheless larger than previously reported values.