Leaf temperature and CO2 effects on photosynthetic CO2 assimilation and chlorophyll a fluorescence light responses during mid-ripening of Vitis vinifera cv. Shiraz grapevines grown in outdoor conditions.

Responses of CO2 assimilation and chlorophyll a fluorescence to light intensity for Shiraz leaves on vines grown outdoors were examined in relation to leaf temperature. The study aimed to assess whether perturbing the carbon source, by manipulating short-term CO2 concentrations, would affect photosynthetic responses to temperature. Strong interactions occurred between leaf temperature and CO2 on photosynthetic and electron transport light responses. Most responses to temperature occurred at low to moderate CO2 and little response to temperature occurred at high CO2 . While assimilation responses accorded with increasing substrate CO2 , electron transport was inhibited by elevated CO2 . By contrast, chlorophyll a fluorescence was not affected by a temperature×CO2 interaction and CO2 had no effect on PSII quantum efficiency or photochemical quenching; whereas there was a moderate effect of temperature. Quantum efficiency of PSII was most severely reduced at low temperatures. Most photochemical quenching also occurred at low temperatures and the least at 40°C, in keeping with the warm to hot growth climate and the apparent assimilation bias towards the higher temperatures of the growing season. No changes in temperature dependency of assimilation were detected at the different CO2 concentrations, confirming sinks have a greater effect on assimilation than does the source.

Changes in photosynthesis and chlorophyll a fluorescence in relation to leaf temperature from just before to after harvest of Vitis vinifera cv. Shiraz vines grown in outdoor conditions.

Harvesting fruit from horticultural species causes a down-regulation of photosynthesis but some species can recover after harvest. The objective of this study was to assess the hypothesis that the impact of fruit removal on the photosynthetic performance of Shiraz grapevines, in relation to CO2 concentration and leaf temperature, would contribute to a depreciation in photosynthetic assimilation. To assess this hypothesis, vines that were continuously vegetative were compared with vines that were harvested when fruit were ripe. These fruiting vines had higher rates of CO2 -limited photosynthesis at all leaf temperatures compared to vegetative vines before harvest but after, photosynthetic rates were highest in vegetative vines. There were few treatment differences in CO2 -saturated photosynthesis before harvest but after, below about 30°C, the harvested vines had higher photosynthesis than the vegetative vines. Maximum rates of ribulose 1,5 bisphosphate (RuBP) carboxylation and regeneration and responses to temperature were unaffected by differences in sink demand but after harvest, maximum rates increased, but markedly more in the vegetative vines, especially at higher temperatures. This conformed to higher photosynthetic rates in the vegetative vines. There were no sink demand effects on chlorophyll a fluorescence, consistent with the evidence that the fruit sink removal probably affected Rubisco activity and performance. The conclusion that sink removal caused a depreciation in photosynthesis was sustained but the temperature had a strong modulating effect through both stomatal and non-stomatal limitations driving the depreciation in assimilation. What was less clear was why assimilation of continuously vegetative vines increased during the harvest time when there were no apparent changes in sink demand.

Impact of climate change on grape berry ripening: An assessment of adaptation strategies for the Australian vineyard.

Compressed vintages, high alcohol and low wine acidity are but a few repercussions of climate change effects on Australian viticulture. While warm and cool growing regions may have different practical concerns related to climate change, they both experience altered berry and must composition and potentially reduced desirable wine characteristics and market value. Storms, drought and uncertain water supplies combined with excessive heat not only depress vine productivity through altered physiology but can have direct consequences on the fruit. Sunburn, shrivelling and altered sugar-flavour-aroma balance are becoming more prevalent while bushfires can result in smoke taint. Moreover, distorted pest and disease cycles and changes in pathogen geographical distribution have altered biotic stress dynamics that require novel management strategies. A multipronged approach to address these challenges may include alternative cultivars and rootstocks or changing geographic location. In addition, modifying and incorporating novel irrigation regimes, vine architecture and canopy manipulation, vineyard floor management, soil amendments and foliar products such as antitranspirants and other film-forming barriers are potential levers that can be used to manage the effects of climate change. The adoption of technology into the vineyard including weather, plant and soil sensors are giving viticulturists extra tools to make quick decisions, while satellite and airborne remote sensing allow the adoption of precision farming. A coherent and comprehensive approach to climate risk management, with consideration of the environment, ensures that optimum production and exceptional fruit quality is maintained. We review the preliminary findings and feasibility of these new strategies in the Australian context.

Interaction effects of temperature and light on shoot architecture, growth dynamics and gas exchange of young Vitis vinifera cv. Shiraz vines in controlled environment conditions.

To examine the interactive effect of temperature and photon flux density (PFD) on growth dynamics and gas exchange of young Vitis vinifera L. cv. Shiraz vines, a controlled environment study was conducted by exposing vines to two different temperatures combined with either high or low PFD. Shoot growth was accelerated and the phyllochron of Shiraz leaves was hastened in the low temperature (25/12°C)×low PFD condition (350µmolm-2s-1). In early emerging leaves, leaf area was responsive to temperature whereas in later emerging leaves it was dependent on light intensity. The high temperature (32/20°C)×high PFD (700µmolm-2s-1) treatment delayed internode extension of early emerging internodes. However, low temperature×high PFD increased leaf gas exchange across the different growth stages. The net shoot carbon balance was greater for the low temperature×high PFD treatment. Dry matter accumulation was also greater in early emerging internodes irrespective of treatment. These results on young Shiraz vines indicate that 25°C is favourable to 32°C, and some growth characteristics are accelerated at low PFD while others favour higher PFD.

Potassium and Magnesium Mediate the Light and CO2 Photosynthetic Responses of Grapevines.

Potassium (K) and magnesium (Mg) deficiency are common stresses that can impact on grape yield and quality, but their effects on photosynthesis have received little attention. Understanding the diffusional and biochemical limitations to photosynthetic constraints will help to guide improvements in cultural practices. Accordingly, the photosynthetic response of Vitis vinifera cvs. Shiraz and Chardonnay to K or Mg deficiency was assessed under hydroponic conditions using miniature low-nutrient-reserve vines. Photosynthesis was at least partly reduced by a decline in stomatal conductance. Light and CO2-saturated photosynthesis, maximum rate of ribulose 1.5 bisphospate (RuBP) carboxylation (Vcmax) and maximum rate of electron transport (Jmax) all decreased under K and Mg deficiency. Likewise, chlorophyll fluorescence and electron transport were lower under both nutrient deficiencies while dark respiration increased. K deficiency drastically reduced shoot biomass in both cultivars, while root biomass was greatly reduced under both Mg and K deficiency. Taken together, these results indicate that the decrease in biomass was likely due to both stomatal and biochemical limitations in photosynthesis. Optimising photosynthesis through adequate nutrition will thus support increases in biomass with carry-on positive effects on crop yields.

Modelling the seasonal changes in the gas exchange response to CO2 in relation to short-term leaf temperature changes in Vitis vinifera cv. Shiraz grapevines grown in outdoor conditions.

Effects of temperature on the photosynthetic response of Vitis vinifera cv. Shiraz leaves to CO2 were investigated across the growing season and modelling was used to determine relationships between photosynthesis and seasonal climate. Results indicated that photosynthetic rates declined from spring to summer, conforming to the deciduous habit of grapevines. Rates of ribulose 1,5-bisphosphate (RuBP) carboxylation and regeneration increased in a temperature dependent pattern throughout the season. However, the maximum rates decreased as the season progressed. There were also marked decreases in temperature sensitivity for each of these processes, consistent with the decreases occurring faster at high compared to low temperatures. There were no correlations between the seasonal climate and each of these photosynthetic processes but the effect of day was significant in all cases. CO2 saturated rates of photosynthesis (Amax) across the season were highly correlated with the maximum rates of RuBP carboxylation and regeneration. The transition temperature between RuBP regeneration and RuBP carboxylation-limited assimilation varied across the growing season, from 23 °C in spring, 35 °C in mid-summer and 30 °C at harvest and were highly correlated with mean day temperature. This suggested dynamic control of assimilation by carboxylation and regeneration processes occurred in these grapevines in tune with the seasonal climate.

Short-term temperature dependency of the photosynthetic and PSII photochemical responses to photon flux density of leaves of Vitis vinifera cv. Shiraz vines grown in field conditions with and without fruit.

Shiraz vines grown outdoors with and without a crop load were used to determine photosynthetic and chlorophyll fluorescence responses to light across a range of leaf temperatures to evaluate the impact of presence/absence of a sink on these responses. Results indicate maximum rates of photosynthesis and light saturation in fruiting vines were biased towards higher temperatures whereas these processes in vegetative vines were biased towards lower temperatures. The maximum efficiency of PSII photochemistry was similarly biased, with higher efficiency for the vegetative vines below 30°C and a higher efficiency for the fruiting vines above. The quantum efficiency of PSII electron transport was generally higher across all temperatures in the fruiting compared with vegetative vines. Photochemical quenching was not sensitive to temperature in fruiting vines but strongly so in vegetative vines, with an optimum at 30°C and marked increases in photochemical quenching at other temperatures. Non-photochemical quenching was not strongly temperature dependent, but there were marked increases in both treatments at 45°C, consistent with marked decreases in assimilation. These results suggest demand for assimilates in fruiting vines induced an acclimation response to high summer temperatures to enhance assimilate supply and this was underpinned by comparable shifts in PSII photochemistry.

Modelling seasonal changes in the temperature-dependency of CO2 photosynthetic responses in two Vitis vinifera cultivars.

A study of photosynthesis of two grapevine cultivars, Vitis vinifera L. cv. Chardonnay and cv. Merlot in relation to the seasonal climate and internal CO2 (Ci) concentration at leaf temperatures from 15 to 45°C was undertaken. Average rates of photosynthesis at saturating CO2 concentrations and all leaf temperatures were higher in Merlot compared with Chardonnay leaves. This was attributable to higher rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and regeneration (Jmax) in Merlot leaves. These differences in photosynthesis were extended as the season progressed, partly because rates of RuBP carboxylation and regeneration of Chardonnay leaves declined markedly whereas rates for Merlot leaves remained high. Although there was no cultivar difference in the seasonal average temperature optima for assimilation (34°C) and the underlying metabolism (40°C for Vcmax and 35°C for Jmax), for temperatures above 35°C, the Merlot leaves had 50% higher rates. Across the season, activation energies of the temperature sensitivity of Vcmax and Jmax declined in response to the seasonal climate but were consistently lower in Merlot than Chardonnay. This suggested some apparent differences in the biochemistry occurred between the two cultivars that limited assimilation in Chardonnay leaves, especially at higher temperatures, but did not limit assimilation in Merlot leaves.

Photosynthetic light responses of apple (Malus domestica) leaves in relation to leaf temperature, CO2 and leaf nitrogen on trees grown in orchard conditions.

Apple trees growing in orchard conditions with two levels of soil nitrogen were used to determine photosynthetic light responses in relation to leaf temperature. In addition, leaves growing along the shoot were measured, and substrate supply for photosynthesis manipulated by altering CO2 concentrations. Results indicated highly significant interactions between leaf temperature, CO2 concentration, leaf position and nitrogen concentrations on attributes of the photosynthetic light responses. Elevated CO2 enhanced the effect of leaf temperature on the light saturated rate (Amax). There were significant effects of leaf position, and apparent photon yield (quantum efficiency) of photosynthesis, Amax and photosynthetic nitrogen use efficiency (ANUE) increased along the shoot, from leaves at the shoot base to those at apical positions; elevated CO2 enhanced these differences. A similar trend occurred with leaf nitrogen, accounting for increases in the photon yield, Amax and ANUE. It was concluded that apical leaves may have accumulated nitrogen which caused the high photosynthetic capacity and nitrogen use efficiency, as these leaves were possibly most exposed. Basal leaves, being possibly exposed to lower light intensities, had low nitrogen concentrations which accounted for their low photosynthetic light responses. This study does demonstrate that the shoot position of the apple leaves had marked effects on the photosynthetic light response and these were correlated with the leaf nitrogen content.

Temperature and CO2 dependency of the photosynthetic photon flux density responses of leaves of Vitis vinifera cvs. Chardonnay and Merlot grown in a hot climate.

Comparisons of the photosynthetic responses to light and temperature between related cultivars are important to understand how well matched they are to the climate where they are grown. Photosynthetic light responses at a range of leaf temperatures and two CO2 concentrations were measured on leaves of two grapevine cultivars (Vitis vinifera L.) Chardonnay and Merlot vines growing in field conditions. The objective was to assess the interaction between photon flux density (PFD), leaf temperature and CO2 on photosynthesis and to compare the two cultivars. Merlot leaves maintained higher light-saturated rates of photosynthesis at all leaf temperatures compared with the Chardonnay leaves. At low temperatures, a reduced photon yield offset with a high stomatal conductance accounted for the low rates of the Chardonnay leaves. At moderate to high temperatures, photon yields, PFDs at light saturation and stomatal conductances did not account for differences between Merlot and Chardonnay leaves. At elevated CO2 (800 µmol mol-1) concentrations, the differences in photosynthetic performance between the cultivars were enhanced, with 30% higher light saturated rates for Merlot compared with Chardonnay leaves. Merlot berries accumulated more sugar, consistent with published data. These results demonstrate Chardonnay, unlike Merlot, appeared to be poorly matched to the hot climate. However, considering the current market and political trends, low alcoholic wines (and, thus, low sugar grapes) should be preferred. Especially in hot climates, it is always hard to obtain such kind of wines and, thus, the most interesting agronomical challenge, especially for Chardonnay vines could be interpreted in an opposite way.

Establishing the temperature dependency of vegetative and reproductive growth processes and their threshold temperatures of vineyard-grown Vitis vinifera cv. Semillon vines across the growing season.

A hydrocooling system provided canopy temperature control of Vitis vinifera L. cv. Semillon vines at set points of 30, 35 and 40°C. The impacts on vegetative and reproductive growth over the growing season were assessed. Dynamics and rates of leaf expansion, bunch biomass and sugar accumulation were strongly affected by canopy temperatures - being highest at 30°C and lowest at 40°C. Leaf and stem biomass accumulation at 40°C was detrimentally affected but was otherwise little affected by temperature. Leaf expansion was earliest, leaf sizes greatest and rates of expansion all optimal at 30°C and all were strongly temperature dependent. Bunch biomass accumulation was earliest at 35°C but amount of biomass in bunches and rates were both highly temperature dependent and optimal at 30°C. Rates of sugar accumulation and total amounts accumulated at harvest were both highly temperature-dependent processes: fastest and greatest at 30°C. Many of the temperature-dependent processes decreased in rates and amounts linearly between 30 and 40°C. Despite the effects of temperature on bunch and berry growth, there were no treatment effects on the yield per vine. The study confirms that the threshold temperature for most processes was 35°C, where some depreciation in dry matter and sugar accumulation occurred, whereas 40°C was detrimental to all growth processes.

Temperature-dependent responses of the photosynthetic and chlorophyll fluorescence attributes of apple (Malus domestica) leaves during a sustained high temperature event.

The objective of this study was to follow changes in the temperature-dependent responses of photosynthesis and photosystem II performance in leaves of field-grown trees of Malus domestica (Borkh.) cv. 'Red Gala' before and after exposure to a long-term heat event occurring late in the growing season. Light-saturated photosynthesis was optimal at 25 °C before the heat event. The high temperatures caused a reduction in rates at low temperatures (15-20 °C) but increased rates at high temperatures (30-40 °C) and a shift in optimum to 30 °C. Rates at all temperatures increased after the heat event and the optimum shifted to 33 °C, indicative of some acclimation to the high temperatures occurring. Photosystem II attributes were all highly temperature-dependent. The operating quantum efficiency of PSII during the heat event declined, but mostly at high temperatures, partly because of decreased photochemical quenching but also from increased non-photochemical quenching. However, a further reduction in PSII operating efficiency occurred after the heat event subsided. Non-photochemical quenching had subsided, whereas photochemical quenching had increased in the post-heat event period and consistent with a greater fraction of open PSII reaction centres. What remained uncertain was why these effects on PSII performance appeared to have no effect on the process of light-saturated photosynthesis. However, the results provide an enhanced understanding of the impacts of sustained high temperatures on the photosynthetic process and its underlying reactions, notably photochemistry.

Seasonal changes in the photosynthetic response to CO2 and temperature in apple (Malus domestica cv. 'Red Gala') leaves during a growing season with a high temperature event.

Changes in the photosynthetic responses of Malus domestica cv. 'Red Gala' leaves to internal CO2 concentrations at leaf temperatures from 15°C to 40°C were followed across the growing season in field-grown trees exposed to a high temperature event in late summer. Light and CO2-saturated photosynthesis (Amax), maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and maximum rates of electron transport (Jmax) were all highest in spring, although Amax was maximal at 30°C, and Vcmax and Jmax were maximal at 40°C. All attributes declined in late summer and reached minima that coincided with the occurrence of the high temperature event. Modelling suggested many of the changes were correlated with the seasonal climate, although the measurement or current temperature had an overriding effect, especially on stomatal conductance. Marked changes in the temperature-dependency of Vcmax and Jmax occurred across the season and appeared to relate to the seasonal changes in climate, especially temperature. The reduction in photosynthesis at high temperatures was, however, partly attributable to a stomatal limitation of up to 60% at the high temperatures. Non-stomatal reductions in photosynthesis during the heat event could be attributed to detrimental effect on RuBP carboxylation and on RuBP regeneration.

Photon flux density and temperature-dependent responses of photosynthesis and photosystem II performance of apple leaves grown in field conditions.

The process of photosynthesis depends on the light, and is modulated by leaf temperature and their interaction is important to understand how climate affects photosynthesis. Photosynthetic and PSII light responses at a range of leaf temperatures were measured on leaves of apple (Malus domestica Borkh. cv. Red Gala) trees growing in field conditions. The objective was to assess the interaction between photon flux density (PFD) and temperature on these processes. Results showed leaf temperature strongly modulated the PFD-dependent response of photosynthesis and PSII performance. An interaction on photosynthesis occurred, with temperature affecting saturated rates as well as PFDs saturating photosynthesis. The efficiency of PSII electron transport (operating and maximum in light) universally declined with increasing PFD but temperature strongly influenced the response. Rates of PSII electron transport at saturating PFDs were affected by temperatures. Both photochemical quenching and non-photochemical quenching also responded strongly to temperature but at high PFDs, photochemical quenching increased linearly with decreasing temperatures while non-photochemical quenching increased curvilinearly with increasing temperatures. Modelling revealed changes in photosynthesis were positively correlated with rates of electron transport. These results greatly enhance our understanding of photosynthesis and the underpinning processes and their responses to temperature and PFD.

Does the hydrocooling of Vitis vinifera cv. Semillon vines protect the vegetative and reproductive growth processes and vine performance against high summer temperatures?

A hydrocooling system applied to Semillon (Vitis vinifera L.) grapevines as a means of protecting the vines from recurrent high temperatures. This system was assessed for impacts on vegetative and reproductive growth and development as well as for carbon economy of vines growing in vineyard conditions. The system maintained canopy temperatures at 35°C over the growing season. Leaf and bunch biomass and yield were all higher in the hydrocooled compared with control vines: the major effect was on dynamics of leaf and berry expansion. Leaf expansion was delayed and occurred over a longer duration whereas berry expansion was advanced and occurred over a longer duration than in control vines. Berry ripening was also faster in the hydrocooled vines and berries had accumulated more sugar at harvest. Leaf photosynthesis along the shoot was also higher in hydrocooled than control vines and there was a significant effect of leaf position on rates of photosynthesis of the hydrocooled vines but not with control vines. However, no differences were observed in the net shoot carbon budget. Lowered canopy temperatures were beneficial for yield and berry composition and, therefore, the cooling system warrants adoption in vineyards at risk from high temperature events during the growing season.

The impact of high temperatures on Vitis vinifera cv. Semillon grapevine performance and berry ripening.

The heat event that occurred in many parts of Australia in 2009 was the worst on record for the past decade, with air temperatures exceeding 40(°)C for 14 days. Our aim was to assess the impacts of this heat event on vine performance, including ripening, yield, and gas exchange of Vitis vinifera cv. Semillon grown in a Riverina vineyard. To assess the affect of high temperatures on Semillon grapevines, the vines were covered with a protective layer to reduce radiant heating and were compared with vines exposed to ambient conditions. The heat event had major effects on ripening; reducing the rate of ripening by 50% and delaying harvest ripeness and causing a high incidence of berry shrivel and sunburn. Yield was not affected. Photosynthesis was reduced 35% by the heat event while transpiration increased nearly threefold and was accounted for by increased stomatal conductance. The conclusion of this study was that heat events delayed ripening in Semillon berries and caused a significant reduction in berry quality. Strategies to minimize the radiant load during heat events are required and this study has confirmed a protective layer can reduce canopy temperatures and enhance berry quality.

Modelling leaf photosynthetic and transpiration temperature-dependent responses in Vitis vinifera cv. Semillon grapevines growing in hot, irrigated vineyard conditions.

BACKGROUND AND AIMS: Grapevines growing in Australia are often exposed to very high temperatures and the question of how the gas exchange processes adjust to these conditions is not well understood. The aim was to develop a model of photosynthesis and transpiration in relation to temperature to quantify the impact of the growing conditions on vine performance. METHODOLOGY: Leaf gas exchange was measured along the grapevine shoots in accordance with their growth and development over several growing seasons. Using a general linear statistical modelling approach, photosynthesis and transpiration were modelled against leaf temperature separated into bands and the model parameters and coefficients applied to independent datasets to validate the model. PRINCIPAL RESULTS: Photosynthesis, transpiration and stomatal conductance varied along the shoot, with early emerging leaves having the highest rates, but these declined as later emerging leaves increased their gas exchange capacities in accordance with development. The general linear modelling approach applied to these data revealed that photosynthesis at each temperature was additively dependent on stomatal conductance, internal CO(2) concentration and photon flux density. The temperature-dependent coefficients for these parameters applied to other datasets gave a predicted rate of photosynthesis that was linearly related to the measured rates, with a 1 : 1 slope. Temperature-dependent transpiration was multiplicatively related to stomatal conductance and the leaf to air vapour pressure deficit and applying the coefficients also showed a highly linear relationship, with a 1 : 1 slope between measured and modelled rates, when applied to independent datasets. CONCLUSIONS: The models developed for the grapevines were relatively simple but accounted for much of the seasonal variation in photosynthesis and transpiration. The goodness of fit in each case demonstrated that explicitly selecting leaf temperature as a model parameter, rather than including temperature intrinsically as is usually done in more complex models, was warranted.

Interactions between light and growing season temperatures on, growth and development and gas exchange of Semillon (Vitis vinifera L.) vines grown in an irrigated vineyard.

High-light intensities and temperatures of the warm climate regions of Australia and elsewhere have a major effect on the growth and development of grapevines (Vitis vinifera L.). The objective of this research was to assess interactions between the light and seasonal temperatures by shading some vines and comparing these with vines exposed to high-light intensities. Canopy temperatures were monitored using infrared radiometers and budbreak, phenology, growth, yield, berry ripening and gas exchange determined over three growing seasons. Results showed canopies were generally about 4 °C cooler than air and shading extended this cooling. Irradiance, irrespective of seasonal temperatures, had no effect on time of budbreak, shoot phenology, stem growth, yield and bunch fresh weights while bunch and leaf dry weights were reduced in low-light. Bunch ripening was initially delayed by low-light but thereafter the ripening process was highly temperature-dependent. Rates increased linearly with increasing temperature in both low and high-light and were optimal at about 35 °C. Maximum photosynthetic capacity was impaired by low irradiance, in accordance with shade leaf attributes, and attributable to stomatal closure. No effects of the low photosynthetic capacity apparently carried-over to sugar accumulation, consistent with the strong sink capacity of bunches.

Modelling photosynthetic responses to temperature of grapevine (Vitis vinifera cv. Semillon) leaves on vines grown in a hot climate.

Field measurements of photosynthesis of Vitis vinifera cv. Semillon leaves in relation to a hot climate, and responses to photon flux densities (PFDs) and internal CO(2) concentrations (c(i) ) at leaf temperatures from 20 to 40 °C were undertaken. Average rates of photosynthesis measured in situ decreased with increasing temperature and were 60% inhibited at 45 °C compared with 25 °C. This reduction in photosynthesis was attributed to 15-30% stomatal closure. Light response curves at different temperatures revealed light-saturated photosynthesis optimal at 30 °C but also PFDs saturating photosynthesis increased from 550 to 1200 µmol (photons) m(-2)s(-1) as temperatures increased. Photosynthesis under saturating CO(2) concentrations was optimal at 36 °C while maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (V(cmax)) and potential maximum electron transport rates (J(max)) were also optimal at 39 and 36 °C, respectively. Furthermore, the high temperature-induced reduction in photosynthesis at ambient CO(2) was largely eliminated. The chloroplast CO(2) concentration at the transition from RuBP regeneration to RuBP carboxylation-limited assimilation increased steeply with an increase in leaf temperature. Semillon assimilation in situ was limited by RuBP regeneration below 30 °C and above limited by RuBP carboxylation, suggesting high temperatures are detrimental to carbon fixation in this species.

Stomatal response of an anisohydric grapevine cultivar to evaporative demand, available soil moisture and abscisic acid.

Stomatal responsiveness to evaporative demand (air vapour pressure deficit (VPD)) ranges widely between species and cultivars, and mechanisms for stomatal control in response to VPD remain obscure. The interaction of irrigation and soil moisture with VPD on stomatal conductance is particularly difficult to predict, but nevertheless is critical to instantaneous transpiration and vulnerability to desiccation. Stomatal sensitivity to VPD and soil moisture was investigated in Semillon, an anisohydric Vitis vinifera L. variety whose leaf water potential (Ψ(l)) is frequently lower than that of other grapevine varieties grown under similar conditions in the warm grape-growing regions of Australia. A survey of Semillon vines across seven vineyards revealed that, regardless of irrigation treatment, midday Ψ(l) was dependent on not only soil moisture but VPD at the time of measurement. Predawn Ψ(l) was more closely correlated to not only soil moisture in dry vineyards but to night-time VPD in drip-irrigated vineyards, with incomplete rehydration during high night-time VPD. Daytime stomatal conductance was low only under severe plant water deficits, induced by extremes in dry soil. Stomatal response to VPD was inconsistent across irrigation regime; however, in an unirrigated vineyard, stomatal sensitivity to VPD-the magnitude of stomatal response to VPD-was heightened under dry soils. It was also found that stomatal sensitivity was proportional to the magnitude of stomatal conductance at a reference VPD of 1kPa. Exogenous abscisic acid (ABA) applied to roots of Semillon vines growing in a hydroponic system induced stomatal closure and, in field vines, petiole xylem sap ABA concentrations rose throughout the morning and were higher in vines with low Ψ(l). These data indicate that despite high stomatal conductance of this anisohydric variety when grown in medium to high soil moisture, increased concentrations of ABA as a result of very limited soil moisture may augment stomatal responsiveness to low VPD.