This article is of interest to all fruit growers even though it is written about citrus. There is a lot of food for thought!

This article was written by Ron Hutton -he is a project team member working with Dr Brian Loveys on aspects of Partial rootzone drying (PRD) as an alternative irrigation technique for citrus. This research project supported by the Land and Water Resources Research and Development Corporation, CSIRO and NSW Agriculture.

The plant responses to PRD that Brian has provided covers the general outcomes from his work with winegrapes.

Citrus response to PRD is quite similar to that of grapes with the exception of improved fruit quality. Research into the impact of extended irrigation intervals on plant water status, shoot and fruit growth in citrus showed that during certain periods of the annual growth cycle, this crop is quite sensitive to water stress.

Phenological studies have also identified that there is no clear separation of fruit and shoot growth in citrus and this observation explains why deficit irrigation in the from of RDI cannot be successfully applied to citrus without exhibiting some negative impact on either yield and/or fruit size. With extended irrigation intervals applied after the onset of Stage II of fruit growth during the summer months, repeated periods of water stress were recorded prior to re-irrigating. This resulted in reduced shoot extension of the summer and autumn growth flushes and reduced fruit size relative to controls at harvest.

However, significant water savings were achieved by applying this technique during the latter half of the irrigation season. The following year, the return crop was heavier than controls in treated trees (more intense spring flush and flowering as a consequence of repeated water stress during summer and autumn of the previous season), although yields were not increased. During the following summer, the higher crop load (set) and reduced fruit growth rate resulting from repeated long interval irrigation treatments resulted in no significant change in yield.

An outcome of this work was to investigate alternating part of the rootzone between wet and dry states such that soil moisture in the wetted part was sufficient to satisfy plant water requirements at all times throughout the growing season, rather than apply cyclical periods of water stress to the entire rootzone during part of the season in an effort to conserve water.

The following is a summary of the NSW Agriculture PRD studies in citrus between 1997 and 2000. The project aimed to define the responsiveness of irrigated citrus, to partial rootzone drying (PRD) under field conditions. This irrigation technique is based on maintaining both wet and dry soil in the root zone. The simple approach of alternate row watering has the potential to save water, increase water use efficiency and reduce accession to water tables during the irrigation season.

Established trees in a mature navel orange orchard at Yanco Agricultural Institute (34o 36' 00''S, 146o 25' 00''E; Elevation 138m) were subjected to alternate row irrigation and modified scheduling to induce partial rootzone drying from 1997. Plant responses to PRD were assessed in terms of growth, transpiration, canopy development, abscisic acid levels, fruit growth and quality. The soil at the experimental site is a medium clay Merungle Loam (Taylor et al., 1979). The water content at field capacity of the top 100cm is 170mm of which 40-60mm is readily available for plant growth in the top 50cm (root zone). Irrigation water supplied from the Murrumbidgee River is of good quality with average salinity levels of 100 mS.cm-1 total dissolved salts.

Methods:

Four irrigation strategies were evaluated from 1977 to 1999. Some of the experimental trees (mature navel orange trees) were converted from permanent furrow flood to drip irrigation. Control flood irrigated trees (watered both sides) received 13.5ML/ha/y in the 1998/99 irrigation season. This was slightly more than the district average. During the same season, drip irrigated trees received only 4.8ML/ha/y. Flood and drip irrigated trees subjected to partial rootzone drying (PRD) treatments, where only one side of the trees were watered for periods of 4 weeks before switching to the alternate side, received even less water. PRD trees received approximately half the respective amounts applied in both flood and drip treatments.

Results:

We have not been able to detect any consistent, statistically significant, difference between drip and PRD drip irrigated trees in terms of water potential or stomatal conductance, although fruit on the PRD trees tended to be slightly smaller.

Water applied for 98-99 season :-

Flood irrigation - 13.5ML/ha - 100% water

Drip irrigation - 4.8ML/ha - 65% water saved

PRD Flood - 8.0ML/ha - 41% water saved

PRD Drip - 2.8ML/ha - 80% water saved Under conditions of extreme evaporative demand during the mid-summer period (Eto >10mm/day), drip irrigated trees were slightly less stressed than those in other treatments. Plant response in PRD Drip (80% water saving) was similar to conventionally flood irrigated trees. Water potential and stomatal conductance measurements provided evidence of water stress in PRD Flood irrigated trees only, and fruit size at harvest was smaller as a result.

Fruit size was increased by 5% in drip irrigated trees, compared to flood irrigation. Where fruit size was reduced (PRD Flood irrigation and standard Flood irrigation) relative to the drip irrigation treatments, both Brix and %acid of juice increased, but Brix/acid ratio decreased. Highest Brix/acid ratio and %juice was recorded for drip irrigated trees. With the exception of PRD flood irrigation, which induced more intense flowering and heavier fruit set classically seen in water stressed trees, irrigation treatments had no apparent effect on yield in either the first or subsequent years of the trial (excluding the effect of alternate cropping cycle).

At this trial site, the majority of the rootzone was limited to the top 40cm of the soil profile due to structural changes at the B horizon forming a physical barrier to root penetration. Maximum water extraction was observed to occur in the top 30cm. In this situation, plant available water was limited to <7% (between 28-35%VWC in the 0-30cm layer). Soil moisture tensions recorded in this VWC range equated to 300+Kpa down to 20Kpa respectively. This indicated a need for frequent low volume applications of irrigation water to maintain optimum soil moisture conditions for maximum plant growth using drip irrigation. Extremely low infiltration capacity was recorded in permanent furrows (4mm/hr), due to repeated compaction occurring over many years. Ponding in furrows remained for up to 2 days, resulting in temporary waterlogging. Water stress (leaf curling) was evident 7-10 days after watering using flood irrigation. These two factors contributed to the slower fruit growth recorded in flood irrigated trees. High infiltration rates were recorded under tree canopies (12-14mm/hr). Use of 2L/hr drippers at 0.5m spacing and run for 5-6 hours wet to the bottom of the rootzone (30-40cm) and lateral spread was limited to 30cm each side of the drip line. This strategy optimised soil moisture availability whilst achieving significant savings in water. Drip irrigated trees performed better than trees watered by the other irrigation treatments. Water savings of up to 80% relative to conventional flood irrigation were achieved in drip irrigated trees where PRD was implemented. Measures of plant water status, fruit growth rate and fruit size at harvest were only slightly less when irrigating by this method relative to full drip irrigation. The physiological basis of this responses remains to be determined. Sap flow measurements in roots confirmed that upward movement of water through the roots subsides as the soil dries out with increasing PRD interval. However, water status measurements in leaves did not reflect this. Evidence was found for rehydration or redistribution of water within trees by reverse flow to the roots at night. This needs further investigation to better understand how PRD works.

Conclusions.

We have shown that by using the principles of partial rootzone drying (PRD) and drip irrigation it is possible to grow oranges with no significant reduction in yield or quality with reduced irrigation input. We believe that these results are sufficiently encouraging to extend the scope of the experiments. It is clear that highly significant increases in water use efficiency have been achieved. What is not so clear is the mechanism whereby this has been achieved. In citrus, we have been unable to demonstrate significant physiological responses typical of plants experiencing PRD. However, huge water savings appear possible. Reduced water application is partly due to the more efficient water delivery of drip compared with flood, but the addition of PRD appears to add another dimension to the water management of citrus trees. Continuing physiological studies are providing some clues as to the mechanisms operating when trees are subject to the PRD treatment and this knowledge will be invaluable in making modification to our irrigation strategies in citrus in the future, but it is clear we do not yet have all the answers.