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Leaf vapor diffusive conductance (gl) and transpiration (T) measurements in an irrigated orchard of the acid lime "Tahiti" were carried out in a subtropical climatic condition in the State of São Paulo, Brazil. Measurements were made using steady-state null-balance porometers and were taken throughout the day, on several occasions from February to November, 1998. During September and October, measurements were performed on exposed and shaded young and fully expanded leaves, as well as on old leaves inside the tree canopy. The old leaves showed lower values of gl and T when compared to the other groups. In the other months, measurements were taken only with exposed and shaded fully expanded leaves. The highest values of gl and T were obtained from February to April. For exposed leaves gl was higher from early morning to midday, decreasing thereafter with some oscillation in the afternoon. Shaded leaves produced a pattern of increasing gl and T from 8:00-9:00 a.m. to midday, followed by a decrease when values matched those of the exposed leaves in the afternoon. In April, the first measurement in midmorning showed low gl and T values, increasing sharply in the exposed leaves until midday and then decreasing until the end of the afternoon. Between May and November, gl and T decreased sharply compared to the previous months. Boundary line and regression analysis were used to find the mathematical relationships between mean values of gl for the tree and photosynthetic photons flux density, temperature, and vapor pressure deficit of the air measured in the orchard. Based on this analysis, the reasons for the gl and T diurnal and annual variation patterns are discussed, with emphasis on the effects of atmospheric variables upon stomatal regulation. Realizaram-se determinações de condutância foliar à difusão de vapor (gl) e de transpiração (T) em plantas de lima ácida 'Tahiti' com o uso de porômetros de equilíbrio dinâmico e balanço nulo, em pomar irrigado em Piracicaba, SP. O clima da região é do tipo subtropical. Efetuaram-se as medidas ao longo do período diurno, em vários dias entre fevereiro e novembro de 1998, e, em setembro e outubro, em folhas em expansão e naquelas totalmente expandidas, tanto ao sol quanto à sombra, bem como em folhas de lançamentos antigos, no interior da copa. As folhas mais antigas apresentaram valores menores de gl e de T em relação aos outros grupos. Nos outros meses, as medidas foram feitas somente em folhas plenamente expandidas ao sol e à sombra. Os maiores valores de gl e T foram observados de fevereiro a abril. Para folhas ao sol, os valores de gl foram altos desde 8-9h até o meio-dia, decrescendo gradualmente à tarde, com oscilações. Para as folhas à sombra, a variação de gl ao longo do dia apresentou crescente aumento desde a primeira medida até o meio-dia, chegando a valores próximos daqueles obtidos nas folhas ao sol e diminuindo à tarde. Em abril, a primeira medida realizada às 9 h mostrou baixos valores de gl e de T para as folhas ao sol e à sombra, sendo crescentes para aquelas ao sol até o meio-dia, com decréscimo à tarde. De maio a novembro, houve uma diminuição acentuada de gl e de T em relação aos meses anteriores. Realizaram-se análises de regressão para estabelecer relações matemáticas entre os valores médios de gl e de densidade de fluxo de fótons fotossinteticamente ativos, temperatura e déficit de pressão de vapor do ar. Com base nessas análises, discutem-se as causas de ocorrência dos padrões de variação de gl e de T observados ao longo do dia e do período experimental, discutindo-se o efeito de cada variável ambiental sobre a resposta estomática.

The first step of the modelling approach consisted in designing a model of leaf diffusive conductance (gs) adapted from Ball, Woodrow and Berry (1987), expressing gs mainly as a function of Anet. The widely accepted biochemical model of C3 leaf photosynthesis of Farquhar, von Caemmerer and Berry (1980) was parameterized for well-lit, young, mature mango leaves using response curves of net photosynthesis to intercellular CO2 concentration (A-Ci curves). To obtain an analytical solution for leaf photosynthesis, we used the following additional conductance relation : Ci = Ca - Anet / gb - Anet / gs, where Ca stands for the ambient CO2 concentration and gb for the conductance across the laminar boundary layer. The resulting model of leaf photosynthesis was tested against three sets of data from gas exchange measurements obtained with a portable infrared gas analyser on leaves of 12 year old mango trees, cv. Lirfa, at two periods of the year : August, just before flowering, and from November to December, during the fruiting stage. The need to take into account the influence of light availability, leaf age and sink activity on photosynthetic capacity to upgrade the model is discussed.

In order to clarify the response of crops to sink restriction, we examined the effects of ear removal on photosynthesis of the flag leaf in wheat. For 11 days there was no difference in CO2 exchange rates between the control and the no-ear plants after ear removal, but after 11 days the rates in the no-ear plants were higher. For 11 days there was no difference in diffusive conductance, apparent quantum yield (AQY) and apparent carboxylation efficiency (ACE) and chlorophyll contents between the plants. Though there was no difference in diffusive conductance, AQY and chlorophyll contents between the plants after 11 days, ACE and ribulose-1, 5-bisphosphate carboxylase/oxygenase contents in the no-ear plants were higher than those in the control. Therefore, it was presumed that activity of CO2 fixation system in the no-ear plants was higher and that flag leaf senescence was delayed due to ear removal.

Our general objective is to design carbon-based models of flowering and fruit quality. The first step of our procedure consisted in modelling carbon gains in mango, cv. Cogshall, using the biochemical model of C3 leaf photosynthesis of Farquhar et al. (1980). Data are presented about the temperature-response of the key parameters of photosynthetic capacity which was assessed in 17-month old mango plants. The temperature-corrected photosynthesis model was then coupled to the model of leaf diffusive conductance (gs) of Ball et al. (1987). Simulation results obtained over one year on recently matured leaves from 13 to 13-year old trees show that our model is apparently robust for leaves from vegetative shoots, in orchard conditions characterized by mild seasonal changes in temperature and non-limiting water supply. However, our model failed to simulate Anet accurately during the flowering period and periods of drought, as well as when the source-sink balance is modified as, for instance, in girdled branches at different fruit loads. Using results obtained over the 5 last years about the effects of flowering, fruiting and waterstress on photosynthesis in mango, we tested three modifications of our model: 1) the fixed parameters of the model of gs were replaced by adapted values for leaves from water-stressed trees and leaves close to inflorescences; 2) the total light-driven photosynthetic electron flux (JT) was corrected to account for the existence of electron fluxes towards alternative sinks in leaves close to inflorescences; and 3) JT was corrected to account for the inhibiting effect of starch accumulation in leaves from girdled branches. Simulation results are presented and discussed in the perspective of our modelling approach. (Resume d'auteur)

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Summary A comparative study was made of the effect of leaf orientation on photosynthesis, transpiration and diffusive conductance of leaves of Festuca gigantea, F. pratensis, F. arundinacea, F. arundinacea var. atlantigena and F. marei. Conductance was also examined in F. drymeja and F. scariosa. Adaxial leaf surfaces ranged from the relatively flat (F. drymeja and F. gigantea) to the heavily corrugated (F. marei and F. scariosa). Stomatal numbers were greatest on the adaxial surface of all species, whilst the frequency on abaxial surfaces ranged from zero in F. gigantea and F. scariosa to more than 60 mm−2 in F. marei. The photosynthesis/transpiration ratio was slightly greater with the abaxial surface uppermost and greater in those species with relatively flat adaxial surfaces. The small change in the ratio with change in leaf orientation was due to the relatively greater reduction in transpiration than in photosynthesis. Species with heavily corrugated leaves transpired faster than those with flat leaf surfaces. Change in leaf orientation did not affect the ranking of the species in terms of transpiration or of leaf conductance but, because of the contrasting responses of F. gigantea in two separate experiments, ranking was not the same. There was no consistent effect of orientation on total leaf conductance but large differences were found in the relative responses of the two surfaces in some species. In those with very few abaxial stomata the adaxial surface always had greatest conductance, whichever surface was uppermost. Where both leaf surfaces had appreciable numbers of stomata, the upper surface always exhibited the greater conductance, irrespective of whether it was the adaxial or abaxial. F. arundinacea showed the greatest change in the ratio of adaxial leaf conductance in response to change in leaf orientation.

The effects of short periods of water shortage on the CO2 and water vapour exchange of attached cassava (Manihot esculenta Crantz 'M Col 72') leaves were measured. Gas-exchange and leaf water potential measurements were made in outdoor grown plants previously subjected to varying periods without water. Leaf water potentials fell to between −0.61 and −1.06 MPa and net photosynthesis and transpiration rates declined as leaf water potential decreased. Estimations of the leaf internal CO2 conductance indicated that the linear relationship established between leaf water potential and net photosynthesis is dominated by, but not completely explained by, stomatal closure.

The effect of leaf water potential on the photosynthetic rate and diffusive conductance in the leaf of rice plants was studied at different levels of leaf-air vapor pressure difference. The photosynthetic rate and diffusive conductance decreased rapidly to 50-60% and 25-40% of its maximum with decreasing leaf water potential from -1.5 to -5 bar, respectively. However, they slowly declined when leaf water potential decreased to lower than -5 bar (Fig. 1). Furthermore, the photosynthetic rate and diffusive conductance in leaves with high leaf water potential decreased with increasing leaf-air vapor pressure difference (Fig. 3) ; air humidity had a direct effect on the photosynthetic rate and diffusive conductance, independent of leaf water potential when leaf water potential was high (Figs. 3 and 4, Table 1). When leaf water potential decreased to lower than -5 bar, the photosynthetic rate and diffusive conductance were not significantly affected by the increase in leaf-air vapor pressure difference (Fig. 3). These results and the relation of diffusive conductance to photosynthetic rate at different leaf water potentials and leaf-air vapor pressure difference (Figs. 2 and 5) suggested that the cause for the decrease in photosynthetic rate differed in leaves with high and low leaf water potentials. In leaves with high leaf water potential where air humidity had a direct effect on the photosynthetic rate and diffusive conductance, the photosynthetic rate decreased due to the decrease of CO2 supply through stomata to mesophyll. On the other hand, in leaves with low leaf water potential where the diffusive conductance was considerably low, the photosynthetic rate decreased due to the decrease of photosynthetic activity in mesophyll in addition to the decrease in the CO2 supply (Fig.6).

The design and construction of a handpiece for the simultaneous measurement of photosynthetic rate and diffusive conductance of cereal leaves in almost natural conditions in the field is described. Photosynthetic rate is found from the assimilation of l4C02 by part of the leaf which is temporarily enclosed in a small hand-held chamber. This chamber also acts as a stirred, steady-state water vapour diffusion porometer, allowing the simultaneous measurement of relative humidity from which the leaf diffusive conductance is estimated. The instrument also measures the leaf and air tempera tures and incident photon flux density. The important criteria of the performance of such a handpiece are discussed, and the sensors which measure the physical variables of humidity, temperature, and photon flux density are described. An automatic sequencing system built from logic circuits which displays the measured values of these variables and times the operations is also described.

Differences in the photosynthetic activity and related leaf water balance were studied in eight rice varieties subjected to various levels of soil moisture tensions, i.e, flooded condition, values of 10, 20, 40 cbar and values at which leaf rolling began (50-62 cbar depending on the variety). The materials consisted of two lowland (IR 8 and Bosque), three tropical upland (Moroberekan, IAC 1246 and Batatais), two Japanese upland (Sensho and Rikuto Norin 24) and one dwarf mutant (Daikoku 1) varieties, respectively. The parameters observed were the photosynthetic rate (P), transpiration rate (T), leaf water potential (LWP), leaf diffusive conductance (1/r1) and leaf water content (W). The P decreased with the decrease in soil moisture with a concomitant decrease of T, LWP, 1/r1 and W, suggesting that the leaf water balance directly affects the P. Based on the responses of these parameters to soil-water stress, the material varieties were differentiated into four types. The lowland varieties performed high P and good leaf-water balance under fiooded condition. But they were very sensitive to the changes of the soil moisture, showing remarkable decreases of these parameters under moderate and severe drought. The tropical upland varieties showed rather low P under flooded condition, but they kept relatively good performance of P and leaf-water balance under moderate and severe drought conditions. The Japanese upland varieties showed intermediate values between the formers under flooded and moderate levels of the soil moisture tension (10-20 cbar), but under severe drought they showed sharp decrease of the values of these parameters as the lowland varieties did. The dwarf mu.tant Daikoku 1 performed comparatively high values of these parameters at every level of the soil moisture tension. The characteristics of these four types were discussed in view of their adaptation and plant-types.