Non-hydraulic Root Signals Research Articles

Foliar application of strigolactones improves the desiccation tolerance, grain yield and water use efficiency in dryland wheat through modulation of non-hydraulic root signals and antioxidant defense

Non-hydraulic root signals (nHRS) are affirmed as a unique positive response to soil drying, and play a crucial role in regulating water use efficiency and yield formation in dryland wheat production. Strigolactones (SLs) can enhance plant drought adaptability. However, the question of whether strigolactones enhance grain yield and water use efficiency by regulating nHRS and antioxidant defense systems in dryland wheat remains unanswered. In this study, pot experiments were conducted to investigate the effects of strigolactones on nHRS, antioxidant defense system, and grain yield and water use efficiency in dryland wheat. The results showed that external application of SLs increased drought-induced abscisic acid (ABA) accumulation and activated an earlier trigger of nHRS at 73.4% field capacity (FC), compared to 68.5% FC in the control group (CK). This phenomenon was mechanically associated with the physiological mediation of SLs. The application of SLs significantly enhanced the activities of leaf antioxidant enzymes, reduced ROS production, and mitigated oxidative damage to lipid membrane. Additionally, root biomass, root length density, and root to shoot ratio were increased under strigolactone treatment. Furthermore, exogenous application of SLs significantly increased grain yield by 34.9% under moderate drought stress. Water use efficiency was also increased by 21.5% and 33.3% under moderate and severe drought conditions respectively, compared to the control group (CK). The results suggested that the application of strigolactones triggered earlier drought-sensing mechanism and improved the antioxidant defense ability, thus enhancing grain yield and water use efficiency in dryland wheat production.

Differentiate responses of tetraploid and hexaploid wheat (Triticum aestivum L.) to moderate and severe drought stress: a cue of wheat domestication

ABSTRACT Differentiate mechanism of wheat species in response to contrasting drought stress gradients implies a cue of its long-term domestication. In the present study, three water regimes including well-watered control (WW, 80% field water capacity (FC)), moderate drought stress (MS, 50% FC,) and severe drought stress (SS, 30% FC) were designed to reveal different responses of eight wheat species (four tetraploid and four hexaploid) representing different breeding decades and genetic origins to drought stresses. The data indicated that 50% FC and 30% FC fell into the soil moisture threshold range of non-hydraulic and hydraulic root signal occurrence, respectively. In general, grain yield, grain number/spike weight per plant, aboveground biomass, harvest index (HI) and water use efficiency (WUE) were significantly higher in hexaploid species than those of tetraploid species under drought stress (P < .05). Particularly, non-hydraulic root signal was triggered and continuously operated under 50% FC, while hydraulic root signal was observed under 30% FC, respectively. Under 80% FC, the allometric exponent (ɑ) of Maboveground vs Mroot decreased from tetraploid to hexaploid (both were of <1), indicating that during the domestication, the hexaploid species allocated less biomass to root system. For the relationship of Mear vs Mvegetative, the ɑ value was significantly greater in the hexaploid species, showing that hexaploid wheat distributed more biomass to ear than tetraploid to improve yield. Under 50% FC, this trend was enhanced. However, under 30% FC, there was no significant difference in the ɑ value between two species. Additionally, correlation analyses on yield formation affirmed the above results. Therefore, drought tolerance tended to be enhanced in hexaploid species under the pressure of artificial selection than that of tetraploid species. When drought stress exceeded a certain threshold, both species would be negatively seriously affected and followed a similar mechanism for better survival.

Differentiate effects of non-hydraulic and hydraulic root signaling on yield and water use efficiency in diploid and tetraploid wheat under drought stress

Non-hydraulic root signals (nHRS) and hydraulic root signals (HRS) are important mechanisms that play a major role in determining plant productivity under drought stress. In the present study, six wheat varieties (three diploid and three tetraploid) were used to investigate the variation of yield, its components, water use efficiency (WUE) and biomass allocation under two signals. At the jointing stage, three water treatments were set up: i) well-watered control (WW, 80 % field water capacity (FC)), ii) moderate water stress (MS, 50 % FC) and iii) severe water stress (SS, 30 % FC). The results showed that around 50 % FC, wheat activated the nHRS to respond to drought. While around 30 % FC, wheat activated the HRS as a response to deeper drought. Under the operation of nHRS, the reduction of yield, aboveground biomass and WUE of diploid species was greater than that of tetraploid ones, but there were no differences under the operation of HRS. It could be argued that two different wheat species followed differentiate positive defense strategy in response to nHRS, but fell into a similar negative adaptive strategy under the HRS. The ploidy level of wheat also had a significant effect on wheat yield composition, WUE and biomass allocation model. As such, grain yield, grain number and spike weight per plant, aboveground biomass, harvest index (HI) and WUE were significantly greater in the tetraploid species than the diploids ones. In addition, the retention rate of aboveground biomass and grain decreased under the operation of nHRS and HRS. Path analysis showed that spikelet number, thousand grain weight and grain number were the main factors affecting yield formation. Grain number had the greatest direct effect on yield, while spikelet number and its grain number exerted the main effect on yield. These results were consistent under both signals.

Arbuscular mycorrhizal fungus alters root-sourced signal (abscisic acid) for better drought acclimation in Zea mays L. seedlings

Inoculation with arbuscular mycorrhizal (AM) fungi can modify stomatal behavior and increase antioxidant enzyme activities and therefore play a pivotal role in plant growth. We hypothesised that inoculation with AM fungi postpones the non-hydraulic root signal and alters physiological and biochemical traits, which enhances drought tolerance and water-use efficiency (WUEB) for plant biomass. Two pot experiments (including progressive soil drying and partial root-zone drying) were conducted to reveal how mycorrhizal colonization altered root signal and its effects on plant growth, biochemical traits and WUEB in maize seedlings in drying soil. In our experiments, inoculation with Funneliformis mosseae improved water absorption and reduced the sensitivity of roots to drought. In addition, it decreased leaf abscisic acid (ABA) content of inoculated plants. Regardless of water conditions, plant biomass production, antioxidant enzyme activity, net photosynthetic rate, stomatal conductance and WUEB were elevated in AM fungal treatments compared to non-AM fungal treatments. Under water-stressed conditions, inoculation with F. mosseae greatly reduced leaf ABA content, and postponed the decline in photosynthetic rate, stomatal conductance and osmotic adjustment. Malondialdehyde (MDA) level was significantly lower in mycorrhizal plants than in non-inoculation plants. However, inoculation with F. mosseae increased antioxidant enzyme activities including peroxidase (POD) and superoxide dismutase (SOD). In this study, inoculation with F. mosseae reduced ABA accumulation that acts as a non-hydraulic root signal and thereby postponed a decline in stomatal conductance and photosynthetic rate, improved water use efficiency and antioxidant enzymes activities, and accordingly reduced proline and MDA content. Thus, inoculation with AM fungi played a role in effective defense for better drought acclimation in water-stressed maize seedlings.

Comparative response to drought in primitive and modern wheat: a cue on domestication.

Primitive wheat follows an opposite metabolic law from modern wheat with regard to leaf biomass/reproductive growth vs above-ground biomass that is under the regulation of non-hydraulic root signals and that influences resource acquisition and utilization. Non-hydraulic root signals (nHRS) are so far affirmed as a unique positive response to drying soil in wheat, and may imply huge differences in energy metabolism and source-sink relationships between primitive and modern wheat species. Using a pot-culture split-root technique to induce nHRS, four primitive wheat genotypes (two diploids and two tetraploids) and four modern wheat ones (released from different breeding decades) were compared to address the above issue. The nHRS was continuously induced in drying soil, ensuring the operation of energy metabolism under the influence of nHRS. We found that primitive wheat followed an opposite size-dependent allometric pattern (logy = αlogx + logβ) in comparison with modern wheat. The relationships between ear biomass (y-axis) vs above-ground biomass (x-axis), and between reproductive biomass (y-axis) and vegetative (x-axis) biomass fell into a typical allometric pattern in primitive wheat (α > 1), and the nHRS significantly increased α (P < 0.01). However, in modern wheat, they turned to be in an isometric pattern (α ≈ 1). Regardless of nHRS, either leaf (i.e., metabolic rate) or stem biomass generally exhibited an isometric relationship with above-ground biomass in primitive wheat (α ≈ 1), while in modern wheat they fell into an allometric pattern (α > 1). Allometric scaling of specific leaf area (SLA) or biomass density showed superior capabilities of resource acquisition and utilization in modern wheat over primitive ones. We therefore proposed a generalized model to reveal how modern wheat possesses the pronounced population yield advantage over primitive wheat, and its implications on wheat domestication.

β-Aminobutyric acid increases abscisic acid accumulation and desiccation tolerance and decreases water use but fails to improve grain yield in two spring wheat cultivars under soil drying.

A pot experiment was conducted to investigate the effect of the non-protein amino acid, β-aminobutyric acid (BABA), on the homeostasis between reactive oxygen species (ROS) and antioxidant defence during progressive soil drying, and its relationship with the accumulation of abscisic acid (ABA), water use, grain yield, and desiccation tolerance in two spring wheat (Triticum aestivum L.) cultivars released in different decades and with different yields under drought. Drenching the soil with 100 µM BABA increased drought-induced ABA production, leading to a decrease in the lethal leaf water potential (Ψ) used to measure desiccation tolerance, decreased water use, and increased water use efficiency for grain (WUEG) under moderate water stress. In addition, at severe water stress levels, drenching the soil with BABA reduced ROS production, increased antioxidant enzyme activity, and reduced the oxidative damage to lipid membranes. The data suggest that the addition of BABA triggers ABA accumulation that acts as a non-hydraulic root signal, thereby closing stomata, and reducing water use at moderate stress levels, and also reduces the production of ROS and increases the antioxidant defence enzymes at severe stress levels, thus increasing the desiccation tolerance. However, BABA treatment had no effect on grain yield of wheat when water availability was limited. The results suggest that there are ways of effectively priming the pre-existing defence pathways, in addition to genetic means, to improve the desiccation tolerance and WUEG of wheat.

Non-hydraulic root-sourced signaling and grain yield stability under shade of maize during progressive soil drying in soybean

Production practice shows shaded soybean in wheat-maize-soybean; relay strip intercropping system has better tolerance to drought as compared with sole cropping soybean. Our researches are to understand the reasons through non-hydraulic root signals (nHRS), yield and the relationships among them. Pot experiments were conducted with two soybean cultivars, under shade of maize (LI) and normal irradiance (HI). nHRS were traced during manipulative progressive soil drying period at branching stage under good soil conditions (HW), water stress treatment (LW), in 2010. Well-watered (WW), light drought (LD), moderate drought (MD) and severe drought (SD) were applied in 2009. In response to soil drying, nHRS appeared earlier in Gongxuan No.1 (GX) than Gongqiudou 05-8 (GQ) under two irradiance treatments, but it disappeared earlier for GX than GQ under normal irradiance. GX exhibited a wider average soil water content threshold range (TR) of nHRS under low irradiance. Drought stress significantly decreased the shoot dry mass, root mass and grain yield (P<0.05), excluding LD and nHRS treatments. Water use efficiency was significantly higher (P<0.05) in GX than GQ under drought conditions in both irradiances. In a conclusion, soybean cultivar GX has wider TR in low irradiance condition, light drought had no significant effect on yield, nHRS maybe beneficial to maintain soybean yield under drought condition. Key words: Soybean, drought, shade, non-hydraulic root-sourced signal, water use efficiency, yield stability.

Biomass partition, leaf gas exchange and water relations of alfalfa and milkvetch seedlings in response to soil drying

This study compared physiological and growth responses to water stress of two legume species during the seedling stage. Potted alfalfa (Medicago sativa L. cv. Algonquin) and milkvetch (Astragalus adsurgens Pall. cv. Pengyang earlymaturing vetch) seedlings were grown under well-watered [soil water content (SWC) maintained at 14.92% daily] or water-stressed conditions (drying) for 15 days. Net photosynthetic rate (P N), transpiration rate (E) and stomatal conductance (g s) of both species decreased parabolically. When SWC decreased to 7.2% and 10.3%, g s values for alfalfa and milkvetch were significantly different from those of the respective well-watered plants (p<0.05). When SWC decreased to 6.6% for alfalfa and 6.8% for milkvetch, leaf water potentials (ψL) were significantly different from those of the well-watered plants (p<0.05). Thus the difference between the SWC thresholds for a nonhydraulic root signal (nHRS) and a hydraulic root signal (HRS) were 0.6% and 3.5% for alfalfa and milkvetch, respectively. Milkvetch had a lower g s than alfalfa for a given SWC (p<0.05). Although alfalfa seedlings had a higher dry mass (DM) and root:shoot ratio (R/S) than milkvetch in both treatments (p<0.05), we concluded that milkvetch seedlings had greater drought tolerance than alfalfa.

The cooperative relation between non‐hydraulic root signals and osmotic adjustment under water stress improves grain formation for spring wheat varieties

Non-hydraulic root signals (nHRS) and osmotic adjustment (OA) are two important adaptive responses of plants to water stress. There is little understanding of their relationships during water stress. The threshold range of soil water potential to occurrence of nHRS, the capacity for OA, grain yield and water use efficiency (WUE) were examined in three spring wheat (Triticum aestivum L.) varieties (two bred after 1975 and one bred before 1900) under water stress conditions. The threshold range of nHRS was significantly correlated with the maintenance rate of grain yield (MRGY) (r = 0.99, P < 0.05) under moderate drought (-0.49 to -0.55 MPa) but not under severe drought (-0.70 to -0.76 MPa). There were similar correlations between OA and the MRGY. However, regulation of nHRS precedes OA during gradual water stress. The threshold range of nHRS and OA was positively correlated (r = 0.93, P < 0.05), suggesting a mechanism for adapting to drought. WUE was higher for modern than for old varieties and was correlated with the root efficiency (full biomass weight including root per root weight, r = 0.78, P < 0.05) and the root water uptake efficiency (water consumption per root weight, r = 0.72, P < 0.05). However, there was a significant negative correlation between WUE and root weight (r = -0.84, P < 0.01). The cooperative relationship between the threshold range of nHRS and OA under water stress was beneficial for improving grain formation for spring wheat varieties.

Performance of wheat crops with different chromosome ploidy: root-sourced signals, drought tolerance, and yield performance

Pot-culture experiments were carried out to estimate the role of non-hydraulic root signals (nHRS) and the relation of these signals to drought tolerance and grain yield formation under drought stress in six wheat varieties. These were two modern hexaploid wheat (Triticum aestivum L., AABBDD) Plateau602 and Longchun8139-2, two diploid wheat (Triticum monococcum L., AB) MO1 and MO4, and two tetraploid wheat (Triticum dicoccum Schuebl L., AABB) DM22 and DM31. In the two diploid relatives, the nHRS was switched on and off at a soil water content (SWC) of approximately 53-45% field water capacity (FWC). In contrast, in the modern hexaploid varieties, Longchun8139-2 and Plateau602 the nHRS occurred between a SWC of about 71 and 35% FWC, a much wider soil moisture range. The two tetraploid relatives, DM22 and DM31, were generally intermediate. The nHRS threshold range in SWC also narrowed as all six varieties went through successive developmental stages from shooting to grain filling. The two hexaploid wheat varieties had the longest duration of survival after the water supply ceased, and the best yield stability under drought stress, similar to with tetraploid wheat varieties; the diploid wheat varieties were least robust. These two parameters were both significantly correlated with the nHRS soil moisture threshold range (r=0.9456** and 0.8608*, respectively). Based on these patterns, we propose a "triple Z" model to describe the features of non-hydraulic stomatal sensitivity versus soil drought in wheat growth.

Effects of different water supply regimes on water use and yield performance of spring wheat in a simulated semi-arid environment

This research explores the limited irrigation strategies based on root-to-shoot communication that exists in spring wheat, and examines the effects of root-sourced signals on water use and yield performance of three genotypes of spring wheat ( Triticum aestivum) under three different irrigation regimes. Four treatments, CT (well-watered management), DIu (supplying water to the upper layer to maintain soil moisture in the entire pot at 50–60% of field water capacity (FWC)), and DId (supplying water to the lower layer to maintain soil moisture in the entire pot at 50–60% FWC), were employed. The treatment DIu was used to simulate frequent post-sowing irrigation with small amount of water in each time, and DId was used to simulate pre-sowing irrigation with the same amount of water. Plants were grown in cylinder pots outdoors. A non-hydraulic root signal was induced from seedling to tillering stage in the treatment DId. But after the jointing stage, the signal resulted in a reduction in root biomass and root length in the upper layer and an increase in root biomass and root length in the middle layer as compared with the treatment DIu. The water use efficiencies of the three genotypes were the highest in the treatment DId and the lowest in the treatment DIu for the genotypes A and C. This suggests that under the conditions of the same amount of water supply frequent post-sowing irrigation to the upper soil layer had lower water use efficiency and grain yield, whereas pre-sowing irrigation to the lower soil layer tended to have higher grain yield and higher water use efficiency.

Root signalling and osmotic adjustment during intermittent soil drying sustain grain yield of field grown wheat

A field study was conducted to investigate the effect of intermittent soil drying on resulting non-hydraulic and hydraulic root signals, leaf gas exchange, leaf growth, day of heading, leaf osmotic adjustment and yield of wheat grown in sand and loam soils in lysimeters. A 40-day-drought treatment was imposed when the flag leaf started to emerge and was terminated close to maturity. Soil water content and soil water potential of various soil layers were measured using the neutron moderation method and tensiometers, respectively. Soil drying in the top soil layers induced increase in both xylem and bulk-leaf abscisic acid (ABA) content and reduced the stomatal conductance and leaf growth even before a measurable change in leaf water potential could be detected in droughted plants when compared with fully watered plants. Further, heading and flowering occurred 4 days earlier in the droughted than in the well-watered plants before any loss in leaf water potential had occurred as compared with the fully watered plants. When more severe drought reduced the leaf water status, further accumulation of leaf ABA occurred and transpiration decreased in addition to gradual osmotic adjustment and senescence of older leaves. The osmotic adjustment sustained leaf turgor pressure during soil drying. At severe drought, the osmotic adjustment at full turgor in the flag leaves was 0.85 MPa. In sand, the kernel dry weight increased and as a result similar grain yield was obtained in both the treatments. In loam which had more water available than sand, no significant reduction in the final yield was induced by the drought. It is concluded that (1) non-hydraulic root signals caused early drought adaptation at mild water stress by reducing leaf growth and stomatal conductance and hastening of heading and flowering; (2) osmotic adjustment sustained turgor maintenance and hence the yield-forming processes during moderate and severe water stress.

Stomatal sensitivity of six temperate, deciduous tree species to non-hydraulic root-to-shoot signalling of partial soil drying

The objectives of this study were to (1) characterize stomatal response of six deciduous tree species to non-hydraulic, root-sourced signals of soil drying, and (2) test whether species sensitivity to non-hydraulic signalling is allied with their drought avoidance and tolerance profiles. Saplings were grown with roots divided between two pots. Three treatments were compared: one half of the root system watered and half droughted (WD), one half of the root system watered and half severed (WS), both halves watered (WW). Drying about half of the root system caused nonhydraulic declines in stomatal conductance (gs) in all species, with gs of WD plants reduced to from 40% to 60% of WS controls. Declines in stomatal conductance were closely related to declining soil matric potential (ψm) between −0.01 and −0.10 MPa. Soil ψm required to cause declines in gs of WD plants to 80% of WS controls varied from a high of −0.013 to a low of −0.044 MPa. Stomatal inhibition varied somewhat with leaf age in half of the species. Leaf osmotic potentials during soil drying were mostly similar among treatments. Although stomatal sensitivity to the nonhydraulic, root-sourced signal (characterized as decline in gs per unit decline in soil ψm) was not closely correlated with previously identified lethal leaf water potentials or capacity for osmotic adjustment, species having the highest stomatal sensitivity also had the least dehydration tolerance. This suggests that stomatal sensitivity to non-hydraulic root signals may be mechanistically linked to a limited extent with other characteristics defining relative species drought tolerance.

Stomatal sensitivity of six temperate, deciduous tree species to non-hydraulic root-to-shoot signalling of partial soil drying

The objectives of this study were to (1) characterize stomatal response of six deciduous tree species to non-hydraulic, root-sourced signals of soil drying, and (2) test whether species sensitivity to non-hydraulic signalling is allied with their drought avoidance and tolerance profiles. Saplings were grown with roots divided between two pots. Three treatments were compared: one half of the root system watered and half droughted (WD), one half of the root system watered and half severed (WS), both halves watered (WW). Drying about half of the root system caused non-hydraulic declines in stomatal conductance (g s ) in all species, with g s of WD plants reduced to from 40% to 60% of WS controls. Declines in stomatal conductance were closely related to declining soil matric potential (Ψ m ) between -0.01 and -0.10 MPa. Soil Ψ m required to cause declines in g s of WD plants to 80% of WS controls varied from a high of -0.013 to a low of -0.044MPa. Stomatal inhibition varied somewhat with leaf age in half of the species. Leaf osmotic potentials during soil drying were mostly similar among treatments. Although stomatal sensitivity to the non-hydraulic, root-sourced signal (characterized as decline in g s per unit decline in soil Ψ m ) was not closely correlated with previously identified lethal leaf water potentials or capacity for osmotic adjustment, species having the highest stomatal sensitivity also had the least dehydration tolerance. This suggests that stomatal sensitivity to non-hydraulic root signals may be mechanistically linked to a limited extent with other characteristics defining relative species drought tolerance.

Short term drying of half the root system reduces growth but not water status or photosynthesis in leaves of passionfruit ( Passiflora sp.)

The impact of soil water deficits and non-hydraulic root signals on the expansion, functioning and water status of leaves and the opening of flowers of passionfruit was studied in a series of pot experiments. Hydraulic and non-hydraulic signals were separated by withholding water from half the root system. The leaves of well-watered plants expanded in a sigmoid pattern to about 65 cm 2 over 15 to 18 days. Drying the whole root system stopped leaf expansion after 6 days, but did not change the pattern of leaf growth. Growth resumed after rewatering but the final leaf size was halved. As leaf growth slowed, the net photosynthesis, P n , of mature leaves fell from 11 ± 1 μmol CO 2 m −2 s −1 to less than zero as the leaves wilted. Leaf water potential, Ψ 1, of mature leaves at midday ranged from −0.9 MPa in well-watered plants to −3.1 to −3.7 MPa in wilted plants. After rewatering, Ψ 1 returned to −0.9 MPa within 1 day, but P n took 3 days to return to control levels. A midday Ψ 1 of −1.5 MPa was associated with a 50% reduction in relative leaf expansion and net photosynthesis. Drying half of the root system reduced leaf expansion by 26%, compared with well-watered plants, and water use by 21 to 27%, without influencing Ψ 1 or P n . Plants with half the root system dry for 13 days flowered earlier than well-watered plants but had the same number of open flowers 1 month after treatments ended. Non-hydraulic root signals affect leaf expansion and flowering in passionfruit.

Relative sensitivity of leaf elongation and stomatal conductance of cucumber plants to changes in leaf and soil water potentials

Identification of drought-resistant cucumber genotypes and optimization of irrigation scheduling require an improved understanding of cucumber plant water relations. Stomatal conductance (ks), relative leaf elongation rate (RLER), and leaf water relations were examined on greenhouse-grown cucumber (Cucumis sativus L. 'Marketmore 80') over two drought cycles. Water was withheld from the entire root system of cucumber plants and ks, leaf length, leaf water (ΨL) and osmotic potential, and soil matric potential were measured periodically. In both drought cycles, ks decreased before, and RLER decreased after predawn ΨL began to decline. No osmotic adjustment was found after one drought cycle and slight osmotic adjustment (−0.14 MPa) was observed in expanding leaves after a second drought cycle. Evidence for a nonhydraulic root signal in cucumber was examined in split-root experiments. ΨL was similar in plants which received water to one (stress treatment) or to both halves (control treatment) of the root system. Stomatal conductance and RLER remained at control levels as soil water potential (Ψ soil) around half the root system declined to −0.25 to −0.30 MPa. No convincing evidence for effects of root signals on ks and RLER at lower Ψ soil was observed. Key words:Cucumis sativus L., leaf elongation, osmotic adjustment, split root, stomatal conductance, water stress

Leaf Expansion in Field‐Grown Sunflower in Response to Soil and Leaf Water Status

AbstractLeaf responses to soil water deficits in controlled environments may be mediated by nonhydraulic root signals (RS). The aim of this study was to evaluate in the field the effects of RS and leaf water potential (Ψ1) on leaf expansion rate (LER) of sunflower (Helianthus annuus L.). Two experiments were performed on a deep sandy‐loam soil (Typic Xerofluvent) in a Mediterranean environment under spring (Exp. 1) and summer conditions (Exp. 2). WET and DRY treatments were established at the 16th‐leaf stage in both experiments. WET plots were irrigated daily with an amount of water equal to the reference evapotranspiration of the previous day. DRY plots received no water during the treatment period which was maintained until LER declined to about 30% of the WET controls. A split‐root (SR) treatment with the root zone divided in dry and wet sections was included in Exp. 2. Soil water content (θa), LER and Ψ1 were measured. Responses of LER to soil drying were better described by Ψ1 than by plant available water (PAW); i.e., the response of LER to PAW depended on evaporative demand, whereas a single regression of Ψ1 on LER fitted all the data (P &lt; 0.001). Leaf expansion rate and Ψ1 of DRY plants began to decline with respect to that of WET plants at similar PAW thresholds. The soil water content in the dry soil zone of the SR plots was similar to that in the DRY plots during the first week of treatment whereas θa in the wet zone allowed SR plants to maintain a water status similar to that of WET controls. Split‐rooted plants did not behave as DRY controls, as expected in the case of a significant root signal effect, but maintained LER similar to the WET plants. It is concluded that, under the conditions of the present experiments, hydraulic effects were probably of more importance than nonhydraulic root signals in the regulation of sunflower leaf expansion.

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a drying soil a non-hydraulic (chemical) signal is produced in the root and transported to the shoot, causing stomatal closure and growth retardation. This study was designed to reveal genetic diversity in wheat response to soil conditions which elicit a root signal, as the first step in the investigation of the genetic control of the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were established in the growth chamber in soil-filled polyvinyl chloride tubes, 120 cm long and of an internal diameter of 10-2 cm. Soil was well fertilized and wet to field capacity at emergence when two treatments were imposed: (1) tubes were watered from the top as needed to eliminate stress (control); and (2) tubes had a constant water table at a soil depth of 100 to 120 cm, with no applied water. Measurements were performed on five dates on leaf water status and stomatal diffusive resistance. Above-ground biomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soil conditions which were previously indicated to elicit a possible root signal. Under these experimental conditions, cultivars differed in their leaf water status, stomatal diffusive resistance (Rs) and plant production. In the control treatment, Rs of cultivars increased with reductions in their relative water content (RWC) and leaf water potential (LWP), indicating the expected control of Rs by leaf water status. Under conditions of a drying top soil, relative water content (RWC) and leaf water potential (LWP) increased in cultivars that had a higher Rs, indicating that stomatal activity was controlling leaf water status. It was therefore suggested that the drying top soil elicited a root signal which caused stomatal closure and reduced plant production. Under such conditions, two cultivars (Bethlehem and V748) consistently maintained relatively low R$ and high plant production, despite their relatively lower RWC and LWP, as compared with cvs C97, V747 and V652. Limited observations suggest that in these two cultivars relatively fewer roots may have been exposed to the drying top soil, as compared with the other three cultivars.

The Effect of a Drying Top Soil and a Possible Non-hydraulic Root Signal on Wheat Growth and Yield

Research has shown that plants are adversely affected by a drying top soil and that this effect may be caused, at least partly, by a non-hydraulic (hormonal) root signal in response to soil water status and/or soil strength. Most studies done to elucidate root signal effects were limited to the short-term, typically 2 to 3 weeks of plant growth. This study was done to detect a possible non hydraulic root signal in response to a drying top soil and to measure its effect on plant growth and production from emergence to maturity. Wheat (Triticum aestivum L.) plants were established in the growth chamber in soil-filled polyvinyl chloride tubes, 120 cm long and of an internal diameter of 10-2 cm. Soil was well fertilized and wet to field capacity at emergence when three treatments were imposed: (1) tubes were watered from the top as needed to eliminate stress (IR); (2) tubes had a constant water table at a soil depth of 100 to 120 cm, with no applied water (WT); and (3) plants grew on stored soil moisture and were rewatered when water stress developed (SW). Soil moisture profiles, soil strength, shoot water status and different shoot developmental traits as well as yield were measured. The upper part of the soil column in WT became dry and hard while roots reached the water table in less than 30 d. There was no significant difference in midday leaf water potential (LWP) and relative water content (RWC) between IR and WT, while SW plants developed comparatively lower LWP and RWC. WT as compared with IR, resulted in earlier heading, smaller flag leaf area, reduced shoot weight, reduced plant height and greater leaf epicuticular wax content. The earliest effect was seen at 20 d after emergence. Tillers developed after this date had less dry weight in WT than in IR. WT also resulted in smaller biomass but a greater harvest index than IR, so that both treatments had the same grain yield per plant. Since IR and WT treatments did not differ in leaf water status, the effect of WT on the plant was intepreted to result from a non-hydraulic root signal in response to the dry and hard top soil.

Mycorrhizal Fungi and Nonhydraulic Root Signals of Soil Drying

We propose that mycorrhizal colonization of roots alters nonhydraulic root to shoot communication of soil drying. Split-root rose (Rosa hybrida L. cv Samantha) plants-one side of the root system colonized by Glomus intraradices Schenck & Smith, the other side nonmycorrhizal-displayed different stomatal conductances upon partial drying, depending upon whether mycorrhizal or nonmycorrhizal roots were dried. No differences in leaf water status were observed among control plants and those whose mycorrhizal or nonmycorrhizal roots were dried.