At some point during summer, available soil moisture from previous winter rains is significantly depleted and shoot tip growth slows; this is about the time many growers start to apply water. The amount of water growers choose to apply from that point to harvest will affect the degree of vine water stress. If a high VPD event occurs, it will usually happen in summer when soil moisture levels have been purposely depleted.

Some growers measure leaf water potential (LWP) weekly during the growing season to monitor the stress in vines over the season and to assist them in making water application decisions. A few people I spoke with noted that LWP measurements made during the heat spell were not much different than the week prior. Researchers have shown that soil moisture availability of deficit irrigated or dry-farmed vines has a stronger effect on LWP than does VPD. Reduced water in the soil profile causes leaf stomata to become smaller which reduces conductance and transpiration. In studies, LWP of moderately stressed vines (LWP less than -12 bars) did not change significantly over a wide range of VPD conditions (Williams and Baeza, 2007, AJEV 58:173).

Take home messages

On very hot, dry days in mid-summer, vines under moderate (or greater) water stress or unirrigated vines will not transpire at significantly greater rates than they would under milder conditions; therefore, they will not lose significantly more water then during “normal” summer days. On the other hand, clusters can lose a significant amount of water by transpiration in those conditions as compared to “normal” summer days, and depending on severity, sunburn or worse may result.

Things to consider

Cluster exposure and thus berry temperature is affected by several factors including row orientation, trellis design, water management, leaf removal and sprinkler cooling. If sunburn is a common problem in a block, then the objective is to reduce direct solar radiation on fruit and to have adequate soil moisture when it is needed. That is easier said than done; however what follows are some things to consider.

Row orientation: This is often determined by several factors including shape and size of the area available for planting, configuration of adjacent blocks, slope, wind direction, land use of adjoining parcels, etc.  N-S rows have the most canopy exposure and E-W the least. Practical considerations and logistics usually have the strongest impact on vineyard design; however, the correct row orientation can reduce the risk of sunburn and heat damage especially in warm, low vigor sites.

Trellis design: A vertically shoot positioned (VSP) trellis is the most common type used in the north coast and is ideal for low to moderate vigor sites because row spacing can be narrow. If row orientation results in excessive cluster exposure, some growers have added cross arms to widen the canopy or modified shoot orientation on the afternoon sun side of the row to allow foliage to better cover clusters. Such adjustments are limited because of the need for tractor space between already narrow rows.  If N-S rows are the only option, do not use a VSP; cross arms will be essential to provide fruit shading.

Water management: Controlling canopy growth by regulating water to achieve yield and quality goals is fundamental to winegrape production. If sunburn and or heat damage regularly causes crop loss, then working to better understand water relations for canopy management purposes in those blocks can benefit fruit quality. Regularly monitoring both soil and vine water status will allow you to see the affect of applied water and soil water disappearance on your specific objectives for shoot growth, berry size and ripening. Knowing the relative amount of water vines can extract in high evaporative demand conditions and how quickly that moisture can disappear will allow you to apply the right amount at the right time to minimize vine stress. Applying water each day of a heat spell will not reduce vine stress if soil moisture content was too low before you started.

Leaf removal: In a region with high winter rains, controlling early season vine growth is accomplished by withholding irrigation until shoot growth slows, and by utilizing cover crops to deplete stored soil moisture. Shoot thinning is required to maintain desirable spur positions and manage crop load. In most sites, those practices alone do not result in the desired level of fruit exposure; however in low vigor sites they may.

The quantity and quality of light on clusters as well as the timing of exposure has significant effects on fruit composition. The goal is to create a light environment in the canopy that achieves fruit quality targets yet avoids excessive heating of clusters. If a site frequently experiences sunburn or heat damage, then the severity of leaf removal must be reduced to improve quality. In cool, foggy regions, leaf removal is essential for disease control and for fruit exposure, as dense canopies are common due to high soil moisture. Because hot periods are more infrequent than in warmer regions but not absent, determining optimal cluster exposure is more challenging in these regions.

Sprinkler cooling: Utilizing sprinklers for evaporative cooling is an option for some growers. When weather forecasts predict consecutive days of expected high ambient temperatures around 100°F or more, sprinklers – either traditional impact or targeted systems - can be used to achieve evaporative cooling. Most research with over-plant sprinklers to achieve cooling has been conducted with impact sprinklers which provide total floor coverage, although over plant targeted sprinklers and under or over plant microsprinklers, misters, etc. have also been used to provide cooling with reduced water use.  Temperature sensors inside “protected” blocks allows determination of when temperatures stabilize after turning on sprinklers and helps growers meet water conservation goals.  The sprinkler irrigation time required to cool a vineyard may be surprisingly short, at which point, there is no further temperature drop by maintaining overhead cooling.  After the system is turned off, the cooling effect will continue as water evaporates off vine surfaces, and can be measured by temperature sensors in the block.

Sources:

Mullins, M.G., A. Bouquet, and L.E. Williams. 1992. Biology of the Grapevine. Cambridge University Press, Cambridge, UK.

Williams, L.E., and P. Baeza. 2007. Relationships among ambient temperature and vapor pressure deficit and stem and leaf water potentials of fully irrigated, field-grown grapevines. Am. J. Enol. Vitic. 58:173-181.