Simon J. Clarke

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.

Nocturnal and daytime stomatal conductance respond to root-zone temperature in ‘Shiraz’ grapevines

Background and Aims Daytime root-zone temperature may be a significant factor regulating water flux through plants. Water flux can also occur during the night but nocturnal stomatal response to environmental drivers such as root-zone temperature remains largely unknown. Methods Here nocturnal and daytime leaf gas exchange was quantified in ‘Shiraz’ grapevines (Vitis vinifera) exposed to three root-zone temperatures from budburst to fruit-set, for a total of 8 weeks in spring. Key Results Despite lower stomatal density, night-time stomatal conductance and transpiration rates were greater for plants grown in warm root-zones. Elevated root-zone temperature resulted in higher daytime stomatal conductance, transpiration and net assimilation rates across a range of leaf-to-air vapour pressure deficits, air temperatures and light levels. Intrinsic water-use efficiency was, however, lowest in those plants with warm root-zones. CO2 response curves of foliar gas exchange indicated that the maximum rate of electron transport and the maximum rate of Rubisco activity did not differ between the root-zone treatments, and therefore it was likely that the lower photosynthesis in cool root-zones was predominantly the result of a stomatal limitation. One week after discontinuation of the temperature treatments, gas exchange was similar between the plants, indicating a reversible physiological response to soil temperature. Conclusions In this anisohydric grapevine variety both night-time and daytime stomatal conductance were responsive to root-zone temperature. Because nocturnal transpiration has implications for overall plant water status, predictive climate change models using stomatal conductance will need to factor in this root-zone variable.

Erratum to: Use of diffusion magnetic resonance imaging to correlate the developmental changes in grape berry tissue structure with water diffusion patterns[ Plant Methods (2014); 4(10),35. doi 10.1186/1746-4811-10-35.

[This corrects the article DOI: 10.1186/1746-4811-10-35.].

Use of diffusion magnetic resonance imaging to correlate the developmental changes in grape berry tissue structure with water diffusion patterns.

BackgroundOver the course of grape berry development, the tissues of the berry undergo numerous morphological transformations in response to processes such as water and solute accumulation and cell division, growth and senescence. These transformations are expected to produce changes to the diffusion of water through these tissues detectable using diffusion magnetic resonance imaging (MRI). To assess this non-invasive technique diffusion was examined over the course of grape berry development, and in plant tissues with contrasting oil content.ResultsIn this study, the fruit of Vitis vinfera L. cv. Semillon at seven different stages of berry development, from four weeks post-anthesis to over-ripe, were imaged using diffusion tensor and transverse relaxation MRI acquisition protocols. Variations in diffusive motion between these stages of development were then linked to known events in the morphological development of the grape berry. Within the inner mesocarp of the berry, preferential directions of diffusion became increasingly apparent as immature berries increased in size and then declined as berries progressed through the ripening and senescence phases. Transverse relaxation images showed radial striation patterns throughout the sub-tissue, initiating at the septum and vascular systems located at the centre of the berry, and terminating at the boundary between the inner and outer mesocarp. This study confirms that these radial patterns are due to bands of cells of alternating width that extend across the inner mesocarp. Preferential directions of diffusion were also noted in young grape seed nucelli prior to their dehydration. These observations point towards a strong association between patterns of diffusion within grape berries and the underlying tissue structures across berry development. A diffusion tensor image of a post-harvest olive demonstrated that the technique is applicable to tissues with high oil content.ConclusionThis study demonstrates that diffusion MRI is a powerful and information rich technique for probing the internal microstructure of plant tissues. It was shown that macroscopic diffusion anisotropy patterns correlate with the microstructure of the major pericarp tissues of cv. Semillon grape berries, and that changes in grape berry tissue structure during berry development can be observed.

Vegetative growth and cluster development in Shiraz grapevines subjected to partial root-zone cooling

Grapevine roots can be exposed to a range of temperatures at any particular moment because the root system can explore large volumes of soil over great depths and distances. A split-pot experiment was designed to assess how vegetative and reproductive development respond to partial and whole root-zone warming following winter dormancy. Simultaneous cooling and warming of parts of the root system slowed shoot elongation, leaf expansion and berry development compared to plants with a fully warmed root-zone, but not to the same extent as those with a fully cooled root-zone.

Techniques for Technetium Scintigraphy in Plants

The accumulation of technetium by plants has been widely studied and reported in the literature from the perspective of the incorporation of environmental 99Tc into the food chain. Pertechnetate (TcO4−) is the most stable surface chemical form of technetium and is known to be extracted by plant roots, transported by the xylem, and reduced in the leaves; however, the mechanism of action is not entirely clear. Measuring the distribution of technetium in plants has been challenging, many questions remaining unanswered. To date, tracer studies for plant physiology (radionuclide and color dye) have relied on destructive sampling, prohibiting repeat-design experimentation. This article explores the technical issues relating to the application of scintigraphic imaging to plant physiology. The benefits and limitations of methods for introducing radiotracers to plants are outlined. Strategies for the successful labeling of various plant organs with 99mTc and several unanticipated artifacts are described. The relevance of these labeling experiments to the study of plant vascular transport is explained, and strategies for optimizing the scintigraphic imaging of plants are outlined. Assessing plant physiology is an emerging frontier, especially given the growing importance of water management and the increased competing demand for crops as biofuels.

Long-distance transport of pertechnetate in the moonflower (Ipomoea alba)

The first research on the transport of metastable-technetium-99 ((99m)Tc) in the form of pertechnetate ((99m)TcO₄⁻) within plants suggested that (99m)TcO₄⁻ may be mobile in the phloem. In contrast, more recent evidence indicates the anion is transported in the xylem. Here we demonstrate that observations of (99m)Tc transport in the test subject of these initial investigations, the moonflower (Ipomoea alba L.), are incompatible with phloem flow. Rather, the presence of only minute amounts of (99m)Tc in typical sinks for phloem solutes and (99m)Tc transport out of labeled leaves when shaded but not when illuminated strongly suggest that the radionuclide is transported in the xylem. The study increases confidence in the identification of (99m)TcO₄⁻ as a xylem mobile compound whose distribution in plants can be visualized using nuclear medicine scintigraphic imaging techniques.

Impact of reduced atmospheric CO2 and varied potassium supply on carbohydrate and potassium distribution in grapevine and grape berries (Vitis vinifera L.)

To assess the robustness of the apparent sugar-potassium relationship during ripening of grape berries, a controlled-environment study was conducted on Shiraz vines involving ambient and reduced (by 34%) atmospheric CO2 concentrations, and standard and increased (by 67%) soil potassium applications from prior to the onset of ripening. The leaf net photoassimilation rate was decreased by 35% in the reduced CO2 treatment. The reduction in CO2 delayed the onset of ripening, but at harvest the sugar content of the berry pericarp was similar to that of plants grown in ambient conditions. The potassium content of the berry pericarp in the reduced CO2 treatment was however higher than for the ambient CO2. Berry potassium, sugar and water content were strongly correlated, regardless of treatments, alluding to a ternary link during ripening. Root starch content was lower under reduced CO2 conditions, and therefore likely acted as a source of carbohydrates during berry ripening. Root carbohydrate reserve replenishment could also have been moderated under reduced CO2 at the expense of berry ripening. Given that root potassium concentration was less in the vines grown in the low CO2 atmosphere, these results point toward whole-plant fine-tuning of carbohydrate and potassium partitioning aimed at optimising fruit ripening.