The Pancharatnam–Berry phase was discovered by Pancharatnam in studies of polarized light and introduced by Berry as a topological phase for matter wave functions. For light, the Pancharatnam–Berry phase is measured in laser interferometers and exploited in optical elements. Excitons are matter waves that directly transform to photons inheriting their coherence and polarization. This makes excitons a unique interface between matter and light and a unique system for exploring the Pancharatnam–Berry phase for matter waves by light interference experiments. Recent studies led to the discovery of polarization textures in light emission of indirect excitons and exciton–polaritons. This connection of the Pancharatnam–Berry phase to polarization makes it an intrinsic phenomenon for polarization textures. An IX is a bound pair of an electron and a hole confined in spatially separated layers. IXs are realized in coupled quantum well structures. Due to their long lifetimes IXs can cool below the temperature of quantum degeneracy and form a condensate in momentum space. IX condensation is detected by measurement of IX spontaneous coherence with a coherence length much larger than in a classical gas. The large coherence length observed in an IX condensate, reaching ~10 μm, indicates coherent IX transport with suppressed scattering, in agreement with theory. A cold IX gas is realized in the regions of the external ring and localized bright spot rings in the IX emission. These rings form on the boundaries of electron-rich and hole-rich regions created by current through the structure and optical excitation, respectively; see ref. and references therein. An LBS is a stable, well defined, square plastic plant pots and tunable source of cold IXs, thus an ideal system for studying coherence and polarization phenomena.
Different LBS offer IX sources of different strength and spatial extension; furthermore, these parameters can be controlled by optical excitation and voltage. This variability gives the opportunity to measure correlations between coherence and polarization. Here, we explore LBS to uncover the Pancharatnam–Berry phase in a condensate of IXs.The experiment shows that the phase shifts correlate with the polarization pattern of IX emission and onset of IX spontaneous coherence. The correlation between the phase shift and the polarization change identifies the phase as the Pancharatnam–Berry phase acquired in a condensate of IXs. This phenomenon is discussed below. The spatial separation of an electron and a hole in an IX reduces the overlap of the electron and hole wave functions suppressing the spin relaxation mechanism due to electron–hole exchange. In a classical IX gas, spin transport in the studied structure is limited by 1−2 μm due to Dyakonov–Perel spin relaxation. As a result, for uncondensed IXs at r < rcoh, the spin relaxation is fast and coherent spin precession is not observed. However, the suppression of scattering in IX condensate results in the suppression of the Dyakonov–Perel and Elliott–Yafet mechanisms of spin relaxation enabling long-range coherent spin transport in IX condensate. Therefore, IX condensation at r > rcoh dramatically enhances the spin relaxation time leading to coherent spin precession and, in turn, precession of the polarization state of IX emission. This precession generates the evolving Pancharatnam–Berry phase of IXs, which is detected as the shift of interference fringes.
Figure 4d shows that no decay of the evolving Pancharatnam–Berry phase is observed over macroscopic lengths exceeding 10 μm. This indicates the achievement of macroscopic long-range coherent spin transport in the IX condensate.According to current climate projections, we face an increase in the intensity, frequency, and duration of heat waves in the coming years . Therefore, it is imperative that the grape and wine industry study the effects of these heat events on different wine growing regions around the world. Extreme temperatures can have detrimental effects on grapevines including but not limited to decreases in yield, unwanted changes in berry composition, and decreases in overall grape quality . High temperatures cause increased water loss via evapotranspirative cooling and overall stress on grapevines, so irrigation practices can be useful in mitigating the negative impacts of HWs by altering vine water status, leaf and berry temperature. Shade cloths, cover crops, rootstock selection, changes in row orientation and trellis system to protect from solar radiation with misting, or increased irrigation throughout HWs, are further strategies being implemented in current wine regions to mitigate their adverse effects . However, with growing water scarcity, a more efficient use of water and a deeper understanding of the effects of HWs and water use during different grapevine phenological stages will be required . Although grapevines are resilient crops that can tolerate drought and extreme temperatures , it is important to explore alternative grape cultivars that may be better suited for these warmer scenarios. Two widely used irrigation methods are regulated deficit irrigation and partial root-zone drying . In RDI, irrigation is reduced or completely stopped for specific periods during the growing season.
A study done in South Australia showed that water deficit after flowering resulted in the “greatest reduction in berry weight compared with that of well-watered vines” . This is important to note because this may not have been the result had water deficit been practiced before flowering, thus showing that timing is crucial. In turn, water deficit after veraison only had a minor effect on berry weight at maturity and berries were not affected by water deficit during the month before harvest. With PRD, half of the root system is maintained in a dry state while the other half is irrigated . The theory behind PRD is that the watered roots maintain a favorable plant water status, while the dry roots result in chemical signals, such as increases in abscisic acid production, that are transported to leaves to reduce growth and therefore vigor. Deficit irrigation has two main effects on grape berry composition: a decrease in berry size and the upregulation of genes in the phenylpropanoid and flavonoid pathways. In terms of berry size, the skin to pulp ratio increases, which causes phenolic compounds to become more concentrated. In terms of transcriptomics, the upregulation of genes in the phenylpropanoid and flavonoid pathways causes an increase in anthocyanin synthesis due to signaling from an increase in abscisic acid . Alternatively, the effect of deficit irrigation on tannins is largely due to a reduction in berry size rather than an impact on their biosynthesis. . It is important to note that the beneficial aspects of deficit irrigation may not be the same in the future due to projected climate warming, and the combined effects of greater heat and water stress with deficit irrigation may be detrimental to berry quality . Berry development follows a double sigmoidal curve, which is divided into three stages . The first phase occurs after fruit set and is characterized by cell division and cell expansion . This is then followed by a lag phase, which is a period of little to no growth, but is characterized by a rapid accumulation of organic acids, particularly malic acid. The last stage is characterized by the onset of veraison . During this second growth phase, berries soften, accumulate sugar, and grow larger. In red varieties, anthocyanins begin to synthesize, allowing for the red and purple pigments to show in the skins . High temperatures affect berry development primarily at post fruit set, veraison, and mid ripening . A decrease in berry size before veraison caused by high temperatures is due to effects on cell division , whereas post veraison, this decrease is likely due to a stop in cell expansion and an increase in transpiration. Closer to maturity, high temperature seems to be linked to cell death, loss in berry mass, and increased water loss, leading to shriveling and sunburn . With global warming, square pot plastic increasing mean temperatures are further correlated with an earlier onset of phenological stages in the grapevine and the shortening of the duration of these stages . Moreover, since most viticultural regions are currently at or near their optimal growing temperatures for the grape cultivars grown there , global warming intensifies the pressure of exploring new varieties that better suit these regions. Additionally, the effects of HWs on grapevines will depend on the timing of the heat event during specific phenological stages. Although the effects of elevated temperatures throughout the growing season vary by cultivar, literature has consistently shown that flowering is a period that is more sensitive to heat, and the length of the interval from bud burst to flowering is more susceptible to a decrease than other phenological intervals. . During grape development, fruit set shows resistance to elevated temperatures, whereas veraison and mid-ripening are more sensitive to heat . From a berry chemistry perspective, exposure to high temperatures has an important impact on primary and secondary metabolite production in the berry.
Studies have shown that grape berry metabolism is sensitive to both day and nighttime temperatures and the magnitude of these diurnal temperature changes . Primary metabolites of grape berries include sugars, amino acids, and organic acids. While they contribute to the support of normal growth and reproduction, secondary metabolites serve ecological functions, such as defense to abiotic or biotic pressures . Among the secondary metabolites produced by the grapevine, phenolic compounds and aromatic compounds are of major interest due to their impact on grape and wine quality. The Shikimate, phenylpropanoid, and flavonoid pathways are responsible for the biosynthesis of the different phenolic compounds that can be found in grapes . Grape phenolics can be divided into two groups: non-flavonoids and flavonoids. Flavonoids are most relevant to wine quality and are divided into three groups: flavan-3-ols, anthocyanins, and flavonols . Flavan-3-ols are mainly present in the form of proanthocyanidins and contribute to the bitterness and astringency of wine . Anthocyanins are responsible for the color of red wine , and flavonols act as UV protectants and copigments . All three groups of flavonoids are affected by environmental factors including high temperature in different ways. Flavonols are synthesized from the flavonoid biosynthetic pathway and also give rise to anthocyanins and proanthocyanidins . They are primarily located in the skin and mainly function as UV protectants and as copigments with anthocyanins to form stable pigments . The major flavonol compounds found in grapes include quercetin, myricetin, and kaempferol . Flavonol synthesis begins at flowering, reaches peak concentrations after veraison, and decreases during development as the berries increase in size . Flavonol synthesis is light dependent, and sunlight has a greater impact on development than temperature does. While shading has modest effects on berry development, it significantly decreases flavonol synthesis . It has been found that high temperatures don’t have a significant impact on flavonol content when compared to other grape berry metabolites . Gouot et al., studied the combined effects of high temperature duration and intensity on phenolic metabolism of Shiraz berries. They found that flavonol content of berries exposed to 46 °C showed no significant difference to those exposed to 35 °C. However, flavonols were degraded in berries exposed to 54 °C. This shows that high temperature has indirect effects on flavonol levels, while sunlight remains the key influencing factor. Anthocyanins are pigmented molecules responsible for the color of red wines and begin their synthesis in grape berries after veraison, reach maximum values close to maturity, and then decrease . Previous studies have shown that high temperatures affect anthocyanin levels in two ways: inhibiting anthocyanin biosynthesis and promoting degradation . Literature suggests that anthocyanin accumulation in grapes is more influenced by temperature rather than light when photon fluxes are above 100 µmol/m2 /s . Anthocyanins reach critical metabolic temperature for synthesis around 30ºC , but signs of inhibition begin beyond this temperature, where reduction in the activity of enzymes involved in the phenylpropanoid and flavonoid pathways, such as phenylalanine ammonia lyase , VIMYBA2, and UDP-glucose flavonoid-3-O-glucosyltransferase , have been observed . Additionally, it has been found that while anthocyanins were suppressed at transcriptional and enzymatic levels, peroxidase activity had increased, suggesting that peroxidase plays a key role in degradation . Studies carried out in Cabernet Sauvignon by Mori et al., found that biosynthesis of anthocyanins was not affected by high temperatures and the decrease of anthocyanin levels was mainly due to chemical and enzymatic degradation. Flavan-3-ols are the most abundant phenolic compounds in grapes and wine , and they are composed of monomeric catechins and oligomeric or polymeric proanthocyanidins . In terms of sensory effects, proanthocyanidins are responsible for astringency and bitterness perceived in grapes and wine . These compounds also have antioxidant properties and interact with anthocyanins to form stable pigments .