DIFFERENT DEVELOPMENTS OF DEFICIT IRRIGATION

II. PRD- An innovative new technique to increase water use efficiency and to improve fruit quality.

PRD, or partial root-zone drying, is a technique originally developed by Mr. Peter Dry and Dr. Brian Lovey to minimize the amount of water needed to irrigate grape crops. However, the potential of PRD has also been demonstrated on citrus crops, increasing the quality, color and flavor of the fruit¹⁶. Lying at the base of this practice is the use of two rows of drip lines instead of the customary one¹⁶. Each line has a dripper positioned on a different side of the crop. The “trick” is to irrigate through one line at a time, several times a week (or according to local conditions) for a couple of weeks¹⁶. For the next couple of weeks, the same practice is carried out for the other drip line – a pattern that lasts throughout the season¹⁶. As a result, at any given time, only half the rootzone is dry, thus the term partial root-zone drying. In response, the dry half produces a hormone called abscisic acid and sends a hormonal message to the rest of the plant relaying the water deficit in that section¹⁶. Consequently, the leaves close their stomata to prevent moisture loss and shoot growth is slowed down in order to conserve water¹⁶. The decrease in vegetative growth due to the inhibition of shoot growth is a positive effect since extensive vegetation blocks the sunlight from most parts of the crops and fruit of lesser quality results. Shoot expansion can be recovered after a while if the situation remains unchanged, but before recovery can occur, the dry side is watered and the watered side is subjected to water stress. This way it is ensured that there will always be a “dry side”¹⁷. The plant is “deceived”, believing it is under water stress and therefore it utilizes the water it has more efficiently than it would under normal irrigation practices¹⁶. Therefore, the basis of the PRD theory is root to shoot signaling and the growers have actually managed to use to their advantage the plants’ signaling system for water conservation in times of stress¹². This irrigation method provides an easier way to control crop growth and water balance than genetic manipulations in the plant¹².

When practicing PRD, it is important to keep the wet side properly watered. Seeing as the dry side is under water stress, it is crucial that the wet side not dry out to a point where the whole plant is stressed otherwise, adverse effects are may be caused¹⁶. Applied correctly, PRD can attain the same fruit yield, and even improve fruit quality, using half the amount of water¹⁶.

Early PRD experiments were conducted on apple trees¹². The roots on the “dry side” were shown to release abscisic acid that caused stomata closure and as a result significant conservation of irrigation water was accomplished. An advantage over other types of irrigation, PRD maintained the trees charged with water, thus the leaves and fruit were not harmed by the water stress¹². On the contrary, fruit flavor seemed to improve since sugars were redirected to the fruit as a result of the hormonally induced suppression of leaf growth¹². Other scientists agree with the finding that PRD contributed to water conservation while improving fruit flavor, however they claim that PRD is in virtually the same method as standard deficit irrigation and that the alternation of sides when irrigating does not amount to much¹⁵. (More about this side of the PRD debate can be found in the PRD in grapes chapter).

In grapes, PRD has enabled grape growers to decrease water use by 40%; furthermore, wine quality has improved as well as fruit color and flavor¹⁶. Furthermore, the vigor of vine decreased, thus, need for pruning lessened and it may even reduce pests and diseases¹⁶. Vigor of vine is a term expressing excessive vegetative growth of a vine¹⁷. As mentioned, excessive vegetative growth decreases the grapes’ exposure to the sun, resulting in poor fruit and wine quality. Thus, reducing vigor by not permitting the dry side to recover shoot function, PRD improves wine quality¹⁷.

In citrus crops, pears and stone fruit, PRD has also bears positive results, allowing a water reduction of up to 60%¹⁶.

A final example showing the beneficial effects of PRD is provided by scientists from the Cukurova University in Turkey who conducted studies examining the effects of PRD on cotton¹². Their results showed that cotton crops treated with PRD produced their yield several weeks in advance, in comparision with the regularly irrigated cotton crops¹². Early cotton yield raises the quality of the yield since the harvest can take place before the rainy season.