Intense Sunlight Weakened Shiraz Vines More Than Heat or Drought, Study Finds

A field study published Monday in the journal OENO One found that intense sunlight, more than heat or drought alone, was the main factor reducing key photosynthetic performance in Shiraz grapevines during hot summer conditions in South Australia.

The research examined how Shiraz vines responded when water deficit, high temperatures and different levels of light exposure occurred at the same time, a combination that is becoming more common as heatwaves and droughts grow more frequent in wine regions. The authors, Walaa Shtai, Marcos Bonada, Everard Edwards, Georg Wohlfahrt, Massimo Tagliavini and Paul Petrie, said the results point to a practical limit of irrigation as a protective tool during extreme weather.

The trial was carried out in the Barossa Valley during the 2022-23 summer on own-rooted Shiraz vines planted in 1998. Researchers compared well-watered vines with water-deficit vines and also took advantage of an east-west row orientation that created two distinct light environments within the same canopy. Leaves on the north side received direct sun for most of the day, with peak radiation around 1,500 to 2,000 micromoles per square meter per second, while leaves on the south side remained more shaded, with peaks around 500 to 700.

Over a 20-day period, the team continuously monitored chlorophyll fluorescence, a way to track the functioning of photosystem II, the part of the plant’s photosynthetic machinery that is especially sensitive to stress. They also measured gas exchange in nearby leaves to assess assimilation, stomatal conductance and transpiration.

The irrigation treatments were clearly separated. Well-watered vines received irrigation five times for a total of 473 liters per vine and were maintained near a midday stem water potential of about -0.8 to -1 MPa. Water-deficit vines were irrigated twice for a total of 164 liters per vine and were kept near -1.3 to -1.5 MPa. During the monitored period, the site also received 6 millimeters of rain.

The weather included six days above 35 degrees Celsius. Three of those were isolated hot days and three consecutive days met the local definition of a heatwave.

The strongest pattern in the data came from canopy side. On hot days, leaves on the north side showed lower maximum efficiency of photosystem II than leaves on the south side. That decline was even greater when those sun-exposed leaves were also under water deficit. During the heatwave itself, irrigation had a significant effect on north-side leaves, with water-stressed leaves showing the lowest values.

The study found that south-side leaves generally maintained better photochemical performance at midday than north-side leaves. North-side leaves showed lower effective photochemical yield and higher non-photochemical energy dissipation, signs that the plant was diverting more absorbed light away from photosynthesis as heat to protect itself. Water stress worsened that pattern on the north side but not on the south side.

Gas exchange measurements added another layer to the findings. Net leaf assimilation, stomatal conductance and transpiration were all higher in north-side leaves than in south-side leaves, especially in well-watered vines. Well-watered plants also had higher assimilation overall than water-deficit plants.

The authors also examined how stomatal conductance tracked with assimilation after normalizing for vapor pressure deficit and carbon dioxide. On north-side leaves, well-watered and water-deficit vines followed similar patterns. On south-side leaves, however, water-deficit vines showed smaller changes in stomatal conductance for the same increase in assimilation than well-watered vines did, a difference the researchers reported as statistically significant.

One of the more striking results came after periods of extreme heat. Following the heatwave, maximum photosystem II efficiency dropped sharply across all treatments and remained low for the final five days of monitoring even though maximum air temperatures had fallen to between 21 and 24 degrees Celsius. That suggests that recovery from combined stress may lag behind cooler weather.

The paper argues that field conditions matter because controlled experiments often isolate one stress at a time and can miss how multiple factors interact in commercial vineyards. In this case, high radiation exposure appeared to be the dominant factor impairing photosystem II performance when compared with heat or water stress alone.

For grape growers and wineries, that matters because vine stress affects canopy function and can eventually influence yield and fruit composition. The findings suggest that irrigation can help sustain vine function during heatwaves, but may not be enough when exposed leaves are also receiving intense radiation. In practice, that could support more precise decisions on irrigation scheduling, row design and protective measures such as shade netting in warm regions where Shiraz is grown.

The study arrives as producers across major wine regions face longer dry periods and more frequent extreme heat events linked to climate change. The authors said understanding when damage occurs in the photosynthetic system can help improve vineyard resilience under those conditions.