Partial root drying : a sustainable irrigation system for efficient water use without reducing fruit yield

Using micro-pressure probe technology we have established that the restriction in fruit growth caused by exposure to soil drying is not a consequence of the hydraulic changes, which occur at the tissue level. We have demonstrated that roots sense soil water status and communicate this information to the shoots, via a signalling mechanism mediated by abscisic acid. This result provides fundamental scientific insight into the regulation of growth during water deficit, with clearly exploitable benefit. Traditional of GM enabled technologies in the future will allow us to manipulate the chemical signalling capability of the plan, such that we can modify a plants' response to soil drying.

The yield differences between treatments were not significant. This means that berry growth was not affected by the mild water stress observed under PRD and DI, during ripening. Because half of the water was used in PRD and DI vines as compared with FI, WUE in terms of fruit production doubled. On the other hand, berry sugar accumulation was not significantly affected under PRD and DI, suggesting that berries were preferential sinks for carbohydrates under deficit irrigation. In the three years of experiments and in the two varieties, skin anthocyanins and total phenols were higher in PRD vines than in DI and FI; however the values were not always significantly different from those of DI. This is an important economical result since the costs with water can be reduced, without negative effects on yield. The increase in quality will improve viticulturists income. Additionally water saving has a very positive impact in the environment. As stated in eTIP 1 and 2, grapevine growers are already using this information.

A reduction in vigour in PRD vines was observed in the two varieties studied. The lower vigour in PRD than in FI (fully irrigated) and DI (Deficit Irrigation) was observed by a decrease in the average individual shoot weight measured at winter pruning, the number of water shoots (developed on the old woody stem), the leaf layer number (LLN) and the total leaf area at harvest. The values observed in PRD were quite similar to those in non-irrigated, rainfed vines (NI). The differences of total leaf area observed between treatments were mainly due to differences in the lateral shoot leaf area. We concluded that PRD was more efficient in reducing vigour than DI, in spite of similar amount of water given to the two treatments. An increased investment in the root system was also observed in PRD vines. As a consequence of the low LLN, the NI and PRD vines showed at veraison a higher percentage of sun exposed clusters than the other irrigated treatments FI and DI. These results have a large impact on the quality of the production, because they enable a better exposure of the clusters. On the other hand, they also lead to a reduction in the costs of manpower for pruning. As in the previous section, this information has already been transferred to the viticulture community, who is willing to apply it.

Our investigations into the effects of irrigation and specifically of PRD on raspberry crops grown in Scotland have demonstrated that irrigation can be an valuable addition for optimal yields; we have shown this to be true even for outdoor raspberry crops in the Scottish climate. Where crops are grown under protection, the irrigation becomes essential. We have demonstrated in both protected cropping and glasshouse situations that PRD can lead to substantial savings in water used without any penalty in terms of yield or quality of the berries produced. This result is potentially very important to the protected raspberry industry, a growing proportion of the total UK and European raspberry crop. Indeed it implies that PRD can be used for a given yield enhancement with less water than can be obtained with conventional irrigation systems. This could be a factor in improved profitability of raspberry growers, especially those that have limited water supplies. The numbers of growers with limited water supplies are increasing with changes in water legislation and as summers become drier with climate change. There are no protectable components from this result. Indeed, it has been widely publicised to growers both nationally and internationally through publication in Grower magazine and through presentation at the Rubus and Ribes congress. Other results have been accepted for publication in an international refereed horticultural Journal. There is evidence of considerable grower and government interest in the technique, with further detailed experimental tests of the technique, including further testing and development for the raspberry crop, to be conducted under a new UK DEFRA-funded project (Partial rootzone drying: delivering water saving and sustained high quality yield into horticulture).

Our theoretical and applied investigations into the potential of thermography for use in detecting plant stress have helped introduce a new technique to this field. Thermography is potentially a rapid method of detecting plant stress and is not labour intensive. In particular, it may be a useful technique for determining the timing of the change over from watering one side of PRD plants to the other. Thus it would be most beneficial in areas prone to drought and in need of irrigation. Extensive studies on a range of crops, but principally with grapevine growing in Portugal, have confirmed the potential of the technique and provided strong evidence of its sensitivity to water deficits in practical horticultural systems. Although no protectable intellectual property has arisen in this area, the results are of great interest to a wide range of growers and other scientists. These preliminary tests have been written up for publication and some have already appeared in the international scientific literature, with other papers yet to appear. Results have been widely publicised to scientist, grower and government audiences in the UK and overseas. The interest has been such that further funding to develop the concepts of thermal sensing for irrigation scheduling has been secured in principle from an industry/government consortium under the UK HortLink scheme. This will hopefully provide an important route for further development of the technique for the benefits of the horticultural industry.

The relative importance of stomatal and metabolic limitations on carbon assimilation was assessed from A/Ci curves, light response curves and key enzymes of the Calvin Cycle. In particular we estimated maximum rate of Rubisco carboxylation (VCmax), maximum electron transport capacity at saturating light (Jmax), the rate of triose-P utilization (TPU) and relative stomatal limitation (RSL) in Moscatel and in Castelão varieties in response to water availability. In the Moscatel variety, Jmax and TPU were more affected by the water treatment and year than VCmax,. Rubisco activity seems to be more resistant to water stress than photophosphorylation. Jmax and TPU were most reduced in NI vines, with PRD and DI vines showing the intermediate and FI vines the highest values. The RSL was higher in NI and PRD vines as compared to DI and FI. In Castelão, only TPU and RSL were affected by water treatments. NI vines showed the lowest values of TPU, PRD and DI vines, the intermediate values, and FI the highest values. However, the RSL was increased only in NI vines. These results confirm the importance of stomatal regulation in carbon metabolism in NI and PRD vines and are an important piece of information to enable modelling of grapevine response to water availability under Mediterranean climate, with practical application in the irrigation management.

Three years of trials in semi-commercial environments has shown that we can deliver sustained yielding in glasshouse tomato crops and potential increases in tomato fruit quality. It appears clear that sustained yield is achieved by a combination of a minimal effect of reduced irrigation on fruit size, coupled with a potential increase in the number of fruit borne by an individual plant. Increases in fruit quality, perceived in terms of analysis of quality parameters such as Brix (a measure of total soluble solid content) and consumer taste testing, offers a very attractive proposition to the UK industry and others within and around the EU involved in commercial tomato production. Dissemination activities underway include writing in the commercial press and attendance at technical meetings, in addition to our own extension service programme within Lancaster. We will shortly start to co-ordinate a £1.1 million Department of Agriculture project to transfer the findings of the IRRISPLIT project into domestic commercial horticulture in England and Wales.

There is no continent of free of soil salinity issue, which is caused largely by mismanaged irrigated agriculture. Agricultural production is under the threat of soil salinity over 100 counties. Twenty millions hectares of agricultural land is abandoned annually because of soil salinity. Results of this work had confirmed that reduced irrigation water application, a general characteristic of the new irrigation technique PRD, means that salt accumulation occurring in plant root zone will relatively be less compared to traditional irrigation practice. If irrigation water is of good quality of EC (electrical conductivity) values far less than 1 dS m-1, there may be no difference regarding to salt accumulation. However, use of irrigation water with high salinity load of EC values as high as 2 to 5 dS m-1 is not uncommon in many arid and semiarid countries of North Africa, West and Central Asia and of Latin America. Salt accumulation is directly proportional to applied water quantity and therefore the less is the irrigation water applied, as practiced under PRD technique, the lower is the salinisation risks of soils under irrigation. It should also be noted that the newly evolving irrigation technique PRD which was field-tested in our work for many crops has the least drainage risk compared to traditional irrigation method. Excess irrigation water application, which is the main cause of drainage problem does not exist under PRD practice, where irrigation water applied barely meets crop water requirement and recharging of ground water during irrigation season is the least. Irrigated agriculture has been questioned for long time regarding its sustainability. Although crop yields increase 10 to 15 times during early phase of irrigated agriculture, soil fertility decreases over the years due to development of salinity and drainage problems. Large sums of capital investment by governments may be needed in reclamation efforts. The PRD technique of irrigation has comparatively less chance of developing salinity and drainage problems than traditional practice of irrigation.

Using transgenic tomato plants which possess a silenced gene required for the production of ethylene we have been able to demonstrate that increasing ethylene production in response to soil drying is a causal mechanism by which the growth rate of leaves is controlled and that suppression of this elevation in concentration, removes the sensitivity of plant growth to soil drying, such that growth continues in relatively dry soil. Clearly this is of significant commercial application. We are currently in discussion with the original patent holders for this transgenic line to see if the original patent claims cover its application in this way.

Partial water stress experienced by plant under PRD facilitates early maturity and harvest. Our results showed that harvest of cotton grown under the PRD practice could be completed nearly two weeks earlier, compared to traditional full irrigation, with no water deficit. Therefore, the PRD practice minimises risks of cotton-boll drops, which causes reduction of yield and deterioration of cotton lint quality owing to early winter rains, common in the Mediterranean countries. Similarly early maturity and harvest realised with tomato, irrigated with PRD practice, can provide considerable market advantage to growers during winter months when tomato largely comes from greenhouses. The growers in our area are very keen for adopting the PRD practice solely for its beneficial effect on crop maturity and resulting early harvest. To this effect, seminars carried out with growers participation will help wide adoption of the PRD practice in the area.

The technique of establishing the PRD effects on row planted vegetable crops may have commercial value in future. We used two drip lines with drippers alternately placed in the two lines, tied together. Only one line was operated in a given irrigation so that only one half of the root zone received water. Operation of the lines can be switched on and off depending on field determined root-drying period of the crop. The two drip lines fused together, with custom-tailored dripper spacing and arrangement, can be manufactured and it is believed that there will surely be market demand on such systems in future. Manufacturers of irrigation hardware have shown close interest in the new technique.

The most important finding of this work was that PRD irrigation treatment showed a beneficial effect on crop water use efficiency in olive trees, tomato and common bean plants. While reducing by 50% the quantity of total water applied, the plant water status and yield were maintained under the PRD. It was also concluded that the application of PRD irrigation technique on horticultural crops such as tomato results in better water status, higher fruit calibre and enhanced fruit quality compared to other irrigation techniques such as the regulated deficit irrigation. This technique may also reveal suitable for other horticultural crops under water scarcity scenarios. These important agronomic findings, if confirmed with further comprehensive field-testing could have a tremendous socioeconomic impact on water management in agriculture, especially in the arid and semi-arid regions of Morocco, where the competition for water resources is gradually increasing. An important aspect of these applications would be to up-scale the research findings of the IRRISPLIT consortium, by testing the PRD irrigation scheme on other crops of major economic importance in the region. Some of the most important tree crops in the Haouz after olive are apricot and Citrus trees, in addition to various horticultural crops. It would be interesting to test this irrigation technique on theses crops and to see if it could improve the water use efficiency (e.g. Citrus, almonds, horticultural crops). The project was implemented in collaboration with other Moroccan agronomic research institutes (INRA, ORMVA), which made it possible to share the experimental protocols and results with all these partners involved in the regional agricultural R & D. However, more experimental work is needed to test PRD effects on the most important species in the region, before to disseminate this technique and promote it with the farmers and to convince them to use it. The collaboration with regional agricultural extension services (DPA, ORMVA) will facilitate this process. The next steps will be to communicate these agronomic results to extension agencies and farmers by organizing field demonstrations in the region.

A newly evolving irrigation technique, partial root drying (PRD), was compared with traditional practice of irrigation where crop-water need was fully met. The technique was tested for vegetables, field and tree-crops. Irrigation water-use efficiency (IWUE) under PRD was nearly two folds higher compared to traditional practice of irrigation. Tomato among the vegetable crops showed the least fruit yield decrease to reduced irrigation water application, if irrigated with the PRD practice. Similar response was obtained for cotton and maize among the field crops tested. Saving of irrigation water was as much as 50% for only marginal yield reductions, 10 to 20%. Results of citrus, among the tree-crops, were similar to field crops, cotton and maize. Saving of 30% irrigation water could easily be achieved without significant fruit-yield reduction. Further decrease in irrigation water caused significant reduction of fruit size, therefore marketable yield decreased. Future studies should include soybean and sunflower among field crops and apple, peach, apricot, almonds and the like among the tree crops. The results imply that adopting the PRD practice for irrigation can be very simple means of increasing crop yields in water scarce areas. What is needed is simply increasing of irrigated areas with however no additional allotment of irrigation water. In areas where lifted irrigation and deep-wells are used, significant savings in fuel costs of pumping can be achieved if PRD practice of irrigation is adopted since irrigation water requirement is decreased. Adoption of the PRD practice in furrow irrigated crops like cotton and maize would result significant savings in irrigation costs. Since the furrows are alternately receive water, labour time needed is proportionally less (50%), compared to normal irrigation practice and therefore it is expected that there will be significant savings in overall irrigation cost. Both farmers and consumers benefit if cost of production decreases. Water demand for industrial and domestic use increases to the detriment of irrigated agricultural production. Noting further that global-fresh water demand increases 10 times for every 100 years, decreasing water allocation for irrigated agriculture is the only option to cope with increasing-water demand of other sectors. It seems that the PRD practice may ease off the problem of future-water shortages that irrigated agriculture will face. It is important that an awareness of the mentioned problem should be created among farmers as of today so that they adopt the new irrigation technologies, including PRD, with high irrigation water-use efficiency.

The short-term and long-term water use efficiency (WUE, measured by the ratio of photosynthesis or productivity and transpiration) were increased in grapevines of the cultivars Moscatel and Castelão subjected to the two deficit irrigation treatments, the PRD (partial root drying irrigation, where 50% of the ETc was supplied to only one side of the root system, alternating sides each 15 day) and the DI (deficit irrigation where 50% of the ETc was supplied to both sides of the row), as compared with full irrigated (FI) vines, where 100% of the ETc was supplied to both sides of the root system. The improved WUE in PRD and DI, in the longer-term, was achieved by the maintenance of production associated with reduced stomatal conductance and transpiration (measured as stem sap-flow). This was corroborated by the higher stable carbon 13-isotope composition (d13C) of leaves and grape berries. The d13C values reflect the balance between the internal and the external CO2 concentration during the carbon uptake process and therefore the degree of leaf stomatal closure integrated across the growing season. So, the higher the d13C, the lower the discrimination against 13C, reflecting more closed stomata. These effects were in many circumstances significantly higher in PRD than in DI grapevines. These results are important as indicators for more efficient water use in grapevines, combining an important ecological impact (with water saving) with an economic impact (by saving costs). Our team has undertaken already very extensive contacts with farmers and wine companies that hold large vineyards, which are very eager to get reliable information on irrigation of vineyards. The aim is to achieve higher productivity without wasting or compromising grape quality.

The physiological mechanisms of plant responses to drought stress are extremely complex, which slows down the progress of genetic improvements of crop drought tolerance and effective management of water resources. The PRD technologies offer the double advantage of making possible the dissection of physiological mechanisms involved in stress response and allowing more efficient schemes for irrigation management and scheduling. Our physiological investigations documented the effects of PRD irrigation on plant water status, leaf biochemical properties such as soluble and cell wall peroxidase activity, phenolic compounds, xylem sap pH and leaf ABA concentration. The analysis of leaf cell wall phenolic compounds showed a significant increase in common bean in the PRD-treated plants. On the other hand, we could not see any ABA signal in PRD treatment after a few days of water deprivation. These preliminary tests open new opportunities for further detailed physiological investigation of drought response mechanisms, especially focusing on possible genetic variability of chemical signalling (ABA, pH) among different plant species. This approach could facilitate the development and implementation of future crop genetic improvement strategies for crop drought tolerance. These physiological studies of PRD effects, which have already been communicated in various international symposia (see list of publications) will be summarized and published in international journals, to contribute in the scientific discussions and advances of stress physiology. They will also resume in future research projects by the systematic comparison of plant physiological and biochemical responses to drought stress.

The PRD irrigation treatment resulted in significant reduction of olive vegetative growth, without any significant effect on yield components and fruit quality parameters. Importantly, the olive oil acidity did not show any significant differences between PRD treatments and well-watered controls, and oil percentage seems even to increase under PRD although not significantly. If validated and confirmed these results could have a major impact on oil production by improving economic water productivity (ratio of the economic return and cost of water). These findings should also be tested and validated in multi-location trials both at national level (other Moroccan regions) and international level (Mediterranean countries). More effort is therefore needed by implementing an applied research and extension program focusing on olive productivity and oil quality in the Mediterranean region, based on IRRISPLIT research findings. A summary of the major outputs of the IRRISPLIT project, including the most salient agronomic results and their potential for application will be communicated to various donor agencies and potential scientific partners, for seeking their interest in supporting a major applied research program focusing on olive economic water productivity.

An extensive theoretical analysis of the use of leaf temperature measurement for estimation of stomatal closure and hence of the need for irrigation has been completed as part of the project aims. In particular this theoretical analysis has highlighted some new possibilities for use of leaf temperature data for irrigation scheduling, and it has provided guidance for the development of novel approaches to the measurement of leaf temperatures using infra-red thermography. The basic predictions of the theoretical analysis were that sensitivity increases as incident radiation increases and as the air humidity decreases. These theoretical predictions as to the sensitivity of the technique for detecting plant water deficits were tested in different experiments using grapevines in Portugal, cotton in Turkey, citrus and eggplant in Cyprus and raspberry in Scotland. These results confirmed the theoretical predictions outlined above. A further theoretical development was the recognition that measurements did not need to be confined to sunlit leaves, as had been thought previously. Indeed the use of thermography on shaded parts of the canopy was shown to have some theoretical advantages. The theoretical analyses also showed that optimal sensitivity requires simultaneous measurements of the temperatures of wet and dry surfaces and that these need to have similar radioactive properties to the leaves. A particularly important aspect of the theoretical analyses of leaf energy balance was the recognition of the importance of leaf orientation in determining the temperatures of individual leaves. Aspects of these theoretical analyses have been published in international refereed journals. Further articles based on these theoretical analyses are in press or preparation. The main impact of these theoretical studies, however, has been on the further development of thermal imaging as a new and exciting tool for irrigation scheduling in a range of crops, and has provided a sound basis for its practical validation.

Studies on N-fertiliser recovery with tomato showed that while significant savings of irrigation water can be attained under PRD practice, with only marginal yield reduction; N﷓fertiliser uptake could also be maintained at the same level as the full irrigation, with no water deficit. Nitrogen uptake rate of tomato showed an apparent increase, following when fruit setting stage was fully in sight. The results suggest that fertilisation should be concerted with plant nutrient demand, which is the highest following fruit setting stage. Experience gained from this project will guide us in future studies on plant nutrient requirement, which should vary depending on plant growth stage. N-fertiliser uptake studies of maize showed that irrigation treatments had no effect on maize N-yields. However, nitrogen fertiliser recovery was the highest and the least mineral nitrogen residue stemming from both the soil and the N-fertiliser was recorded under the PRD treatment. Since under PRD practice, one side of the plant rows stays proportionally dry, all agronomic practices such as chemical spraying, fertiliser applications and the like which need heavy agricultural machinery can be carried out timely and with the least soil disturbance. It was further noted that air humidity under canopy of cotton, irrigated with PRD, was lower compared to traditional irrigation where crop-water need was fully met. It is therefore expected that spread of pests (e.g., thrips, different bollworms, white fly, aphids) and fungal disease are lower, and comparatively less chemical sprays are needed if PRD is extensively adopted. The results therefore imply that the PRD practice not only provides significant savings in irrigation water, but should also be valued with its environmental friendly character as regard to fertiliser practice and lesser needs of chemical sprays.

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