Partial Root-zone Drying Plants Research Articles

Effects of partial root-zone drying and deficit irrigation on yield, irrigation water-use efficiency and some potato (Solanum tuberosum L.) quality traits under glasshouse conditions

Agriculture water resources are expected to decline due to increasing water demand and ongoing climate change. In this context, water-saving irrigation techniques, such as partial-root zone drying (PRD) and deficit irrigation (DI) were assessed under glasshouse conditions on potato cultivar Mondial. Four irrigation levels were applied: 50, 70, 80 and 100% of field capacity during 2016 and 2017 growing seasons. The results showed that the yield penalty with PRD was similar to that caused by DI. Nevertheless, PRD plants had higher number of stems and were shorter than DI plants. Sugar and protein contents of tubers gradually decreased with water restriction, however, remained higher in PRD than DI tubers. In contrast, the amounts of polyphenols and antioxidants increased in tubers with decreasing irrigation levels. Untargeted metabolite analysis revealed higher metabolite content of PRD than DI tubers with less decrease in glucose and fructose concentrations and with double amount of mannitol. Transcript level of key-genes involved in carbohydrate metabolism was elevated at 20% water-saving in PRD tubers, but not in DI tubers. We assumed that the detected changes in tubers reflect better adaptation of plants to water-saving irrigation under PRD than DI.

Prolonged drying cycles stimulate ABA accumulation in Citrus macrophylla seedlings exposed to partial rootzone drying

Partial rootzone drying (PRD) establishes discrete wet and dry parts of the rootzone (for example using parallel drip lines on either side of the crop row), and alternates them to stimulate root growth and root-to-shoot ABA signalling. To assess whether alternation frequency affects plant physiological responses, Citrus macrophylla Wester seedlings were grown with the root system split between two pots and 5 irrigation treatments applied: Control, PRD-Fixed (where wet and dry parts of the rootzone were not alternated) and three alternate PRD treatments where the wet and dry parts were swapped at 3 (PRD1), 6 (PRD2) and 12 (PRD3) days intervals, to dry the soil to different degrees before alternating the irrigation. Water was equally distributed between both pots in Control plants, whereas only one pot was watered and the other allowed to dry in PRD plants, with all plants receiving the same irrigation volume. After 24 days, soil water content (θv), leaf water potential (Ψleaf), root water potential (Ψroot), abscisic acid (ABA) concentration in roots ([ABA]root), leaves ([ABA]leaf) and shoot xylem sap ([X-ABA]shoot), biomass allocation and leaf area were measured. Higher soil water availability of the dry side (PRD1 and PRD2) had no significant effects on leaf water relations, ABA status and plant biomass allocation. However, increasing the duration of exposure of part of the root system to dry soil (PRD3 and PRD-Fixed) further decreased Ψroot and stimulated root ABA accumulation, while decreasing Ψleaf and increasing [ABA]leaf of PRD3 plants compared to the other treatments. Differences in physiological response between PRD3 and PRD-Fixed plants were attributed to differences in the proportion of root mass exposed to drying soil: PRD3 plants had a lower Ψleaf and a higher [ABA]leaf with a smaller proportion of their root mass in wet soil. Since long drying cycles were required to alter plant biomass allocation and physiological responses in PRD plants, these should be implemented in designing suitable PRD strategies for field application.

Plasticity in stomatal size and density of potato leaves under different irrigation and phosphorus regimes

The morphological features of stomata including their size and density could be modulated by environmental cues; however, the underlying mechanisms remain largely elusive. Here, the effect of different irrigation and phosphorus (P) regimes on stomatal size (SS) and stomatal density (SD) of potato leaves was investigated. The plants were grown in split-root pots under two P fertilization rates (viz., 0 and 100mgkg−1 soil, denoted as P0 and P1, respectively) and subjected to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation regimes. Results showed that SS and SD were unresponsive to P but significantly affected by the irrigation treatment. FI plants had the largest SS, followed by DI, and PRD the smallest; and the reverse was the case for SD. Compared to FI and DI, PRD plants had significantly lower values of specific leaf area (SLA) and leaf carbon isotope discrimination (Δ13C) under P0. Midday leaf water potential (Ψleaf) and stomatal conductance (gs) was similar for DI and PRD, which was significantly lower than that of FI. Leaf contents of C, N, K, Ca and Mg were higher in PRD than in DI plants, particularly under P0. When analyzed across the three irrigation regimes, it was found that the P1 plants had significantly higher leaf contents of P and Mg, but significantly lower leaf K content compared to the P0 plants. Linear correlation analyses revealed that SS was positively correlated with Ψleaf and Δ13C; whereas SD was negatively correlated with Ψleaf, Δ13C and SLA, and positively correlated with leaf C, N and Ca contents. And gs was positively correlated with SS but negatively correlated with SD. Collectively, under low P level, the smaller and denser stomata in PRD plants may bring about a more efficient stomatal control over gas exchange, hereby potentially enhance water-use efficiency as exemplified by the lowered leaf Δ13C under fluctuating soil moisture conditions.

Drying/rewetting cycles of the soil under alternate partial root-zone drying irrigation reduce carbon and nitrogen retention in the soil–plant systems of potato

Dry/wet cycles of soil may stimulate mineralization of soil organic carbon (C) and nitrogen (N) leading to increased N bioavailability to plants but potentially also increased C and N losses. We investigated the effects of partial root-zone drying (PRD) and deficit irrigation (DI) on C and N retention in the soil–plant systems of potato. Potato plants were grown in 20L split-root pots with three N-fertilization rates, viz., 1.4 (N1), 2.5 (N2), and 4 (N3) gNpot−1soil, respectively. At tuber initiation and earlier tuber bulking stages, the plants were subjected to PRD and DI treatment for five weeks. For each N rate, the PRD and DI plants received a same amount of water, which allowed re-filling one half of the PRD pots close to full water holding capacity. The results showed that plant dry biomass, plant water use, and water use efficiency were increased with increasing N-fertilization rate but were unaffected by the irrigation treatment. As compared with DI, PRD significantly decreased soil C and N contents, which could have been due to an enhanced soil organic C and N mineralization. PRD did not influence plant C content but significantly increased plant N content in relation to DI. However, owing to substantial decreases of C and N contents in the soil, the amounts of C and N retained in the soil–plant systems were lower in PRD than in DI. Although the C gain in the soil–plant systems of potato was positive due to production of plant biomass, the dry/wet cycles of the soil under the PRD treatment stimulated the mineralization of soil organic C and N leading to greater C and N losses; thus PRD might not be a sustainable irrigation practice in terms of C and N sequestration in the soil–plant systems.

Differential responses of stomatal morphology to partial root-zone drying and deficit irrigation in potato leaves under varied nitrogen rates

The effects of partial root-zone drying (PRD) as compared with deficit irrigation (DI) on stomatal morphology of potato (Solanum tuberosum L.) under varied nitrogen (N) rates were investigated. The plants were grown in split-root pots under three N rates, viz., 70 (N1), 125 (N2), and 200 (N3)mgNkg−1soil, respectively. For each N rate, PRD and DI plants received the same amount of water, which allowed re-filling one half of the PRD pot to 100% water holding capacity. Across the three N rates, guard cell size was larger in DI than in PRD, whereas stomatal pore aperture area (SA) was similar between the two irrigation treatments. Stomatal density (SD) was affected by both N rate and irrigation treatment and was lower in PRD than in DI under N2 and N3, whereas the reverse was the case under N1. Plant leaf area increased with increasing N rate, but was unaffected by the irrigation treatment. SD positively correlated with leaf N concentration and xylem sap ABA concentration for the DI plants, but not for the PRD plants. Nonetheless, negative linear relationships of SD to the mean soil water content in the pots and the carbon isotope discrimination in the leaves were found across all treatments. Regression analyses showed that it was SA rather than SD positively correlated with the stomatal conductance and the transpiration rate per unit leaf area in the DI; however such relationships were not evident in the PRD. In conclusion, compared to DI, PRD led to a more conservative control in plant water use via modulating stomatal morphology; the smaller stomata combined with a lower SD in the plants had efficiently reduced plant water use under high N rate, which maintained a better soil water moisture condition in the PRD pots.

Photosynthetic, chlorophyll fluorescence and growth changes in hot pepper under deficit irrigation and partial root zone drying

There is increasing evidence that the spatial distribution of water within the root zone, as well as total soil water status, determines plant physiological and agronomic responses. To examine the response of photosynthesis, chlorophyll fluorescence and growth of hot pepper (Capsicum annuum L.) to deficit irrigation (DI50, 50% of the control) and partial root-zone drying (PRD, with half of the root system exposed to soil drying and the other half watered with 50% irrigation water of the control), two water deficit treatments were imposed on greenhouse grown hot pepper during the growing period in 2006. Control plants received irrigation to both halves of the root system when soil water content was < 80% of field capacity. Both PRD and DI50 treatments decreased total dry mass by 33 to 44%, shoot biomass by 31 to 44% compared to the control. These treatments increased root-shoot ratio by 35 to 44% in relation to the control, with significant differences between PRD, DI50 and the control. Deficit irrigation led to a relative leaf water content of about 77.91 to 92.71%. Two water deficit treatments reduced photosynthetic rate (Pn) slightly and transpiration rate (Tr) significantly, thus improving leaf water use efficiency (WUEL, defined as the ratio of Pn to Tr) by 24 to 26%. During water stress, a down-regulation of PSII activity was observed along with some impairment of photochemical activity, as revealed by decreases in the maximum quantum yield of PSII (Fv/Fm). Although Fv/Fm did not significantly differ between the deficit treatments and the control, Fv/Fm of PRD plants was higher than that of DI50 plants. The mean values of Fv, Fm, qP at four stages decreased and Fo and qN increased in the soil drought environment. Development of non-radiative energy dissipation mechanisms was evidenced during stress by increases in non-photochemical quenching and decreases in efficiency of excitation capture by open centers. Key words: Photochemical parameters, relative leaf water content, DI50, partial root-zone drying (PRD).

Alternate partial root-zone drying irrigation improves nitrogen nutrition in maize ( Zea mays L.) leaves

The effects of alternate partial root-zone drying (PRD) irrigation as compared with conventional deficit irrigation (DI) and full irrigation (FI) on leaf nitrogen (N) accumulation were investigated in maize ( Zea mays L.) grown under three N-fertilization rates (1.5, 3.0, and 6.0 g N pot −1). The plants were grown in split-root pots in a glasshouse and were exposed to FI, PRD, and DI treatments from the seventh leaf to tasselling/silking stage. The results showed that neither irrigation nor N-fertilization treatments influenced shoot biomass and plant leaf area; while PRD plants had the highest root biomass and root to shoot ratio compared to DI and FI plants. Increase of N rate significantly increased leaf N accumulation; across the N-fertilization rates, PRD and FI plants accumulated significantly greater amount of N in leaves than did DI plants. Leaf δ 15N decreased significantly with increasing N-fertilization rate, and was significantly higher in PRD and FI than in DI plants. Water use efficiency (WUE) was the highest in PRD, followed by DI and the lowest in FI; while N-fertilization rate had no effect on WUE. It was concluded that an enlarged root system and an enhanced soil N availability under PRD might have contributed to the greater N accumulation in maize leaves.

Soil microbial response, water and nitrogen use by tomato under different irrigation regimes

The objectives of this study were to investigate the effects of full irrigation (FI), deficit irrigation (DI) and partial root-zone drying (PRD) on plant biomass, irrigation water productivity (IWP), nitrogen use efficiency (NUE) of tomato, and soil microbial C/N ratio. The plants were grown in pots with roots split equally between two soil compartments in a climate-controlled glasshouse. During early fruiting stage, plants were exposed to FI, DI, and PRD treatments. In FI, both soil compartments were irrigated daily to a volumetric soil water content of 18%; in PRD, only one soil compartment was irrigated to 18% while the other was allowed to dry to ca. 7–8%, then the irrigation was shifted; in DI, the same amount of water used for the PRD plants was equally split to the two soil compartments. The results showed that, the FI treatment produced significantly higher dry biomasses of leaves, stems, and fresh weight of fruit and water productivity of aboveground dry biomass production than either DI or PRD, however, fruit IWP in DI was 25% higher than that of FI, and harvest index in DI and PRD were 50% and 22% higher than FI, respectively, for the 26% and 23% less water used in the DI and PRD, respectively, than the FI treatment. The DI treatment caused the smallest losses of N and highest N use efficiency by fruit. Both DI and PRD caused a significant increase in the soil microbial C/N ratio, meaning ratio of fungal biomass was high at low soil water contents. The result indicates that more work is needed to link the aboveground N uptake and the underground microbially mediated N transformation under different water-saving irrigation regimes.

Partial root-zone drying increases WUE, N and antioxidant content in field potatoes

Partial root-zone drying (PRD) is a new water-saving irrigation strategy which requires that the roots are simultaneously exposed to both dry and wet soil zones. This technique is now undergoing extensive trials with a range of agricultural crops. These results show significant benefit in increasing water-use efficiency. The field potato ( Solanum tuberosum L. cv Liseta) experiments were conducted during 2007 and 2008. Subsurface drip irrigation was used. In 2007 season PRD plants received 70% of full irrigation (FI). To further enhance water saving during the last 3 weeks of the irrigation period, PRD using 70% of FI was replaced with PRD using 50% of FI in 2008. By five harvests during the season N content, fresh and dry matter (DM) of leaves, stems and tubers were followed. At final harvest the effects of PRD and FI irrigation on total and marketable yield and yield quality were investigated. Also, the irrigation water-use efficiency (IWUE) was calculated. As compared to FI, PRD treatment saved 33% (2007) and 42% (2008) of irrigation water while maintaining similar yield. This resulted in 38% and 61% increase in IWUE for the 2007 and 2008 seasons, respectively. In both years the PRD treatments resulted in the increase of N, starch content and antioxidant activity in potato tubers. The latter novel findings on the effect of PRD irrigation on tubers quality might be favorable for the health-promoting potato value.

MEASURING SAP FLOW IN "TWO ROOT-ONE SHOOT" GRAFTED PLANTS TO MODEL SHOOT XYLEM ABA CONCENTRATION DURING PARTIAL ROOTZONE DRYING

Quantifying sap fluxes and ABA concentrations of xylem saps collected from different parts of the root system may assist in modelling leaf xylem ABA concentration ([X-ABA](leaf)) of plants grown with heterogeneous soil moisture distribution. To investigate the contribution of different parts of the root system to total sap flow and [X-ABA](leaf), individual shoots were grafted onto the root systems of two plants grown in two separate compartments and sap flow sensors placed on each hypocotyl below the graft union. Two different irrigation regimes were imposed: conventional deficit irrigation (DI) where both pots received the same volumes of water, and partial rootzone drying (PRD) where only one pot (designated wet) was watered and other (designated dry) dried the substrate. In plants grown in 3 L pots that were vertically divided into two watertight compartments, soil water content (theta) and sap flow were continuously monitored. In plants grown in 2 appressed 0.43 L pots, xylem sap was collected independently from each root system (after severing the shoot below the graft union) to measure root xylem ABA concentration. During PRD, sap flow from roots in drying soil decreased: this relationship was described by a threshold theta(dry) (at which sap flow from roots in drying soil started to decrease) and sensitivity (the rate of decline in sap flow). The threshold theta(dry) varied with both theta(wet) and soil volume. As theta(wet) increased, the threshold theta(dry) increased but the sensitivity of sap flow to theta(dry) was independent of theta(wet). At a similar theta(wet), as soil volume increased, the threshold theta(dry) decreased. Root xylem ABA concentration increased as theta declined in both DI and PRD plants. A simple model that weighted the ABA contributions of wet and dry root systems to [X-ABA](leaf) according to the sap flow from each, showed that with moderate soil drying, simulated [X-ABA](leaf) was higher under PRD than DI.

Comparative effects of partial root-zone drying and deficit irrigation on nitrogen uptake in potatoes (Solanum tuberosum L.)

The effects of partial root-zone drying (PRD) as compared with deficit irrigation (DI) and full irrigation (FI) on nitrogen (N) uptake and partitioning in potato (Solanum tuberosum L.) were investigated. Potato plants were grown in split-root pots and were exposed to FI, PRD, and DI treatments at tuber bulking stage. Just before onset of the irrigation treatment, each plant received 0.6 g N (in the form of urea) with 5% of which was 15N-labeled. After 4 weeks of irrigation treatments (i.e., one drying/wetting cycles completed in the PRD treatment), the plants were harvested and plant dry mass and N content were determined. The results showed that although the plant dry mass was not affected by the irrigation treatments, due to a reduced water use by the plant, both the PRD and DI treatments significantly increased crop water use efficiency. Compared with the FI and DI plants, PRD plants had significantly higher N contents in the leaves, stems and tubers; whereas, the 15N content in the plant organs was similar for the FI, PRD, and DI plants. It is suggested that not the root N uptake efficiency but the soil N availability was enhanced by the PRD treatment.

Effect of partial rootzone drying on growth, gas exchange, and yield of tomato ( Solanum lycopersicum L.)

Three levels of partial rootzone drying (PRD) were evaluated and compared with conventional irrigation, in terms of gas exchange, water relations, growth, yield, fruit quality, and water use efficiency in tomato ( Solanum lycopersicum L.) saladette-type plants growing in pots with volcanic material and drip irrigation. There were four treatments, a control, in which available water (AW) in the whole root system was allowed to fall to 90%; PRD90-30, in which on one side of the divided root AW was allowed to fall to 90% and on the other side it was allowed to fall to 30%; PRD70-30, in which on one side of the divided root AW was allowed to fall to 70% and on the other side it was allowed to fall to 30%; PRD50-30, in which on one side of the divided root AW was allowed to fall to 50% and on the other side it was allowed to fall to 30%. When each treatment reached the desired AW level they were then irrigated. At the same time, when on one side AW reached values ≤30% irrigation was shifted to the other side. Results showed a significant decrease of leaf water potential up to 14% in PRD plants compared with control. Shoot dry weight and leaf area decreased in relation to substrate available water in PRD plants. Stomatal conductance and transpiration rate were lower, up to 31 and 18%, respectively, in plants with PRD compared with control. However, CO 2 assimilation rate was similar among treatments which along with the reduction of transpiration in PRD plants, increased instantaneous water use efficiency by 28, 25, and 33% in PRD90-30, PRD70-30, and PRD50-30 treatments, respectively, compared to control. Yield, number of fruits and fruit total soluble solids content were similar among treatments. An increase of 25% in fruit titratable acidity was reached in PRD50-30. Fruit firmness increased up to 31% in PRD treatments. PRD treatments allowed a water irrigation saving up to 46%.

Comparative effects of partial rootzone drying and deficit irrigation on growth and physiology of tomato plants

The effects of partial rootzone drying (PRD), deficit irrigation (DI), and full irrigation (FI) on tomato physiology were investigated. In PRD and DI plants, leaf water potential values and stomatal conductance were significantly lower, while xylem ABA concentration was greater compared to FI plants. Photosynthesis was similar for all treatments. Water use efficiency was improved by PRD and DI, which reduced fruit dry weight, but had no effect on dry weight of leaves and stems.

Changes in antioxidant activities and phenol content in tomato plants subjected to partial root drying and regulated deficit irrigation

Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) are water saving irrigation systems that have been developed to increase water use efficiency (WUE) without significant yield reduction. To examine whether tomato responded differently to RDI and PRD, we compared the changes in antioxidative defenses in tomato plants using a split-root system. Tomato plants were grown for 21 days under controlled conditions with their roots separated equally between two soil compartments. Three irrigation treatments were imposed: Control, receiving an amount of water equivalent to 100% of plant transpiration; PRD in which one compartment was watered with 50% of the amount of water supplied to the controls, allowing one-half of the root system to be exposed to dry soil, and switching irrigation between sides weekly; RDI in which 50% of the amount of water given to the controls was supplied, half to each side of the root system. Relative water content (RWC), midday leaf Ψ and chlorophyll content decreased largely in RDI-treated plants, whereas the PRD plants exhibited relatively higher Ψ and RWC values. An enhanced level of lipid peroxidation in both roots and leaves indicated that PRD and RDI caused oxidative stress in tomato plants. In leaves, superoxide dismutase (SOD), soluble peroxidase (POX) and polyphenol oxidase (PPO) activities showed an increase in the early phase of water deficit, and then decreased in the remaining phase of the drying cycle. However, the increase was more pronounced under RDI. Catalase (CAT) activity declined continuously from the onset of PRD and RDI treatments to below the control level, and the reduction was less under PRD than RDI. POX cell-wall associated activities exceeded the control level by 450% and 230%, respectively, under RDI and PRD. At the root level, while CAT activity also decreased under both PRD and RDI, the activities of SOD, POX and PPO significantly increased and their activities showed an alternating increase/decrease paralleling the alternating irrigation in PRD-treated roots. As a result of the difference in POX and PPO activities between the two water treatments applied, PRD-treated plants accumulated more soluble and cell-wall bound phenolic compounds.

Abscisic acid signalling when soil moisture is heterogeneous: decreased photoperiod sap flow from drying roots limits abscisic acid export to the shoots

To investigate the contribution of different parts of the root system to total sap flow and leaf xylem abscisic acid (ABA) concentration ([X-ABA](leaf)), individual sunflower (Helianthus annuus L.) shoots were grafted onto the root systems of two plants grown in separate pots and sap flow through each hypocotyl measured below the graft union. During deficit irrigation (DI), both pots received the same irrigation volumes, while during partial root zone drying (PRD) one pot ('wet') was watered and another ('dry') was not. During PRD, once soil water content (theta) decreased below a threshold, the fraction of sap flow from drying roots declined. As theta declined, root xylem ABA concentration increased in both irrigation treatments, and [X-ABA](leaf) increased in DI plants, but [X-ABA](leaf) of PRD plants actually decreased within a certain theta range. A simple model that weighted ABA contributions of wet and dry root systems to [X-ABA](leaf) according to the sap flow from each, better predicted [X-ABA](leaf) of PRD plants than either [X-ABA](dry), [X-ABA](wet) or their mean. Model simulations revealed that [X-ABA](leaf) during PRD exceeded that of DI with moderate soil drying, but continued soil drying (such that sap flow from roots in drying soil ceased) resulted in the opposite effect.

Involvement of abscisic acid and polyamines in berry ripening ofVitis vinifera(L.) subjected to water deficit irrigation

Background and Aims: Partial rootzone drying (PRD) is an irrigation system that permits better control of vegetative growth without reducing fruit yield. However, little is known about how grapevine reproductive development is regulated when irrigated under PRD. Therefore, the aim of this work was to investigate the relationship between abscisic acid (ABA) and polyamines (PAs) in leaves and berries during ripening, and how hormonal balance is affected under different irrigation regimes. Methods and Results: The study was carried out using container-grown Superior Seedless grapevines with a split-root system and grown under controlled conditions in a greenhouse. Three irrigation treatments were imposed: control (well watered), and two forms of deficit irrigation – PRD and sustained deficit irrigation (SDI). PRD plants have higher yield and berry size than SDI vines. At the onset of veraison, PRD berries have higher ABA, free PAs and free to bound PA ratio than other treatments. Conclusions: Although the restriction of vegetative growth in PRD might be partly a response to the volume of water applied and therefore similar to a SDI response, our results suggest that specific hormonal factors may explain the effects on reproductive growth and yield. Significance of the Study: The study demonstrates that changes induced in berry ABA during PRD conditions could be counterbalanced by changes in PA metabolism so that berry size and yield are maintained.

Measurement and modelling of ABA signalling in potato ( Solanum tuberosum L.) during partial root-zone drying

The objective of this study was to develop a simple mechanistic model to predict the magnitude of ABA signalling ([X-ABA]) of potatoes ( Solanum tuberosum L.) exposed to partial root-zone drying (PRD). Potatoes were grown in pots in a glasshouse with the roots split equally between two soil columns. At tuber initiation stage, plants were subjected to three irrigation treatments: (1) both soil columns were fully irrigated (FI) daily to a volumetric soil water content ( θ) of 18.0%; (2) PRD, in which one soil column was irrigated daily to 18.0% while the other was allowed to dry, and the irrigation was shifted between columns when the θ of the drying soil column had decreased to 7–8%; (3) non-irrigation (NI), where irrigation was withheld after onset of treatments and lasted for 5 days until θ had decreased to 7%. In the PRD plants, the fraction of soil water extraction (FSWE) by the dry roots declined exponentially with declining soil water potential ( Ψ soil-dry); however, after shifting of irrigation, the previously dried roots immediately recovered the full capacity of water uptake. During the first PRD drying cycle, FI plants had the highest stomatal conductance ( g s), and followed by PRD plants and NI plants had the lowest g s. Photosynthesis ( A) was similar for FI and PRD plants, and was significantly lower for the NI plants only on 3–4 days after treatment. In the NI plants, a linear relationship between Ψ soil and [X-ABA] was obtained. Based on these relationships, a simple model predicting [X-ABA] in the PRD plants ([X-ABA] PRD) was developed. Assuming that a constant [X-ABA] of 115 nM (similar to that found in the FI plants) originated from the wet roots; the simulation results indicated that irrigation should be shifted between the two sides when Ψ soil-dry had decreased to ∼−80 kPa, and [X-ABA] PRD had reached a peak of ca. 150 nM. However, the [X-ABA] PRD predicted by the model was significantly lower than the measured value; whilst a simple average of [X-ABA] from the wet and the dry soil columns based on the [X-ABA]– Ψ soil relationship better predicted [X-ABA] PRD.

Water relations, photosynthesis, growth and water-use efficiency in tomato plants subjected to partial rootzone drying and regulated deficit irrigation

Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) are water-saving irrigation systems that have been developed to increase water-use efficiency (WUE) without significant yield reduction. In order to investigate whether a high-value horticultural crop such as tomato responded differently to RDI and PRD, we compared the physiological and growth responses of tomato plants using a split-root system. Plants were grown in a greenhouse under controlled conditions with their roots separated equally between two soil compartments. Three irrigation treatments were imposed: (i) Control, receiving an amount of water equivalent to 100% of plant transpiration; (ii) PRD, in which one compartment was watered with 50% of the amount of water supplied to the controls, allowing one-half of the root system to be exposed to dry soil and switching irrigation between sides weekly; and (iii) RDI, in which 50% of the amount of water given to the controls was supplied, half to each side of the root system. Leaf RWC and midday leaf Ψ decreased substantially in RDI-treated plants, while the PRD plants exhibited relatively higher Ψ and RWC values. Both PRD and RDI treatments reduced by about 30% the total plant dry mass compared with the control. However, plant transpiration was reduced by about 50% in both PRD and RDI, allowing a significant improvement in whole-plant WUE. Stomatal conductance (Gs) and leaf growth were also significantly reduced by PRD and RDI. These results may be related to a significant increase in xylem sap pH and leaf apoplastic pH. Generally, the photosynthetic apparatus of tomato leaves had a high resistance to restricted water availability. In fact, the decreased Gs had no major negative impact on carbon assimilation. However, V cmax, i.e. Rubisco efficiency, was significantly decreased in RDI plants with respect to control ones. This may imply that, although the differences between the PRD and RDI treatments in our study were subtle, they may become more marked with a more prolonged and severe water deficit.

Soil moisture heterogeneity during deficit irrigation alters root-to-shoot signalling of abscisic acid.

The effects of different irrigation techniques on leaf xylem ABA concentration ([X-ABA]leaf) were compared in tomato (Lycopersicon esculentum Mill.). During partial rootzone drying (PRD), water was distributed unevenly to the root system such that part was irrigated while the remainder was allowed to dry the soil. During conventional deficit irrigation (DI), plants received the same volume of water as PRD plants, but water was distributed evenly to the entire root system. When the plant root system was allowed to explore two separate soil compartments, DI plants had a higher [X-ABA]leaf than PRD plants with moderate soil drying, but PRD plants had a higher [X-ABA]leaf than DI plants as the soil dried further. The difference in [X-ABA]leaf between the two sets of plants was not because of differences in either whole pot soil water content (θpot) or leaf water potential (Ψleaf). To investigate the contribution of different parts of the root system to [X-ABA]leaf, individual shoots were grafted onto the root systems of two plants grown in two separate pots, so that the graft union had the appearance of an inverted 'Y'. After sap collection from detached leaves, removal of the shoot below the graft union allowed sap collection from each root system. Again, DI plants had a higher [X-ABA]leaf than PRD plants when the soil was relatively wet, but the opposite occurred as the soil dried. Root xylem ABA concentration ([X-ABA]root) increased exponentially as soil water content (θ) declined. In DI plants, [X-ABA]root from either pot (or the arithmetic mean of [X-ABA]root) accounted for a similar amount of the variation in [X-ABA]leaf. In PRD plants, [X-ABA]root from the watered side underestimated [X-ABA]leaf, whereas [X-ABA]root from the dry side overestimated [X-ABA]leaf. The arithmetic mean of [X-ABA]root best explained the variation in [X-ABA]leaf, implying continued sap flow from the dry part of the root system (Jdry) at soil water potentials (Ψsoil) at which Jdry had ceased in previous studies of PRD plants (Yao et al. 2001). Evaluating the relationship between Jdry and Ψsoil may assist in maintaining export of ABA (and other growth regulators) from the drying part of the root system, to achieve desirable horticultural outcomes during PRD.

Water relations and yield of lysimeter-grown strawberries under limited irrigation

Abstract The effects of partial root-zone drying (PRD), as compared to deficit irrigation (DI) and full irrigation (FI), on strawberry (cv. Honeoye) berry yield, yield components and irrigation water use efficiency (WUE I ) were investigated in a field lysimeter under an automatic rain-out shelter. The irrigation treatments were imposed from the beginning of flowering to the end of fruit maturity. In FI the whole root zone was irrigated every second day to field capacity viz. volumetric soil water content ( θ ) of 20%; while in DI and PRD 60% water of FI was irrigated to either the whole or one-half of the root system, respectively, at each irrigation event. In PRD, irrigation was shifted from one side to the other side of the plants when θ of the drying side had decreased to 8–11%. Compared to FI plants, leaf water potential was significantly lower in DI and PRD plants in 3 out of 10 measurement occasions, while stomatal conductance was similar among the three treatments. Leaf area, fresh berry yield (FY), individual berry fresh weight, berry water content, and berry dry weight (DW) were significantly lower in DI and PRD plants than those of FI plants; whereas the total number of berry per plant was similar among treatments. Compared with FI, the DI and PRD treatments saved 40% of irrigation water, and this led to a 28 and 50% increase of WUE I based on berry FY and DW, respectively, for both DI and PRD. Conclusively, under the conditions of this study PRD had no advantage compared to DI in terms of berry yield and WUE I . DI and PRD similarly decreased berry yield and yield components and thus cannot be recommended under similar conditions.