Alternate Partial Root-zone Drying Research Articles

Biochar amendment modulates xylem ionic constituents and ABA signaling: its implications in enhancing water-use efficiency of maize (Zea mays L.) under reduced irrigation regimes

While biochar amendment is known to enhance plant productivity and water-use efficiency (WUE), particularly under water-limited conditions, the specific mechanisms driving these benefits remain largely elusive. Thus, the present study was conducted to elucidate the synergistic effects of biochar and reduced irrigation on maize (Zea mays L.) plants, focusing on xylem composition, root-to-shoot signaling, stomatal behavior, and WUE. Maize plants were cultivated in split-root pots filled with clay loam soil, amended by either wheat-straw (WSB) or softwood (SWB) biochar at 2% (w/w). Plants received full (FI), deficit (DI), or alternate partial root-zone drying (PRD) irrigation from the fourth leaf to the grain-filling stage. Our results revealed that WSB amendment significantly enhanced plant water status, biomass accumulation, and WUE under reduced irrigation, particularly when combined with PRD. Although reduced irrigation inhibited photosynthesis, it enhanced WUE by modulating stomatal morphology and conductance. Biochar amendment combined with reduced irrigation significantly increased xylem K+, Ca2+, Mg2+, NO3-, Cl-, PO43-, and SO42- while decreased Na+, which in turn lowered xylem pH. Moreover, biochar amendment and especially WSB amendment further resulted in increased abscisic acid (ABA) contents in both leaf and xylem sap under reduced irrigation conditions due to changes in xylem ionic constituents and pH. The synergistic interactions between xylem components and ABA led to refined adjustments in stomatal size and density, thereby affecting stomatal conductance and ultimately improving WUE of maize plants at different scales. The combined application of WSB and PRD can therefore emerge as a promising approach for improving the overall plant performance of maize plants with increased stomatal adaptations and WUE especially under water-limited conditions.

The effect of alternate partial root-zone drying and deficit irrigation on the yield, quality, and physiochemical parameters of milled rice

While paddy fields produce a high yield, they also require a large amount of water and produce a significant amount of methane. Therefore, the adoption of water-saving irrigation techniques for rice cultivation is critical. Furrowed rice farming may be a viable alternative to paddy rice cultivation. The objective of the present study was to evaluate the impact of alternate partial root-zone drying irrigation on rice yield, milled rice quality, and cooking quality under furrow rice cultivation. A two-year field trial was conducted on a local rice cultivar, Tarom Hashemi, in 2015 and 2016. Seven water regimes, including three levels of regulated deficit irrigation (RDI), three levels of alternate partial root-zone drying irrigation (APRDI), and conventional flooding irrigation (CFI), were used in this study. In RDI and APRDI treatments, plots were irrigated when soil matric potential had reached −0.1 (RDI1 and APRDI1), −0.3 (RDI3 and APRDI3), and −0.6 bar (RDI6 and APRDI6). RDI1 and APRDI1 treatments produced milled rice yield similar to the CFI, while irrigation water productivity (IWP) was significantly higher by 22.9% and 45.7%, respectively. Regardless of the soil water potential, the IWP in APRDI treatments was 16% higher than that of RDI treatments. Severe water stress (RDI6 and APRDI6) caused a marked increase in amylose content and alkali spreading value of milled rice resulting in improved cooking quality. Nitrogen uptake in APRDI treatments was 2% higher than that of RDI treatments. On average, methane emission per milled grain yield declined by 77.9% and 78.7% in RDI and APRDI treatments, respectively. Our data indicate that the expensive and laborious practice of puddling can be avoided to increase water productivity and improve rice quality without sacrificing yield. The results also show that furrow rice cultivation could significantly reduce the methane emission contribution of rice production.

The effect of irrigation management on rice grain yield, irrigation water productivity and methane emissions in northern Iran

AbstractThis experiment aims to study the effect of water‐saving irrigation on rice production and methane emissions. Three irrigation methods, that is, flooding irrigation, deficit irrigation (DI) and alternate partial root‐zone drying (APRD), were employed in a 2‐year field trial. The experiment was conducted in a randomized complete block design with three replications. For the DI and APRD treatments, plots were not irrigated until the soil matric potential reached −10 (DI10 and APRD10), −30 (DI30 and APRD30) and −60 kPa (DI60 and APRD60). In the APRD treatments, two adjacent furrows of each plant row were alternately irrigated in consecutive irrigation events. The DI and APRD treatments did not affect yield components when the soil matric potential was −10 kPa. Applying APRD treatments increased irrigation water productivity by 45%, 51% and 31% for soil matric potentials of −10, −30 and −60 kPa, respectively. On average, water stress treatments reduced methane emissions by 78% compared to flooding treatment. On average, the APRD treatments decreased the water input by 10% compared with the DI treatments while achieving a 2.5% higher yield. This is the first study to outline the effect of APRD on methane emissions and rice production. These findings suggest that APRD could be a valuable strategy to mitigate methane emissions without risking rice yield reduction.

Biochar amendment alters root morphology of maize plant: Its implications in enhancing nutrient uptake and shoot growth under reduced irrigation regimes.

Biochar amendment provides multiple benefits in enhancing crop productivity and soil nutrient availability. However, whether biochar addition affects root morphology and alters plant nutrient uptake and shoot growth under different irrigation regimes remain largely unknown. A split-root pot experiment with maize (Zea mays L.) was conducted on clay loam soil mixed with 2% (w/w) of wheat-straw (WSP) and softwood (SWP) biochar. The plants were subjected to full (FI), deficit (DI), and alternate partial root-zone drying (PRD) irrigation from the fourth leaf to the grain-filling stage. The results showed that, compared to plants grown in unamended soils, plants grown in the biochar-amended soils possessed greater total root length, area, diameter, volume, tips, forks, crossings, and root length density, which were further amplified by PRD. Despite a negative effect on soil available phosphorus (P) pool, WSP addition improved soil available nitrogen (N), potassium (K), and calcium (Ca) pool and cation exchange capacity under reduced irrigation. Even though biochar negatively affected nutrient concentrations in shoots as exemplified by lowered N, P, K (except leaf), and Ca concentration, it dramatically enhanced plant total N, P, K, Ca uptake, and biomass. Principal component analysis (PCA) revealed that the modified root morphology and increased soil available nutrient pools, and consequently, the higher plant total nutrient uptake might have facilitated the enhanced shoot growth and yield of maize plants in biochar-added soils. Biochar amendment further lowered specific leaf area but increased leaf N concentration per area-to-root N concentration per length ratio. All these effects were evident upon WSP amendment. Moreover, PRD outperformed DI in increasing root area-to-leaf area ratio. Overall, these findings suggest that WSP combined with PRD could be a promising strategy to improve the growth and nutrient uptake of maize plants.

Aquaporin expression and changes in fatty acid levels in apple leaves subjected to partial root-zone irrigation

ABSTRACT Partial root-zone drying (PRD) can induce changes in levels of the membrane fatty acids, transporter channels such as aquaporins (AQPs) and drought stress-responsive transcription factors (TFs) expression. The apple (Malus domestica (Suckow) Borkh. cv. Braeburn) trees were exposed to different PRD treatments for two consecutive years. The plasma-membrane intrinsic proteins (PIP1;1) and tonoplast-intrinsic proteins (TIP1;1) AQPs presented the highest expression in alternate partial root-zone drying (APRD75) and fixed partial root-zone drying (FPRD75) treatments than FPRD50 and control throughout the entire irrigation season, which was 120-140-fold higher in these treatments during some times. The TFs were highly expressed in APRD75 and FPRD75 treatments. The highest unsaturated fatty acid content was observed in APRD75 and FPRD75 treatments, and the lowest saturated fatty acid content was obtained in FPRD50 in both years. In addition to improving the water use efficiency (WUE) and maintaining the yield, APRD75 and FPRD75 treatments had a higher jasmonic acid (JA) content than the control. These results implied that changes in levels of the membrane fatty acids, AQPs, JA and TFs induced by APRD75 and FPRD75 treatments may be one of the possible ways for apple trees to adapt to the changing environmental conditions caused by PRD.

INFLUENCE OF PARTIAL ROOT-ZONE DRYING ON WATER USE EFFICIENCY AND PHOSPHORUS UPTAKE OF CORN (ZEA MAYS L.) IN A GYPSIFEREOUS SOIL

Pot experiment was conducted to study the alternate and fixed partial root zone drying on irrigation water use efficiency and phosphorus absorption efficiency of corn (Zea mays L.) in a gypsiferous soil during the spring growing season. The experiment included three irrigation techniques, which were conventional irrigation (C1), alternating partial root-zone drying (APRD) and fixed partial root-zone drying (FPRD), and three levels of phosphate fertilizer application (40, 80, 120 Kg h-1), for three stages of plant growth, jointing stage (40 days from planting), booting stage (60 days from planting) and maturing stage (105 days from planting). The experiment was carried out with a complete randomized design with nine replications. Yellow corn seeds variety DKC6664 were planted on 20/3/2020. During the aforementioned growth stages, the development of the root system and vegetative parts of corn plants were studied and the water use efficiency was calculated for each growth stage. Results showed a decrease in the ratio of root to the vegetative weight under the alternating and fixed root-zone drying for the three fertilization levels and the three-growth stage (jointing, booting, maturing). The conventional irrigation treatment gave a grain yield of 40.53, 51.44 and 57.71 g pot-1 for the fertilization levels of 40, 80 and 120 kg P h-1 with a significant difference compared to alternate and fixed partial root-zone drying treatments. Alternate partial root-zone drying give the highest water use efficiency of 0.90, 0.93 and 0.98 kg m-3. Higher values of phosphorus absorption was recorded with CI treatment. The phosphorus absorption was high in the jointing and booting stages of corn growth. Phosphorous reduced some of the negative effects of water shortage on plant growth and yield and the reason for an increase in water use efficiency.

Role of alternate and fixed partial root-zone drying on water use efficiency and growth of maize (Zea mays L.) in gypsiferous soils

Alternate partial root-zone drying (APRD) is a water-saving method but can regulate crop physiological responses. A pot experiment has been conducted to study the efficiency of partial and fixed root-zone drying on the growth and production of maize (Zea mays L.) in addition to the water use efficiency in soils with different gypsum content. The experimental treatments include three irrigation treatments, i.e. Conventional Irrigation (CI), Alternate Partial Root-zone Drying (APRD) and Fixed Partial Root-zone Drying (FPRD), and three soils with different gypsum content " (60.0 [G1], 153.7 [G2], and 314.2 [G3] g kg−1)". The vegetative growth, root dry mass and physiological indices (leaf relative water content, carotenoid concentration, proline) have been studied during three stages of maize plant growth (jointing, tasselling, and maturing). The Results showed that compared to CI, APRD and FPRD increased water use efficiency by 38.93 and 14.94% based on dry seed yield. In addition, compared to CI, APRD increased maize seed yield by 4.62–20.71%, while FPRD decreased yield by 19.24–5.28% for the gypsiferous soils G2 and G3, respectively. APRD has a slight effect on leaf water potential, leaf relative water content, carotenoid and proline activities from jointing to maturing stages at the three gypsiferous soils. Results suggest that APRD could make maize plants use water even more productively with better adaptation to water shortages in the gypsiferous soils.

Effect of alternate partial root-zone drying (PRD) on soil nitrogen availability to alfalfa

Available soil N is critical for N fertilizer management in crop production but the effects of alternate partial root-zone drying (PRD) on the dynamics of N and the impact of these effects on perennial crops such as alfalfa (Medicago sativa L.) have not been evaluated. A field experiment was conducted to compare two irrigation modes (PRD and conventional furrow irrigation (CFI)) in combination with four irrigation levels (70% ETc (evapotranspiration), 85% ETc, 100% ETc, and 115% ETc). PRD improved the mineralization of organic matter to increase available N and the NO3−-N/NH4+-N ratio. Thus, it increased N uptake, water productivity and maintained seasonal forage yield. The combination of PRD and irrigation level (100% Etc) produced maximum NO3−-N stock. These results present an evaluation of how PRD influences soil N availability to N-fixing perennial crops.

Partial root-zone drying irrigation increases water-use efficiency of tobacco plants amended with biochar

Modulation of stomatal morphology and behavior is associated with changes in water use of the plant due to varied input of water, yet the underlying mechanism of such association remains largely elusive. We investigated the effects of reduced irrigation on leaf gas exchange and stomatal morphology of tobacco (Nicotiana tabacum L.) plants grown in a Ferralsol and an Anthrosol amended by softwood (SWB) and wheat (Triticum aestivum L.) straw biochar (WSB). Compared to plants grown under full irrigation (FI), deficit irrigation (DI) and especially alternate partial root-zone drying irrigation (PRD) significantly decreased the stomatal conductance (gs) while marginally reducing the leaf photosynthetic rate (An), thereby enhancing intrinsic water-use efficiency (WUEi=An/gs), whereas the effects of biochars were not evident. Moreover, reduced irrigation, particularly PRD, increased the concentration of abscisic acid in the leaf ([ABA]leaf) compared to FI, which was further amplified by WSB addition. Leaf carbon isotope discrimination (Δ13Cleaf) responded similarly to irrigation and biochar addition as gs. Compared to FI, reduced irrigation (DI and PRD) lowered stomata size (SS) but increased their density (SD) and this effect was more pronounced for PRD coupled with WSB. Moreover, SD was significantly positively correlated with [ABA]leaf, while negatively correlated with gs, soil water content, and Δ13Cleaf. The opposite correlations between SS and these variables were observed. The results collectively suggest that ABA-mediated the effects of PRD combined with WSB amendment resulted in higher SD and lower SS and gs, contributing to the increased WUEi and long-term WUE of tobacco plants.

EVALUATION OF PARTIAL ROOT ZONE DRYING IRRIGATION UNDER GATED PIPE FOR CORN CROP

In recent time, Egypt is suffering from the scarcity of fresh water. In addition, Egypt is one of the event states under the water poverty line, who is identified as less than 1000 m3 per capita per year due to the existence of dry climatic conditions in most parts of the country and limited available water resources, therefore optimization and saving of water consumption have vital importance. The main goal of this research is to study the effect of Partial Root zone Drying (PRD) on corn yield, water distribution efficiency and water use efficiency compared with conventional irrigation (CI).Field experiment was conducted during summer season of 2017 in the Agricultural research station, Etay El-Baroud, El-Behera Governorate (Etay-El-Baroud region is 6m above sea level, 30˚ 88 / N and 30˚ 66 / E).Corn plants were planted under different irrigation regimes which were Alternate Partial Root zone Drying (APRD),Fixed Partial Root zone Drying (FPRD),and Conventional Irrigation (CI) comparing with control irrigation. The irrigation regimes were carried out under two levels of land leveling (0.05% - 0.1%), and three levels of water cutting times. The experimental treatments were irrigated by 152mm diameter of PVC gated pipes system. The results indicated that, the APRD treatment achieved the highest value of corn production (7.85Mg ha-1) when the applied water was reached to the end of furrow plus 5 minutes storage (Q3) under furrow slope of 0.1% and improving water use efficiency (WUE) when applied water was reached 5m before the end of furrow length (Q2) by 1.73 kg m-3 under furrow slope of 0.1%. Also, the APRD regime saved 37.16% of applied water under Q2 and furrow slope of 0.1%.

EVALUATION OF PARTIAL ROOT ZONE DRYING IRRIGATION UNDER GATED PIPES SYSTEM FOR CORN CROP

In recent time, Egypt is suffering from the scarcity of fresh water. In addition, Egypt is one of the event states under the water poverty line, who is identified as less than 1000 m3 per capita per year due to the existence of dry climatic conditions in most parts of the country and limited available water resources, therefore optimization and saving of water consumption have vital importance. The main goal of this research is to study the effect of Partial Root zone Drying (PRD) on corn yield, water distribution efficiency and water use efficiency compared with conventional irrigation (CI).Field experiment was conducted during summer season of 2017 in the Agricultural research station, Etay El-Baroud, El-Behera Governorate (Etay-El-Baroud region is 6m above sea level, 30˚ 88 / N and 30˚ 66 / E).Corn plants were planted under different irrigation regimes which were Alternate Partial Root zone Drying (APRD),Fixed Partial Root zone Drying (FPRD),and Conventional Irrigation (CI) comparing with control irrigation. The irrigation regimes were carried out under two levels of land leveling (0.05% - 0.1%), and three levels of water cutting times. The experimental treatments were irrigated by 152mm diameter of PVC gated pipes system. The results indicated that, the APRD treatment achieved the highest value of corn production (7.85Mg ha-1) when the applied water was reached to the end of furrow plus 5 minutes storage (Q3) under furrow slope of 0.1% and improving water use efficiency (WUE) when applied water was reached 5m before the end of furrow length (Q2) by 1.73 kg m-3 under furrow slope of 0.1%. Also, the APRD regime saved 37.16% of applied water under Q2 and furrow slope of 0.1%.

Partial root-zone drying irrigation in orange orchards: Effects on water use and crop production characteristics

Abstract We have studied the effects of partial root-zone drying (PRD) on plant physiological response, plant-soil water dynamics, yield and fruit quality of young orange trees during the irrigation seasons 2013 and 2014 in an orchard located in Eastern Sicily (Southern Italy). The irrigation treatments included: (i) full irrigation (T1), with trees irrigated by supplying 100% of crop water demand using micro-irrigation systems; and (ii) alternate partial root-zone drying (T4), with trees irrigated at 50% of crop water demand. Minimally invasive electrical resistivity tomography (ERT) was adopted to help quantify root-water-uptake (RWU) processes at the finer (decimetric) spatial scale. Results show that, compared with the full irrigation treatment, PRD at 50% of crop water demand (ETc) increased the fruit yield by 20% in 2013 and 10% in 2014. The PRD irrigation treatment, which induces a reduction of the wetted soil volumes, had also obvious positive effects on water use efficiency (WUE), compared to full irrigation. From the results of this study, we concluded that when water resources are limited, PRD at 50% level of ETc is an efficient water saving strategy to increase WUE, while other physiological and growth parameters are practically unaffected.

Alternate partial root-zone drying irrigation improves fruit quality in tomatoes

 Alternate partial root-zone drying (PRD) irrigation and deficit irrigation (DI) are water-saving irrigation strategies. Here, comparative effects of PRD and DI on fruit quality of tomato (Solanum lycopersicum L.) were investigated. The results showed that the irrigation treatments had no effect on tomato yield but significantly affected several organic and mineral quality attributes of the fruits. Compared to DI, PRD significantly increased the fruit concentrations of Ca and Mg, and fruit juice concentrations of total soluble solid, glucose, fructose, citric and malic acid, P, K and Mg. It is concluded that PRD is better than DI in terms of improving fruit quality, and could be a promising management strategy for simultaneous increase of water use efficiency and fruit quality in tomatoes.

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.

Effect of water stress in partial root zone on response of photosynthesis of Aconitum barbatum

To study the effect of different water stress mode on photosynthesis characteristics of Aconitum barbaturnm, aimed at providing the theoretical basis for cultivating this kind of plant for the arid area. The transplanted seedlings of A. barbatum were processed with fixed partial root zone drying(TI) ,alternate partial root zone drying(T2) and both partial root zone irrigation (T3) for three cycles,and relative parameters were measured. The net photosynthetic rate (Pn) in the treatment of T2 gradually increased and was higher than that in the treatment of T3 about 6.53%, mainly effected by non-stomatal factors. The water use efficiency (WUE) was higher than that in the treatments of T3 about 25.21%, while the transpiration rate (Tr) and the stomatal conductance was lower than those in the treatment of T1 and T3 separately. The maximum net photosynthesis rate (Pmax),the apparent quantum yield (AQY) and the dark respiration rate (Rd) were higher than those in comparison condition slightly. The alternate partial root zone drying is worth promoting vigorously, and it shall be a rational way to cultivate the A. barbatum under woodland.

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.

Vine Water Relations and Quality of ‘Muscat of Alexandria’ Table Grapes Subjected to Partial Root-zone Drying and Regulated Deficit Irrigation

The effects of several irrigation strategies on vine water relations and the quality of table grapes, Vitis vinifera L. ‘Muscat of Alexandria’, were evaluated from veraison to harvest. The treatments included: (1) standard practice irrigation (control): re-irrigation when the soil moisture tension reached 15 kPa; (2) regulated deficit irrigation (RDI): re-irrigation 4 to 7 days after reaching a soil moisture tension of 15 kPa; (3) fixed partial root-zone drying (FPRD): one half of the root system was re-irrigated when the soil moisture tension reached 15 kPa; and (4) alternate partial root-zone drying (APRD): one half of the root system was re-irrigated when the soil moisture tension reached 15 kPa, and every week the irrigated half was switched. Treatments were continued for 7 weeks until harvest. During the experiment, RDI vines received 58% less irrigation water than the controls, while 33% less irrigation water was applied to FPRD and APRD vines. The results showed that only RDI vines had a significantly lower midday stem water potential compared with the control. Vines of FPRD and APRD treatments had the highest efficiency of water use (photosynthetic rate/stomatal conductance). At harvest, RDI fruit had markedly higher TSS, sugars, and amino acids, a similar acidity, and lower firmness and smaller size compared with the control. FPRD and APRD fruits had slightly higher TSS, sugars, and amino acids, a similar firmness and size, and lower acidity compared with the control. Aroma concentrations were high in RDI and FPRD fruits.

Alternation of wet and dry sides during partial rootzone drying irrigation alters root-to-shoot signalling of abscisic acid.

Partial rootzone drying (PRD) is an irrigation technique where water is distributed unevenly to the root system such that part is irrigated while the remainder is allowed to dry the soil. Tomato (Lycopersicon esculentum Mill.) plants were grown with their roots in two soil columns to compare the physiological consequences of alternation of wet and dry columns during PRD irrigation (alternate PRD, PRD-A) with retention of the same wet and dry columns (fixed PRD, PRD-F). When PRD plants received 50% less water than well-watered (WW) plants, xylem ABA concentration ([X-ABA]) increased and stomatal conductance decreased relative to WW plants. Although both sets of PRD plants received the same amount of water, [X-ABA] of PRD-A plants increased up to 2-fold above that of PRD-F plants, which further decreased stomatal conductance. Differences in [X-ABA] were detected within an hour of alternation, but did not persist beyond the photoperiod of alternation. [X-ABA] increased linearly as whole-pot soil water content (θpot) and leaf water potential (Ψleaf) declined, but the difference in [X-ABA] between the two sets of PRD plants was not due to differences in either θpot or Ψleaf. In PRD-F plants, the unwatered part of the root system contributes proportionally less to the transpiration stream as the soil progressively dries (Yao et al. 2001, Plant, Cell & Environment 24, 227-235). In PRD-A plants, we hypothesise that re-watering the dry part of the root system allows these roots to contribute proportionally more to total sap flux, thus liberating a pulse of ABA to the transpiration stream as the root ABA pool accumulated during soil drying is depleted. Since the enhancement of [X-ABA] caused by PRD-A increased as θpot and Ψleaf declined, an optimal frequency of alternation to maximise the cumulative physiological effects of this ABA pulse must consider possible negative impacts of leaf water deficit as soil water status declines.

The effects of partial rootzone drying on root, trunk sap flow and water balance in an irrigated pear ( Pyrus communis L.) orchard

Partial rootzone drying (PRD) means that part of the root system is watered as in full irrigation while the rest is exposed to soil drying. This practice is predicted to influence field hydrological circle. We studied the effect of this practice on soil water distribution, root and trunk sap flow, water consumption of pear trees, and capillary contribution from ground water table and water balance for three months in an irrigated orchard with a shallow ground water table. The irrigation treatments included: (a) conventional flooded irrigation (CFI), (b) fixed partial rootzone drying (FPRD), and (c) alternate partial rootzone drying (APRD). Root and trunk sap flows were monitored using a heat-pulse sap flow meter. The results showed that there were significant differences of soil water content in both sides of rootzone under partial drying. The capillary contribution from ground water table was significantly increased in APRD and FPRD when compared with CFI. More significantly, the total irrigation amount was greatly reduced, by 43.64 and 45.84%, respectively, for APRD and FPRD. The two PRD treatments used more soil-stored water while CFI had more drainage. The root sap flow on the wet side was substantially enhanced as a result of PRD, and was greater than that from same side in CFI. The trunk sap flow in FPRD and APRD was smaller than that in CFI. On average, both APRD and FPRD reduced plant daily water consumption by about 9.96 and 17.97%, respectively, when compared to CFI during the PRD period. Daily root water flow was a significant function of the reference evapotranspiration. The daily trunk water flow was also related to the reference evapotranspiration but the CFI carried more water than APRD and FPRD under the same evaporation demand, suggesting a restriction of transpirational water loss in the PRD trees. CFI needed a higher soil water content to carry the same amount of trunk flow than the PRD trees, suggesting the hydraulic conductance of roots in PRD trees enhanced, and the roots had a greater water uptake capacity than in CFI when the average soil water content in the rootzone was the same.