PRD in grapes
A REMINDER
As discussed earlier, PRD, or Partial root-zone drying, is an irrigation method developed in Australia, in which crops are irrigated one side at a time, leaving the other side dry, under mild water stress. The root-zones alternate every few weeks between dry and irrigated. It has been hypothesized that the mild water stress induced on one side of the plant will send signals stimulating the whole plant to utilize water more efficiently. In addition, it has been shown that fruit quality is enhanced as a result of PRD application16.
THE EXPERIMENT
A study was conducted in semi-arid conditions investigating the effects of PRD on grapevines, specifically, Vitis vinifera L. var. Shiraz3.
THE TREATMENTS
Four different treatments were applied to the vines3:
THE MEASUREMENTS
Stomatal conductance, leaf water potential and total vine leaf area were measured regularly throughout the growing season3.
THE RESULTS
No significant differences in leaf water potential were observed between the four treatments at 1000 hours or at 1500 hours3. The total leaf vine area was also not influenced by the different treatments. Stomatal conductance however decreased in both the PRD treatments and in the 50% control crops in comparison to the 100% control crops. This finding is somewhat unusual since with what we know about PRD as a method requiring alternation of the drip line between both sides of the vine row, it would be expected that the 50% control group would behave in a manner more similar to the 100% control crops than to the PRD treated crops.
CONCLUSION
Since the PRD treatment halved the water use of the 100% control crops, without affecting yield, there is an obvious increase in the ratio of yield to irrigation applied, or in other words the water use efficiency (WUE) was improved3. Other studies5, 7 have shown improvement in grape and wine quality in response to PRD irrigation, thus giving yet another reason to apply PRD commercially and to conserve water while improving crop quality.
That said, one cannot ignore the finding that the crops in the 50% control group (who received the same amount of water the PRD crops did) showed a trend similar to the PRD treated crops even though the former were only irrigated with one drip line on one side of the vine row, without alternating between vine row sides. This inconsistency raises certain questions regarding the PRD system, among them: is PRD really more effective than regular deficit irrigation or are they essentially the same thing? What role does the switching of the drip line from side to side of the vine row play? Sanliang Gu15 conducted an experiment aiming to answer these questions and more. In his study, the effect of the overall water volume, using both the PRD system and a simple one drip line system, was examined. One group of Sauvignon Blanc vines was irrigated to 80% evapotranspiration via traditional irrigation or PRD. A second group of vines were watered to 50% evapotranspiration also via conventional irrigation or by PRD15. According to the results, the difference between the vines was in the amount of water supplied to them and not in technique through which the water was administered15. The vines reacted to the water cutback similarly under PRD and under standard irrigation, thus the factor responsible for inducing water stress, decreasing transpiration and vegetative growth and improving fruit quality is the irrigation amount and not PRD15.
A study with similar results was conducted in a vineyard in Kibbutz Yiftakh, Israel on Merlot grafted to 100R rootstock8. This study included two PRD treatments and one control treatment8. The important distinction in this study (also present in Gu’s study) compared to earlier PRD studies, is that the control treatments received the same amount of water as the PRD treatments, however the irrigation was applied to both sides of the vine. Measurements of crop level, pruning weight, crop load, number of clusters per vine, berry number, berry weight, Brix, berry juice TA, berry juice pH, water potential of the stem, stomatal conductance and CO2 uptake were obtained, soil water content was measured and a GC-MS (gas chromatophore and mass spectrometry) analysis was carried out8.
No significant effect of PRD on the any of the above factors was observed8. However, differences due to the amount of water applied via both types of irrigation (control and PRD) were detected8. The authors concluded that conventional drip irrigation standards are based on partial drying of roots and thus it is not expected that irrigation via alternation of vine sides will have an advantage over regular deficit irrigation8.
Yet another study demonstrating the similarity between PRD and regular deficit irrigation and was conducted on Chardonnay grapevines in Australia and consisted of many different PRD and control treatments10. According to the results, the improvement in the WUE was due to a decrease in the amount of irrigation water and not attributed to a specific irrigation method10.
These claims do not discredit PRD as an effective irrigation method, however they do show that PRD is not more effective than regular deficit irrigation and in a sense the two are the fundamentally the same method15. Thus, studies that demonstrate the positive influences of PRD on crop quality and water reduction are by no means bogus, however, implementing deficit irrigation on the same crops are likely to bear very similar results.
All the while, it is important to keep in mind that RDI is a different method than PRD, in the same way that RDI differs from traditional deficit irrigation: RDI is strategically aimed at accomplishing different goals15. Timing the deficit irrigation according to the specific crop is the important distinction between RDI and PRD or regular deficit irrigation, allowing RDI to enhance fruit quality as well as conserve water15.