Abscisic Acid Content In Roots Research Articles

Wind disturbance-based tomato seedlings growth control

Wind-disturbance is a potential eco-friendly technique for tackling leggy seedlings. This study uses orthogonal experimental design and seedlings vigour assessment by strong seedling index (SSI) to investigate wind-disturbance on regulating tomato seedlings growth. Changes in endogenous hormone levels and biomechanical properties of tomato seedlings were investigated using enzyme-linked immunosorbent assay and uniaxial tension tests. Results showed that factors influencing significantly SSI, in descending order, are wind disturbance time (T), seedling age at the onset of wind disturbance (SA), wind velocity (V), and interval time (I). The wind-disturbance effect was found to be optimal with SSI = 0.126 for a condition where V, T, I and SA values are 3 m s−1, 1 min, 30 min, and 15 days, respectively. V and T were positively correlated with the ethylene and abscisic acid content in seedling leaves, abscisic acid and auxin content in stems, cytokinin and ethylene content in roots, and the elastic moduli of stems and roots but negatively associated with the cytokinin content in stems and leaves, auxin and abscisic acid content in roots, and leaves’ elastic modulus. Wind disturbance mechanism for controlling seedlings growth involved eliciting the accumulation of abscisic acid in stems and leaves and reduction of the auxin content in roots to about the optimal threshold for roots growth thereby reducing seedling stems and leaves development and promoting a better roots growth and a high SSI. This work offers theoretical insights and technical guidance for utilising wind-disturbance as a sustainable seedling cultivation and personalised seedling management approach.

Differential roles of abscisic acid in maize roots in the adaptation to soil drought

AbstractMaize yield reduction occurs frequently due to soil drought. Abscisic acid (ABA) is an important hormonal signal indicating drought. However, it remains unclear about the specific roles of root ABA in the adaptation to soil drought in maize. This study applied five different soil water potentials (SWP), referring to maintaining SWP at −15, −30, −45, −60, and −75 kPa, respectively, and investigated the ABA content in roots and the activities and gene expression levels of key enzymes involved in the ABA biosynthesis, the leaf photosynthetic properties, root traits, and kernel yield. The results showed that maize root ABA content, the activities and gene expressions of key enzymes involved in the ABA biosynthesis were increased with the decrease of SWP. The leaf transpiration rate, and root dry weight, length, volume, surface area, and activity, and kernel yield of maize were increased first and then decreased with the severity of soil drying. The leaf photosynthetic rate was not significantly reduced with ABA accumulation at a low ABA content in roots. Root ABA content was significantly positively correlated with leaf transpiration efficiency and root activity when ABA content was relatively low (9.03–22.02 nmol g−1 DW), whereas root ABA content was negatively correlated with leaf photosynthetic rate, leaf transpiration efficiency, root volume, and root activity when ABA content was high (28.32–39.23 nmol g−1 DW). The results indicate that maize root ABA exhibit differential roles in the adaptation to soil drought, and can positively regulate the drought‐resistance of maize at an appropriate level.

Short-term severe drought influences root volatile biosynthesis in eastern white pine (Pinus strobus L).

Climate change-related drought stress is expected to shift carbon partitioning toward volatile organic compound (VOC) biosynthesis. The effect of drought stress on VOC synthesis remains unknown in several tree species. Therefore, we exposed eastern white pine (Pinus strobus) plants to severe drought for 32 days and performed physiological analysis (chlorophyll content, leaf water content, and root/shoot index), biochemical analysis (non-structural carbohydrates, proline, lipid peroxidation, and antioxidant assay), and total root VOC analysis. Drought stress decreased the relative water and soil moisture contents. Root proline accumulation and antioxidant activity increased significantly, whereas leaf chlorophyll synthesis and fresh weight decreased significantly in drought-treated plants. A non-significant increase in sugar accumulation (leaves and roots), proline accumulation (leaves), antioxidant activity (leaves), and lipid peroxidation (leaves and roots) was observed in drought-treated plants. Drought stress caused a non-significant decline in root/shoot ratio and starch accumulation (leaves and roots) and caused a significant increase in root abscisic acid content. Drought-treated plants showed an increase in overall monoterpene synthesis (16%) and decline in total sesquiterpene synthesis (3%). Our findings provide an overall assessment of the different responses of VOC synthesis to severe water deficit that may help unravel the molecular mechanisms underlying drought tolerance in P. strobus.

Integrated Metabolomic and Transcriptomic Analyses Reveal the Basis for Carotenoid Biosynthesis in Sweet Potato (Ipomoea batatas (L.) Lam.) Storage Roots.

Carotenoids are important compounds of quality and coloration within sweet potato storage roots, but the mechanisms that govern the accumulation of these carotenoids remain poorly understood. In this study, metabolomic and transcriptomic analyses of carotenoids were performed using young storage roots (S2) and old storage roots (S4) from white-fleshed (variety S19) and yellow-fleshed (variety BS) sweet potato types. S19 storage roots exhibited significantly lower total carotenoid levels relative to BS storage roots, and different numbers of carotenoid types were detected in the BS-S2, BS-S4, S19-S2, and S19-S4 samples. β-cryptoxanthin was identified as a potential key driver of differences in root coloration between the S19 and BS types. Combined transcriptomic and metabolomic analyses revealed significant co-annotation of the carotenoid and abscisic acid (ABA) metabolic pathways, PSY (phytoene synthase), CHYB (β-carotene 3-hydroxylase), ZEP (zeaxanthin epoxidase), NCED3 (9-cis-epoxycarotenoid dioxygenase 3), ABA2 (xanthoxin dehydrogenase), and CYP707A (abscisic acid 8'-hydroxylase) genes were found to be closely associated with carotenoid and ABA content in these sweet potato storage roots. The expression patterns of the transcription factors OFP and FAR1 were associated with the ABA content in these two sweet potato types. Together, these results provide a valuable foundation for understanding the mechanisms governing carotenoid biosynthesis in storage roots, and offer a theoretical basis for sweet potato breeding and management.

H2O2 participates in ABA regulation of grafting-induced chilling tolerance in cucumber.

Rootstock provides more abscisic acid (ABA) content to scions to increase the chilling tolerance of seedlings. H2O2 is involved in ABA regulation of grafting-induced chilling tolerance of cucumber. Here we examined the role of ABA in the response of grafted cucumber to chilling stress. The data showed chilling induced an increase in leaf and root ABA content and there was a positive correlation between ABA content and the chilling tolerance of the varieties. The increase of ABA content and NCED mRNA abundance in the leaf of both Cs/Cs (self-root) and Cs/Cm (grafted with pumpkin as rootstock) showed a delay under aerial stress compared with those under whole plant and root-zone stress. Intriguingly, an increase in ABA in xylem was found under whole-plant and root-zone chilling stress but was not detected under aerial stress, implying the increases in ABA content in leaves were mainly from root ABA transportation. Compared to Cs/Cs, a higher ABA content and NCED mRNA abundance were observed in Cs/Cm, which showed that Cm could output more ABA than Cs. The removal of endogenous ABA decreased the difference in chilling tolerance induced by Cm, as evidenced by the observed similar oxidative stress levels and photosynthetic capacity between Cs/Cs and Cs/Cm after chilling stress. Moreover, we found that the H2O2 signal in grafted cucumber could respond to chilling stress earlier than the H2O2 signal in self-rooted cucumber. The inhibition of endogenous H2O2 decreased the chilling tolerance of grafted cucumber induced by ABA by reducing photosynthesis and the mRNA abundance of CBF1 and COR. Thus, our results indicate that H2O2, as the downstream signal, participated in the rootstock-induced chilling tolerance of grafted seedlings induced by ABA.

Crosstalk of phenylpropanoid biosynthesis with hormone signaling in Chinese cabbage is key to counteracting salt stress

Salt stress limits the regional distribution of Chinese cabbage, yet the molecular factors that confer tolerance to salt stress in Chinese cabbage remain poorly understood. To further investigate this issue, salt-tolerant and salt-sensitive Chinese cabbage cultivars were selected, and their responses to salt stress were tested. Then, comparative transcriptome profiling was performed on the roots of salt-tolerant and salt-sensitive Chinese cabbage cultivars under salt stress. The results showed that Qinghua and Biyu are considered to be salt-tolerant and salt-sensitive Chinese cabbage cultivars, respectively. Qinghua had higher water content and lower Na+/K+ in roots and leaves, better root development, higher lignin content and higher abscisic acid content in roots than Biyu. Comparative transcriptome profiling showed that the abscisic acid and ethylene signaling pathways of Qinghua roots were more actively expressed than those of Biyu. During root adaptation to salt stress, abscisic acid crosstalk with suberin biosynthesis and ethylene signaling crosstalk with lignin biosynthesis were observed. Suberin development and lignin accumulation in the Qinghua roots were found to be important strategies for resisting salt stress in this study. These strategies are not independent, but rather coordinate to promote salt tolerance in Chinese cabbage. The increased lignin content in the roots of Qinghua under salt stress may prevent embolism. Embolism prevention can maintain the flow of water transport, and thus compensate for the reduction in water permeability of these suberized cells in a process that occurs concurrently with greater sodium ion limitation. These results provide novel insight into the crosstalk of phenylpropanoid biosynthesis with hormone signaling under salt stress, and facilitate further dissection of molecular mechanism of adaptive response to salt stress in Chinese cabbage.

The Apocarotenoid Zaxinone Is a Positive Regulator of Strigolactone and Abscisic Acid Biosynthesis in Arabidopsis Roots.

Carotenoids are ubiquitous precursors of important metabolites including hormones, such as strigolactones (SLs) and abscisic acid (ABA), and signaling and regulatory molecules, such as the recently discovered zaxinone. Strigolactones and ABA are key regulators of plant growth and development, adaptation to environmental changes and response to biotic and abiotic stress. Previously, we have shown that zaxinone, an apocarotenoid produced in rice by the enzyme zaxinone synthase (ZAS) that is common in mycorrhizal plants, is required for normal rice growth and development, and a negative regulator of SL biosynthesis. Zaxinone is also formed in Arabidopsis, which lacks ZAS, via an unknown route. In the present study, we investigated the biological activity of zaxinone in Arabidopsis, focusing on its effect on SL and ABA biosynthesis. For this purpose, we quantified the content of both hormones and determined the levels of related transcripts in Arabidopsis (Arabidopsis thaliana), roots upon zaxinone treatment. For SL quantification, we also employed Striga seed germination bioassay. Results obtained show that zaxinone application to hydroponically grown Arabidopsis seedlings enhanced transcript levels of key biosynthetic genes of both hormones, led to higher root ABA and SL (methyl carlactonoate, MeCLA) content, and increased SL release, even under sufficient phosphate supply. Using the SL insensitive (max2-1) and the ABA deficient (aba1-6, aba2-1, and nced3) mutants, we also show that zaxinone application reduced hypocotyl growth and that this effect is caused by increasing ABA content. Our results suggest that zaxinone is a regulatory metabolite also in Arabidopsis, which triggers the biosynthesis of both carotenoid-derived hormones, SLs and ABA, in roots. In the non-mycotrophic plant Arabidopsis, zaxinone does not increase growth and may be perceived as a stress signal, while it acts as a growth-promoting metabolite and suppressor of SL biosynthesis in rice.

Root-specific NF-Y family transcription factor, PdNF-YB21, positively regulates root growth and drought resistance by abscisic acid-mediated indoylacetic acid transport in Populus.

Root growth control plays an important role in plant adaptation to drought stress, but the underlying molecular mechanisms of this control remain largely elusive. Here, a root-specific nuclearfactorY (NF-Y) transcription factor PdNF-YB21 was isolated from Populus. The functional mechanism of PdNF-YB21 was characterised by various morphological, physiological, molecular, biochemical and spectroscopy techniques. Overexpression of PdNF-YB21 in poplar promoted root growth with highly lignified and enlarged xylem vessels, resulting in increased drought resistance. By contrast, CRISPR/Cas9-mediated poplar mutant nf-yb21 exhibited reduced root growth and drought resistance. PdNF-YB21 interacted with PdFUSCA3 (PdFUS3), a B3 domain transcription factor. PdFUS3 directly activated the promoter of the abscisic acid (ABA) synthesis key gene PdNCED3, resulting in a significant increase in root ABA content in poplars subjected to water deficit. Coexpression of poplar NF-YB21 and FUS3 significantly enhanced the expression of PdNCED3. Furthermore, ABA promoted indoylacetic acid transport in root tips, which ultimately increased root growth and drought resistance. Taken together, our data indicate that NF-YB21-FUS3-NCED3 functions as an important avenue in auxin-regulated poplar root growth in response to drought.

Moderate water stress in rice induces rhizosheath formation associated with abscisic acid and auxin responses.

The rhizosheath is known to be beneficial for drought resistance in many plants, but the regulation of rhizosheath formation in rice plants is unclear. Here, we investigate rhizosheath formation in different rice varieties and root hair mutants. Our results showed that moderate water stress in rice induced rhizosheath formation. The soil porosity and water content were higher in the rice rhizosheath than in the rice bulk soil under moderate water stress. Additionally, rhizosheath formation in short root hair mutants was lower than in wild-type rice under moderate water stress. Moreover, transcriptomic results indicated that abscisic acid (ABA) and auxin were involved in root and root hair responses in rhizosheath formation. Further, blocking ABA and auxin pathways in wild type and in rhl1-1, the shortest root hair mutant, rhizosheath formation and root hair length were significantly decreased under moderate water stress. However, wild type plants maintained a higher root ABA content, root basipetal auxin transport, root hair length, and amount of rhizosheath than did rhl1-1. Our results suggest that moderate water stress in rice induces rhizosheath formation by modulating the ABA and auxin responses to regulate root and root hair growth, which may be used to breed rice varieties resistant to drought.

Exogenous application of abscisic acid to shoots promotes primary root cell division and elongation

Root-derived abscisic acid (ABA) is known to regulate shoot physiology, such as stomata closure. Conversely, the basipetal regulatory effect of shoot-derived ABA is poorly understood. Herein, we report that simulation of shoot-ABA accumulation by exogenous application of ABA to shoots basipetally stimulates primary root (PR) growth. ABA applied to shoots accelerates root cell division, as evidenced by the increase in meristem size and cell number and the intensity of CYCB1;1::GFP (a mitosis marker). Root ABA content was not changed following shoot ABA application, although the ABA reporter line RAB18::GFP showed an increase in ABA in the cotyledons. Shoot-ABA application increases basipetal auxin transport by 114 %. Shoot-ABA-promoted PR growth can be abolished by attenuating basipetal auxin flux using 2,3,5-triiodobenzoic acid (TIBA, an auxin transport inhibitor), demonstrating that ABA promotes PR growth by increasing basipetal auxin transport. Root cell elongation, evaluated by the total length of the first 7 cells in the elongation zone (EZ), was increased by 56 % following shoot-ABA application. The cell walls of the root EZ were alkalinized by ABA, as exhibited by 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt staining. Higher pH promotes both PR growth and cell elongation. Thus, shoot-ABA promotes cell elongation by alkalinizing the cell wall. In light of our results, we provide a representative detailed model of the basipetal regulatory effect of ABA on PR growth.

Expression of maize calcium-dependent protein kinase (ZmCPK11) improves salt tolerance in transgenic Arabidopsis plants by regulating sodium and potassium homeostasis and stabilizing photosystem II.

In plants, CALCIUM-DEPENDENT PROTEIN KINASES (CDPKs/CPKs) are involved in calcium signaling in response to endogenous and environmental stimuli. Here, we report that ZmCPK11, one of maize CDPKs, participates in salt stress response and tolerance. Salt stress induced expression and upregulated the activity of ZmCPK11 in maize roots and leaves. Activation of ZmCPK11 upon salt stress was also observed in roots and leaves of transgenic Arabidopsis plants expressing ZmCPK11. The transgenic plants showed a long-root phenotype under control conditions and a short-root phenotype under NaCl, abscisic acid (ABA) or jasmonic acid (JA) treatment. Analysis of ABA and JA content in roots indicated that ZmCPK11 can mediate root growth by regulating the levels of these phytohormones. Moreover, 4-week-old transgenic plants were more tolerant to salinity than the wild-type plants. Their leaves were less chlorotic and showed weaker symptoms of senescence accompanied by higher chlorophyll content and higher quantum efficiency of photosystem II. The expression of Na+ /K+ transporters (HKT1, SOS1 and NHX1) and transcription factors (CBF1, CBF2, CBF3, ZAT6 and ZAT10) with known links to salinity tolerance was upregulated in roots of the transgenic plants upon salt stress. Furthermore, the transgenic plants accumulated less Na+ in roots and leaves under salinity, and showed a higher K+ /Na+ ratio in leaves. These results show that the improved salt tolerance in ZmCPK11-transgenic plants could be due to an upregulation of genes involved in the maintenance of intracellular Na+ and K+ homeostasis and a protection of photosystem II against damage.

Deep root growth, ABA adjustments and root water uptake response to soil water deficit in giant reed.

Giant reed (Arundo donax L.) is a deep-rooted crop that can survive prolonged dry periods probably as a result of its capacity to uptake water from below ground, but specific information on the functioning of deep/shallow roots is missing. The objective of this study was to understand the dynamic interrelationships of root water acquisition, canopy water conservation and abscisic acid (ABA) signals from both shallow and deep roots. In transparent split top-bottom rhizotron systems (1-m-high columns), where hydraulically isolated and independently watered layers were created with the aid of calibrated soil moisture sensors, water uptake trends were monitored. Rooting patterns were traced on the walls of the rhizotrons. Leaf gas exchange was determined using a portable infrared gas analyser. Leaf and root ABA concentrations were monitored. Under well-watered conditions, water uptake from both upper and deeper soil layers was similar. Water uptake from deeper soil layers increased gradually by up to 2.2-fold when drought stress was imposed to upper layers compared to the control conditions. Despite the significant increase in water uptake from deeper layers, surface root length density of drought-treated plants remained unchanged, suggesting increased root water uptake efficiency by these roots. However, these adjustments were not sufficient to sustain photosynthesis and therefore biomass accumulation, which was reduced by 42 %. The ABA content in shallower drought-treated roots increased 2.6-fold. This increase closely and positively correlated with foliar ABA concentration, increased intrinsic water use efficiency and leaf water potential (LWP). Giant reed is able to change its water sources depending on water availability and to maximize water uptake efficiency to satisfy canopy evapotranspirative demands. The regulation of deep root functioning and distribution, adjustment of canopy size, and root/foliar synthesized ABA play a central role in controlling LWP and leaf transpiration efficiency.

Root Abscisic Acid Contributes to Defending Photoinibition in Jerusalem Artichoke (Helianthus tuberosus L.) under Salt Stress.

The aim of the study was to examine the role of root abscisic acid (ABA) in protecting photosystems and photosynthesis in Jerusalem artichoke against salt stress. Potted plants were pretreated by a specific ABA synthesis inhibitor sodium tungstate and then subjected to salt stress (150 mM NaCl). Tungstate did not directly affect root ABA content and photosynthetic parameters, whereas it inhibited root ABA accumulation and induced a greater decrease in photosynthetic rate under salt stress. The maximal photochemical efficiency of PSII (Fv/Fm) significantly declined in tungstate-pretreated plants under salt stress, suggesting photosystem II (PSII) photoinhibition appeared. PSII photoinhibition did not prevent PSI photoinhibition by restricting electron donation, as the maximal photochemical efficiency of PSI (ΔMR/MR0) was lowered. In line with photoinhibition, elevated H2O2 concentration and lipid peroxidation corroborated salt-induced oxidative stress in tungstate-pretreated plants. Less decrease in ΔMR/MR0 and Fv/Fm indicated that PSII and PSI in non-pretreated plants could maintain better performance than tungstate-pretreated plants under salt stress. Consistently, greater reduction in PSII and PSI reaction center protein abundance confirmed the elevated vulnerability of photosystems to salt stress in tungstate-pretreated plants. Overall, the root ABA signal participated in defending the photosystem’s photoinhibition and protecting photosynthesis in Jerusalem artichoke under salt stress.

Rapid changes in root HvPIP2;2 aquaporins abundance and ABA concentration are required to enhance root hydraulic conductivity and maintain leaf water potential in response to increased evaporative demand.

To address the involvement of abscisic acid (ABA) in regulating transpiration and root hydraulic conductivity (LpRoot) and their relative importance for maintaining leaf hydration, the ABA-deficient barley mutant Az34 and its parental wild-type (WT) genotype (cv. Steptoe) were grown in hydroponics and exposed to changes in atmospheric vapour pressure deficit (VPD) imposed by air warming. WT plants were capable of maintaining leaf water potential (ψL) that was likely due to increased LpRoot enabling higher water flow from the roots, which increased in response to air warming. The increased LpRoot and immunostaining for HvPIP2;2 aquaporins (AQPs) correlated with increased root ABA content of WT plants when exposed to increased air temperature. The failure of Az34 to maintain ψL during air warming may be due to lower LpRoot than WT plants, and an inability to respond to changes in air temperature. The correlation between root ABA content and LpRoot was further supported by increased root hydraulic conductivity in both genotypes when treated with exogenous ABA (10-5 M). Thus the ability of the root system to rapidly regulate ABA levels (and thence aquaporin abundance and hydraulic conductivity) seems important to maintain leaf hydration.

English

The effect of drought stress on Abscisic acid (ABA) on roots and root characteristics of three alfalfa varieties (Aohan, Zhongmu No.1 and Suntory) was studied. The study was conducted in greenhouse of the Chinese Academy of Agricultural Sciences (CAAS), Beijing, China from September 2014 to May 2015. Alfalfa varieties were exposed to four irrigation levels of water-holding capacity viz 100% (w1) (control); 85% (w2); 70% (w3) and 55% (w4), which were considered as deficit irrigation treatments. The results showed that ABA was significantly affected by deficit irrigation treatments during different plant growth stages. With the exception of 90 and 105 days after transplanting, ABA increased after transplanting up to harvest in all treatments. Increasing deficit irrigation showed significant increase in ABA content in roots of alfalfa. The minimum content of ABA was obtained from w1 (44 ng/g.FW) and then increased to 56.6, 64.6 and 94.4 ng/g.FW for w2, w3 and w4 respectively on 105 days after transplanting. Moreover, the results showed that ABA content was differently affected among different varieties of alfalfa under different water stress levels. The maximum ABA content was 83.2, 61.7 and 49.9 ng/g.FW obtained with Aohan, Suntory and Zhongmu No.1 varieties respectively, at 105 days after transplanting. Highest water stress w4 (55%) reduced root length by 20.92%, lateral roots by 20.71%, root fresh biomass by 43.79% and root dry biomass by 37.96%. The root to shoot ratio was 1.9 times higher in water stressed plants compared with the control, indicating that water stress in alfalfa is affected more shoot growth than root one.   Key words: alfalfa, deficit irrigation, Abscisic acid (ABA), root characteristics.

Biochemical and Physiological Effect of Excess Manganese (Mn) in Bean (Phaseolus vulgaris L. cv. Strike)

The present work aimed to study the physiological effects of manganese (Mn) in bean (Phaseolus vulgaris L. cv. Strike). Bean seedlings were exposed to increasing doses (0.1, 0.2, 0.3 and 0.4 mM) of manganese chloride (MnCl2·H2O) for 10 days. After 10 days of heavy metal treatment, seedlings were harvested for pigment, proline, protein, ABA (abscisic acid) and vitamin content determinations. Elevated Mn levels increased the ABA content in roots and leaves of seedlings. An increase of proline and vitamin in leaves of seedlings and decrease of chlorophyll (a and b) and total protein contents (p < 0.05 or p < 0.01) occurred on exposure to Mn. Mn level at certain critical points negatively affected seedling growth. These results are important to understand the biological consequences of heavy metal contamination particularly in soil reserved to organic agriculture.

Effects of temperature and watering regime on growth, gas exchange and abscisic acid content of canola ( Brassica napus) seedlings

Global climate change, including global warming, will likely reduce soil moisture in many parts of the world. Few studies have examined the combined effects of temperature and watering regime on plant growth and physiological parameters, on time-course basis in early developmental stages. We grew canola ( Brassica napus L. cv. 45H72) plants under lower (22/18 °C) and higher (28/24 °C) temperature regimes in controlled-environment chambers. One half of the plants were watered to field capacity and the other half at wilting point. In three separate experiments, we determined growth, gas exchange and endogenous abscisic acid (ABA) content in 1-week-old seedlings grown for another 1, 5 or 10 days under the above-mentioned conditions. Higher temperature decreased stem height, leaf area, leaf area ratio (LAR) and water-use efficiency (WUE), but increased specific leaf weight (SLW), leaf weight ratio (LWR) and transpiration ( E). Water stress reduced stem height, leaf number, leaf area, dry matter of individual organs and whole plant, LAR, net CO 2 assimilation, E and WUE, but increased SLW and LWR. Overall, plant growth, dry matter, shoot:root weight ratio and ABA content increased with exposure duration, especially by day 10. However, higher temperature inhibited water stress-induced increases in ABA content. Interaction between watering regime and exposure duration was significant for all parameters of growth and dry matter as well as for LWR, WUE and ABA. No significant interaction was found between temperature and watering regime, between temperature and exposure duration, or among temperature, watering regime and exposure duration for any of the measured parameters. This study revealed that water stress strongly, but higher temperature to a lesser extent, affected canola seedlings, leading to reduced growth and dry matter, and the negative effects increased with time.

Characteristics of Endogenous Hormone Variations in the Roots of Alfalfa ( Medicago sativa L.) Cultivars of Different Fall Dormancies During Spring Regrowth Stage

Regrowth traits of alfalfa ( Medicago sativa L.) in spring are closely related to its fall dormancy before winter. In order to determine the relationship between fall dormancy (FD) grade and hormone variation pattern and provide academic references for the variety improvement and production of alfalfa, the variations of gibberellins (GA 3), indole-3-acetic acid (IAA), and abscisic acid (ABA) in alfalfa roots during regrowth period in spring were examined by high performance liquid chromatography (HPLC). The study involved seven alfalfa cultivars that belonged to four fall dormant grades, i.e., 2, 4, 6, and 8. The results showed that the differences in spring regrowth among the alfalfa cultivars were partially associated with their root hormone levels. The alfalfa cultivars that belonged to the same dormancy grades presented similar variation trends in endogenous hormone content in their roots during the spring regrowth stage. At the early regrowth stage, cultivars with a higher dormant grade had a higher GA 3 concent and a lower ABA content in their roots than the cultivars with a lower dormant grade; and IAA content in roots of non- and semi-fall dormancy cultivars was higher than that of fall dormancy cultivars. During the whole period of spring regrowth, the root ABA content of fall dormancy alfalfa cultivar is significantly higher than those of semi- and non-fall dormancy cultivars. GA 3 contents in the roots of all cultivars under study showed a double-peak dynamic curve; root IAA contents of the studied cultivars presented a downward trend. But the trend did not significantly differed among the different fall dormant cultivars. The higher GA 3 content and lower ABA content in root of non-fall dormancy alfalfa lead to its earlier regrowth. Regrowth time and rate of alfalfa can be regulated by exogenous GA 3 or ABA at the early regrowth stage to meet producing requirement.

Hydraulic and Chemical Responses of Citrus Seedlings to Drought and Osmotic Stress

In this work we investigated the function of abscisic acid (ABA) as a long-distance chemical signal communicating water shortage from the root to the shoot in citrus plants. Experiments indicated that stomatal conductance, transpiration rates, and leaf water potential decline progressively with drought. ABA content in roots, leaves, and xylem sap was also increased by the drought stress treatment three- to sevenfold. The addition of norflurazon, an inhibitor of ABA biosynthesis, significantly decreased the intensity of the responses and reduced ABA content in roots and xylem fluid, but not in leaves. Polyethylene glycol (PEG)-induced osmotic stress caused similar effects and, in general, was counteracted only by norflurazon at the lowest concentration (10%). Partial defoliation was able to diminish only leaf ABA content (22.5%) at the highest PEG concentration (30%), probably through a reduction of the active sites of biosynthesis. At least under moderate drought (3–6 days without irrigation), mechanisms other than leaf ABA concentration were required to explain stomatal closure in response to limited soil water supply. Measurements of xylem sap pH revealed a progressive alkalinization through the drought condition (6.4 vs. 7.1), that was not counteracted with the addition of norflurazon. Moreover, in vitro treatment of detached leaves with buffers iso-osmotically adjusted at pH 7.1 significantly decreased stomatal conductance (more than 30%) as much as 70% when supplemented with ABA. Taken together, our results suggest that increased pH generated in drought-stressed roots is transmitted by the xylem sap to the leaves, triggering reductions in shoot water loss. The parallel rise in ABA concentration may act synergistically with pH alkalinization in xylem sap, with an initial response generated from the roots and further promotion by the stressed leaves.

Primary Root Elongation Rate and Abscisic Acid Levels of Maize in Response to Water Stress

Agricultural crops are exposed to greater water deficits as production regions receive less rainfall and producers move into more arid landscapes to meet demand. Maintenance of root elongation is vital for seedling establishment in these types of climates. Some maize (Zea mays L.) lines can maintain primary root elongation under severe water stress (SS) conditions. Twelve maize inbred lines were examined at the seedling stage before leaf expansion to assess the phenotypic diversity in primary root elongation rate and root elongation zone abscisic acid (ABA) content at three water potentials, −0.03 MPa, −0.3 MPa, and −1.6 MPa. A phylogenetic tree was constructed using 93 simple sequence repeat (SSR) markers to examine genetic and phenotypic relationships among the lines in relation to stress response. Statistical analysis revealed phenotypic diversity in both primary root elongation rate and ABA content. No correlations were detected between the amount of ABA present in the root tip and the rate of root elongation in any of the treatments. Histograms of primary root elongation response to the varying water deficits suggest multiple mechanisms may be responsible for the response to water stress observed in different maize lines. Different phenotypic responses toward water stress were observed for lines with small genetic distances, which will aid in identification of specific alleles for maintenance of primary root growth under water deficits.