Dennis H. Greer

Heat stress affects flowering, berry growth, sugar accumulation and photosynthesis of Vitis vinifera cv. Semillon grapevines grown in a controlled environment

High temperatures during the growing season characterise many grape growing regions in Australia and elsewhere in the world, and impact on many processes including growth and berry development. To quantify the impact of heat on the Vitis vinifera L. cv. Semillon, potted vines were grown in controlled environments and exposed to a temperature regime of 40/25°C at flowering, fruit set, veraison and mid-ripening stages. Vegetative and reproductive development was measured throughout and leaf photosynthesis and stomatal conductance tracked during heat exposures. Accumulation of soluble solids was determined during ripening. Leaf growth and stem extension were unaffected by heat whereas flowers completely abscised. Berries treated at fruit set developed normally and those treated at veraison and mid-ripening stopped expanding and sugar content stopped increasing. Photosynthesis was also affected on each occasion, with rates declining by 35% and taking 12 days to recover. Up to 10 mg carbon g (berry dry weight)–1 day–1 was required for ripening after veraison. For vines heat treated at veraison and mid-ripening, net carbon acquisition rates fell to below 4 mg carbon g (leaf dry weight)–1 day–1, which is inadequate to supply berry carbon requirements. This suggests that the impacts of heat on the ripening process can be traced back to the supply of carbon.

Does night-time transpiration contribute to anisohydric behaviour in a Vitis vinifera cultivar?

The hypothesis that vines of the Semillon wine grape variety show anisohydric behaviour was tested, i.e. that tissue hydration is unstable under fluctuating environmental conditions. Stomatal conductance and transpiration rates from leaves were measured during the day and at night. Leaf water potential (Ψl) in Semillon was negatively correlated to vapour pressure deficit (VPD) both predawn and during the day. Furthermore, Ψl fell to significantly lower values than in any of the nine other varieties examined. Night-time values of stomatal conductance (gn) and transpiration (En) in Semillon were up to four times higher than in other varieties; plants enclosed in plastic bags overnight to reduce En resulted in better plant–soil equilibration so that predawn Ψl in Semillon was the same as in Grenache. These data indicate that the hypothesis is supported, and that night-time transpiration contributes significantly to the low Ψl values in Semillon during warm, dry nights. The other contributing factor is daytime stomatal conductance (gday), which in Semillon leaves was higher than in other varieties, although the decline in gday with increasing VPD was greater in Semillon than in Shiraz or Grenache. The high values of gday were associated with high rates of transpiration (Eday) by Semillon through a day when VPD reached 4.5 kPa. When compared to other varieties, Semillon was not unusual in terms of root length density, stomatal density, xylem sap abscisic acid, or leaf electrolyte leakage. Night-time and daytime water loss and insufficient stomatal regulation therefore account for the tendency to anisohydric behaviour shown by Semillon.

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.

Changes in photosynthetic efficiency and carotenoid composition in leaves of white clover at different developmental stages

Abstract Changes in photosynthetic capacity and efficiency, and in carotenoid composition have been examined during leaf ontogeny in the pasture legume white clover, Trifolium repens (L.). Leaf chlorophyll, measured as μg–1 FW, was used as an indicator of leaf maturity, with maximum levels of the pigment denoting the mature-green phase of leaf development, and an observed decrease in chlorophyll content accompanying leaf senescence. For chlorophyll a and b, a constant ratio (a/b) between the two pigments was observed in mature-green leaves, and in the early stages of senescence while the ratio increased during the later stages of leaf senescence. Measurement of the net photosynthetic CO2 assimilation rate (PN) as μmol m–2 s–1 revealed a decrease in the photosynthetic rate that correlated with the decrease in total chlorophyll content. Measurement of chlorophyll fluorescence in vivo revealed that the optimal quantum efficiency of PSII (Fv/Fm) did not decline significantly, but the effective quantum efficiency of PSII in the light ((Fm′ – Ft)/Fm′) did decrease significantly, with a concomitant increase in non-photochemical quenching (NPQ). In terms of changes in the accessory pigments during senescence, the total pool of carotenoids decreased when expressed per unit leaf area, but not as rapidly as total chlorophyll such that the chlorophyll/carotenoid ratio decreased. The relative abundance of the carotenoids comprising the xanthophyll cycle, zeaxanthin (Z), violaxanthin (V) and antheraxanthin (A) altered during leaf ontogeny. In mature-green leaves, violaxanthin was the more abundant pigment, but as leaf senescence progressed, zeaxanthin became the most abundant pigment, and the ratio of (Z + A)/(Z + A + V) increased. These results are discussed in terms of the role of the xanthophylls cycle in the protection of PSII during leaf senescence.

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.

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Effect of temperature

Photoinhibition of photosynthesis was induced in attached leaves of kiwifruit grown in natural light not exceeding a photon flux density (PFD) of 300 μmol·m-2·s-1, by exposing them to a PFD of 1500 μmol·m-2·s-1. The temperature was held constant, between 5 and 35° C, during the exposure to high light. The kinetics of photoinhibition were measured by chlorophyll fluorescence at 77K and the photon yield of photosynthetic O2 evolution. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Photoinhibition followed pseudo first-order kinetics, as determined by the variable fluorescence (Fv) and photon yield, with the long-term steady-state of photoinhibition strongly dependent on temperature wheareas the observed rate constant was only weakly temperature-dependent. Temperature had little effect on the decrease in the maximum fluorescence (Fm) but the increase in the instantaneous fluorescence (Fo) was significantly affected by low temperatures in particular. These changes in fluorescence indicate that kiwifruit leaves have some capacity to dissipate excessive excitation energy by increasing the rate constant for non-radiative (thermal) energy dissipation although temperature apparently had little effect on this. Direct photoinhibitory damage to the photosystem II reaction centres was evident by the increases in Fo and extreme, irreversible damage occurred at the lower temperatures. This indicates that kiwifruit leaves were most susceptible to photoinhibition at low temperatures because direct damage to the reaction centres was greatest at these temperatures. The results also imply that mechanisms to dissipate excess energy were inadequate to afford any protection from photoinhibition over a wide temperature range in these shade-grown leaves.

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.

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Recovery and its dependence on temperature

Recovery of photoinhibition in intact leaves of shade-grown kiwifruit was followed at temperatures between 10° and 35° C. Photoinhibition was initially induced by exposing the leaves for 240 min to a photon flux density (PFD) of 1 500 μmol·m-2·s-1 at 20° C. In additional experiments to determine the effect of extent of photoinhibition on recovery, this period of exposure was varied between 90 and 400 min. The kinetics of recovery were followed by chlorophyll fluorescence at 77K. Recovery was rapid at temperatures of 25–35° and slow or negligible below 20° C. The results reinforce those from earlier studies that indicate chilling-sensitive species are particularly susceptible to photoinhibition at low temperatures because of the low rates of recovery. At all temperatures above 15° C, recovery followed pseudo first-order kinetics. The extent of photoinhibition affected the rate constant for recovery which declined in a linear fashion at all temperatures with increased photoinhibition. However, the extent of photoinhibition had little effect on the temperature-dependency of recovery. An analysis of the fluorescence characteristics indicated that a reduction in non-radiative energy dissipation and repair of damaged reaction centres contributed about equally to the apparent recovery though biochemical studies are needed to confirm this. From an interpretation of the kinetics of photoinhibition, we suggest that recovery occurring during photoinhibition is limited by factors different from those that affect post-photoinhibition recovery.

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

Grapevines growing in Australia suffer from high temperatures which have major effects on photosynthesis and transpiration. To learn more, gas exchange was measured over several seasons and then modelled across temperatures from 20 to 45°C and validated with independent data.

Photosynthetic and fluorescence light responses for kiwifruit (Actinidia deliciosa) leaves at different stages of development on vines grown at two different photon flux densities

Potted kiwifruit, Actinidia deliciosa [(A.Chev.) C.F.Liang et A.R.Ferguson], vines were grown at two controlled photon flux densities (PFD) of 250 and 1100 mol m–2 s–1 for 110 d to examine ontogenetic and PFD responses of photosynthesis. Oxygen evolution of detached leaf disks and PFD responses of both photosynthesis and chlorophyll fluorescence of attached leaves were measured. A range of leaves, at different nodal positions on the vines, were used and measurements were repeated over time on the same leaves as they expanded. Results showed that PFD responses of photosynthesis of the high- and low-PFD-grown vines are typical for sun/shade differentiation in Pmax and in the PFD for light saturation. The low-PFD-grown vines saturated at a PFD of 680 mol m–2 s–1 and had a mean maximum rate of 12.0 mol m–2 s–1 while high-PFD-grown vines saturated at 960 mol m–2 s–1, with a mean maximum rate of 15.2 mol m–2 s–1. There were similar differences between the two growth regimes in the electron transport rate (ETR) but non-photochemical quenching (NPQ) was higher in the low than in the high-PFD-grown vines. Young expanding leaves were characterised by low efficiency of both photochemistry and photosynthesis, low capacity for both electron transport through photosystem II and CO2 fixation capacity but by high respiration and a high capacity for non-radiative thermal dissipation. When the leaves had fully expanded, there were marked shifts towards higher photon yields, ETR and Pmax and low respiration and NPQ. In comparison with the effects of growth PFD, ontogenetic effects on development of photosynthetic competence had greater influences on the PFD responses of photochemistry and photosynthesis.