Merlot responses to irrigation

Introduction Irrigation may be used for manipulating growth and yield to obtain specific wine quality objectives. In most grape growing regions, water resources are limited. Therefore, irrigation must be applied judiciously. This implies that the least amount of water should be applied to obtain a specific wine quality objective. In this regard, various aspects play an important role. Drip irrigation, for example, requires less irrigation water than micro-sprinklers to obtain comparable growth, yield and wine quality. Another approach that could improve irrigation water use efficiency (WUE) expressed as kg grapes per m3 water, is partial root zone drying (PRD). With this approach, one half of the root system is well-watered, while the other half is allowed to dry out. After approximately 14 days irrigation is switched to the dry side, while the wet side is allowed to dry out. By repeating this switching throughout the growing season, approximately 50% water may be saved without compromising yield and wine quality. The objective of the study was to compare low frequency drip irrigation (less than 10 irrigations per season) and PRD to dryland (rain fed) cultivation. Experimental background The field experiment was carried out with Merlot/99R near Wellington in the Coastal Region during the 2004/05 and 2005/06 seasons. Six irrigation treatments were applied as indicated in Table 1. In the case of the three irrigations per season, irrigations were applied (i) at pea size berries, (ii) véraison and (iii) post-harvest. For the five irrigations per season, irrigations were applied (i) at pea size berries, (ii) between pea size berries and véraison, (iii) at véraison, (iv) during ripening and (v) post-harvest. These two groups of low frequency irrigations were either applied on the grapevine rows or subsurface in the middle of the work rows. In the case of the PRD approach, dripper lines were also installed subsurface in the middle of the work rows. Irrigation was applied in alternating rows. After approximately 14 days, irrigation was switched to the dry work rows. One to two irrigations per week were required to keep the soil moist. The irrigation switching between alternating work rows was applied from pea size berries until harvest. Each irrigation treatment was replicated four times. Irrigation volumes applied and measured grapevine responses are listed in Table 1 and Figure 1. For more details of the experiment layout and measurements, refer to Myburgh (2011a & b). Results and discussion Plant water constraints According to stem water potential measured around 13:00 by means of a pressure chamber, the dryland grapevines experienced high water constraints during berry ripening (Fig. 1). Water constraints only tended to be lower where three irrigations were applied compared to dryland cultivation. Where five irrigations were applied, grapevines experienced moderate water constraints. Subsurface irrigation in the middle of the work rows did not affect grapevine water status, irrespective of the number of irrigations. Grapevines irrigated according to the PRD approach, experienced only low water constraints (Fig. 1).

TABLE 1. Effect of number of irrigations, position of dripper lines and partial root zone drying (PRD) on the performance of Merlot/99R near Wellington. Water use efficiency (WUE) is grapes produced per unit irrigation water. Results are means of two seasons. FIGURE 1. Effect of irrigation on midday stem water potential in Merlot/99R during ripening near Wellington. Dashed lines indicate thresholds of water constraint classes for Merlot. FIGURE 2. Effect of (A) dryland cultivation and (B) PRD irrigation on the foliage of Merlot/99R during berry ripening near Wellington.

Shoot growth

Cane mass at pruning increased with the number of conventional drip irrigations compared to dryland cultivation (Table 1). During berry ripening, leaves of the dryland grapevines showed yellowing (Fig. 2A). This typical response to water constraints was not observed in the irrigated grapevines. Irrigation in the work rows did not affect growth vigour, irrespective of the number of irrigations. During berry ripening, limited active shoot growth tips occurred on the PRD grapevines (Fig. 2B). Despite this trend, the PRD approach did not increase cane mass compared to five irrigations. In another study, the PRD approach had a similar effect on cane mass of Merlot in the Breede River Valley (Lategan & Howell, 2010a). The results confirmed that PRD can restrict shoot growth in spite of higher irrigation volumes.

Yield

At harvest, berry mass increased with the number of irrigations compared to dryland cultivation (Table 1). During berry ripening, berries on the dryland grapevines showed crimping and shrivelling. Irrigation in the work rows did not have any effect on berry mass, irrespective of the number of irrigations. In the case of PRD, more irrigation increased berry mass compared to five irrigations. The effect of the irrigations on berry mass clearly reflected in bunch mass and yield (Table 1). Yield of Merlot in the Breede River Valley also declined as water constraints increased (Lategan & Howell, 2010b).

Water use efficiency

The WUE decreased as the irrigation volumes increased (Table 1). The WUE of the PRD grapevines was the lowest. Likewise, PRD did not improve WUE of Merlot in the Breede River Valley (Lategan & Howell, 2010b). These results confirmed that PRD will not necessarily improve WUE compared to conventional, single line drip irrigation. In many of the previous PRD experiments, the control grapevines were also irrigated by means of double dripper lines. Consequently, the control grapevines received double the volume irrigation compared to PRD, where irrigation was only applied via alternating dripper lines. Due to the biased comparison, WUE of the PRD approach was substantially higher than the other irrigated treatments.

Sugar, acidity and pH

The objective was to pick the grapes at approximately 25°B (Table 1). In the case of PRD, berry ripening was slower compared to the other treatments. Consequently, the PRD grapes had to be picked at 24°B to reduce the risk of yield losses due to rot. The total titratable acidity decreased with the volume of irrigation water (Table 1). In general, irrigation tended to increase juice pH compared to dryland cultivation. This could possibly be due to more leaves being shaded as irrigation increased grapevine vigour, which resulted in lower total titratable acidity and higher pH (Iland, 1989).

Wine sensorial characteristics

In comparison to dryland cultivation, sensorial wine colour decreased with the number of irrigations, irrespective of where the water was applied (Table 1). The high irrigation volumes, which induced the low water constraints in the PRD grapevines, resulted in the poorest wine colour. More prominent berry character was obtained with dryland cultivation and three irrigations compared to five irrigations and PRD. The PRD approach produced the poorest overall wine quality (Table 1). The combination of better colour and more prominent berry character caused overall wine quality to be better where grapevines experienced higher water constraints. Previous studies in the Breede River Valley (Lategan & Howell, 2010b) and the Lower Olifants River Region (Myburgh, 2011c) also showed that higher water constraints improved Merlot wine quality. For more detailed results and discussion refer to Myburgh (2011a & b).

Conclusions and recommendations

Under the prevailing conditions, Merlot grape yield responded positively to increasing volumes of irrigation water compared to dryland cultivation. In contrast, less irrigation increased grapevine water constraints, sensorial colour, berry character and overall wine quality. If high wine quality is the objective, up to three irrigations can be applied, and to ensure higher yields compared to dryland cultivation. If yield is the primary objective, Merlot should be irrigated when the midday stem water potential reaches -1 MPa to maintain a balance between yield and wine quality. The low WUE and poor wine quality obtained with the PRD approach clearly did not meet the expectations. Therefore, this practice cannot be recommended for irrigation of wine grape vineyards.

Summary

The effect of low frequency drip irrigation and PRD (partial root zone drying) on grapevine responses were compared to dryland cultivation, i.e. rain fed, in a warm climate. The field trial was carried out with Merlot/99R near Wellington in the Coastal Region. Under the prevailing conditions, grape yield responded positively to the increased volume of irrigation water applied compared to dryland cultivation. In contrast, higher grapevine water constraints under dryland cultivation or only three irrigations improved sensorial wine quality. If high wine quality is the objective, three irrigations, i.e. at pea size, véraison and post-harvest should be applied. In addition to high wine quality, this irrigation approach will also sustain higher yields. Five or more irrigations should be applied if high yield is the objective if adequate irrigation water is available. In the latter case, Merlot must be irrigated when midday stem water potential reaches -1 MPa to achieve a balance between yield and quality. The low water use efficiency, as well as low wine quality of the PRD did not meet the expectations. Therefore, PRD cannot be recommended for wine production.

References

Iland, P., 1989. Grape berry composition – the influence of environmental and viticultural factors (Part 2): Solar radiation. Australian and New Zealand Grapegrower & Winemaker, April 1989, 74 – 76.

Lategan, E.L. & Howell, C.L., 2010a. Partial root zone drying (PRD) of Merlot in the Breede River Valley (Part 1): Irrigation volumes, plant water stress and vigour. Wynboer Technical Yearbook 2010, 19 – 21.

Lategan, E.L. & Howell, C.L., 2010b. Partial root zone drying (PRD) of Merlot in the Breede River Valley (Part 2): Yield, water use efficiency and wine quality. Wynboer Technical Yearbook 2010, 22 – 24.

Myburgh, P.A., 2011a. Response of Vitis vinifera L. cv. Merlot to low frequency drip irrigation and partial root zone drying in the Western Cape Coastal Region (Part 1): Soil and plant water status. South African Journal of Enology & Viticulture 32, 89 – 103.

Myburgh, P.A., 2011b. Response of Vitis vinifera L. cv. Merlot to low frequency drip irrigation and partial root zone drying in the Western Cape Coastal Region (Part 2): Vegetative growth, yield and quality. South African Journal of Enology & Viticulture 32, 104 – 116.

Myburgh, P.A., 2011c. Effect of different irrigation strategies on vineyards in sandy soil in the Lower Olifants River Region (Part 3): Growth, yield and quality of Merlot. Wynboer Technical Yearbook 2011, 30 – 31.

This article originates from research funded by Winetech and the final report of project WW 04-23, “Determining optimum irrigation schedules for drip irrigated vineyards in the Breede River Valley and the Coastal Region”, can be downloaded from http://www.sawislibrary.co.za/dbtextimages/Winetech2010_06.pdf.

– For more information, contact Philip Myburgh at myburghp@arc.co.za.