Rootstock – scion signalling: factors that mediate scion performance and graft compatibility (abstract)

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Cultivar-dependent differences in tuber growth cause increased soil resistance in potato fields

Since soil compaction of potato fields delays shoot emergence and decreases total yield, the causes and effects of this compaction need to be better understood. In a controlled environment trial with young (before tuber initiation) plants, roots of cv. Inca Bella (a phureja group cultivar) were more sensitive to increased soil resistance (3.0 MPa) than cv. Maris Piper (a tuberosum group cultivar). Such variation was hypothesized to cause yield differences in two field trials, in which compaction treatments were applied after tuber planting. Trial 1 increased initial soil resistance from 0.15 MPa to 0.3 MPa. By the end of the growing season, soil resistance increased three-fold in the upper 20 cm of the soil, but resistance in Maris Piper plots was up to twice that of Inca Bella plots. Maris Piper yield was 60% higher than Inca Bella and independent of soil compaction treatment, whilst compacted soil reduced Inca Bella yield by 30%. Trial 2 increased initial soil resistance from 0.2 MPa to 1.0 MPa. Soil resistance in the compacted treatments increased to similar, cultivar-dependent resistances as trial 1. Maris Piper yield was 12% higher than Inca Bella, but cultivar variation in yield response to compacted soil did not occur. Soil water content, root growth and tuber growth were measured to determine whether these factors could explain cultivar differences in soil resistance. Soil water content was similar between cultivars, thus did not cause soil resistance to vary between cultivars. Root density was insufficient to cause observed increases soil resistance. Finally, differences in soil resistance between cultivars became significant during tuber initiation, and became more pronounced until harvest. Increased tuber biomass volume (yield) of Maris Piper increased estimated mean soil density (and thus soil resistance) more than Inca Bella. This increase seems to depend on initial compaction, as soil resistance did not significantly increase in uncompacted soil. While increased soil resistance caused cultivar-dependent restriction of root density of young plants that was consistent with cultivar variation in yield, tuber growth likely caused cultivar-dependent increases in soil resistance in field trials, which may have further limited Inca Bella yield

Exogenous monoterpenes mitigate H2O2-induced lipid damage but do not attenuate photosynthetic decline during water deficit in tomato

Although monoterpenes are suggested to mediate oxidative status, their role in abiotic stress responses is currently unclear. Here, a foliar spray of monoterpenes increased antioxidant capacity and decreased oxidative stress of Solanum lycopersicum under water deficit stress. The foliar content of monoterpenes increased with spray concentration indicating foliar uptake of exogenous monoterpenes. Exogenous monoterpene application substantially decreased foliar accumulation of hydrogen peroxide (H2O2) and lipid peroxidation (malondialdehyde). However, it appears that monoterpenes prevent the accumulation of reactive oxygen species rather than mitigating subsequent reactive oxygen species-induced damage. Low spray concentration (1.25 mM) proved most effective in decreasing oxidative stress but did not up-regulate the activity of key antioxidant enzymes (superoxide dismutase and ascorbate peroxidase) even though higher (2.5 and 5 mM) spray concentrations did, suggesting a complex role for monoterpenes in mediating antioxidant processes. Furthermore, soil drying caused similar photosynthetic limitations in all plants irrespective of monoterpene treatments, apparently driven by strong reductions in stomatal conductance as photosystem II efficiency only decreased in very dry soil. We suggest that exogenous monoterpenes may mitigate drought-induced oxidative stress by direct quenching and/or up-regulating endogenous antioxidative processes. The protective properties of specific monoterpenes and endogenous antioxidants require further investigation

Long-distance ABA transport can mediate distal tissue responses by affecting local ABA concentrations

Environmental stresses that perturb plant water relations influence abscisic acid (ABA) concentrations, but it is unclear whether long-distance ABA transport contributes to changes in local ABA levels. To determine the physiological relevance of ABA transport, we made reciprocal- and self-grafts of ABA-deficient flacca mutant and wild-type (WT) tomato plants, in which low phosphorus (P) conditions decreased ABA concentrations while salinity increased ABA concentrations. Whereas foliar ABA concentrations in the WT scions were rootstock independent under conditions, salinity resulted in long-distance transport of ABA: flacca scions had approximately twice as much ABA when grafted on WT rootstocks compared to flacca rootstocks. Root ABA concentrations were scion dependent: both WT and flacca rootstocks had less ABA with the flacca mutant scion than with the WT scion under conditions. In WT scions, whereas rootstock genotype had limited effects on stomatal conductance under conditions, a flacca rootstock decreased leaf area of stressed plants, presumably due to attenuated root-to-shoot ABA transport. In flacca scions, a WT rootstock decreased stomatal conductance but increased leaf area of stressed plants, likely due to enhanced root-to-shoot ABA transport. Thus, long-distance ABA transport can affect responses in distal tissues by changing local ABA concentrations

Chronic tropospheric ozone exposure reduces seed yield and quality in spring and winter oilseed rape

Oilseed rape (Brassica napus L.) is cultivated worldwide, producing 11.5% of global oilseeds at an economic value of 38 billion USD in 2020. It is sensitive to phytotoxic damage from exposure to tropospheric ozone (O3), a major air pollutant, which disrupts plant physiological processes and thus decreases biomass accumulation. As background ozone concentrations continue to increase globally, we investigated the impact of ozone exposure on seed and oil yield of a shorter-lived spring (cv. Click) and a longer-lived winter (cv. Phoenix) oilseed rape cultivar to ozone levels (treatments with peaks of 30, 55, 80, 110 ppbv) representative of typical European conditions where these cultivars are common. Thousand Seed Weight (TSW), an important measure of final yield, decreased more in Phoenix (40%) than Click (20%) with increasing ozone exposure. Click produced more racemes and many small seeds while Phoenix produced fewer racemes and larger seeds. However, seed quality declined more substantially in Click than Phoenix. The oil content in Click's seed significantly decreased with increased ozone exposure, while less desirable components (moisture, chlorophyll, ash) increased. Scaled to field-level, our findings imply substantial economic penalties for growers, with potential losses of 175–325 USD ha−1 in Click and 500–665 USD ha−1 in Phoenix under ozone concentrations typical of spring and summer periods in Europe. Decreased total yield would likely outweigh the benefits of any improvement in animal oilseed cake quality (increased protein and key micronutrients for livestock feed). Neither cultivar sustained visible injury at earlier growth stages, and Phoenix sustained photosynthesis even under high exposure, thereby making ozone an invisible threat. Our findings of reduced oilseed quantity and quality threaten oilseed rape production, but differences between the cultivars may also offer an opportunity for breeders and agronomists to identify and exploit variation in ozone tolerance in oilseed rape

Decreased irrigation volume, not irrigation placement, promotes accumulation of multiple hormones in cotton leaves during partial rootzone drying

While ABA is often assumed to mediate partial stomatal closure as the soil dries, other plant hormones and hydraulic signals may also be involved. We tested whether irrigation volume (% of crop evapotranspiration, ET) and placement (partial rootzone drying [PRD] or deficit irrigation [DI], which irrigate part or all of the rootzone respectively) affect this signalling by measuring stomatal conductance (gs), leaf and shoot water potential (Ψleaf, Ψshoot), shoot xylem sap ABA concentration ([X-ABA]shoot) and various foliar hormones (ABA, IAA, SA, JA, JA-Ile and cis-OPDA) in cotton plants exposed to different irrigation volumes (100%ET or 50%ET) and placements (DI or PRD). Partial rootzone drying caused stomatal closure coincident with sustained foliar ABA accumulation and minimal changes in Ψshoot, but continued soil drying of the dry compartment reversed partial stomatal closure (with gs of 100%ET PRD plants sometimes greater than well-watered plants). With 100%ET PRD, partial stomatal closure correlated with decreased soil moisture of the dry compartment and increased [ABA]leaf, but neither Ψleaf nor [X-ABA]shoot. Irrespective of irrigation placement, 50%ET significantly decreased gs, Ψleaf and Ψshoot, but significantly increased [ABA]leaf, [X-ABA]shoot, [SA]leaf, [IAA]leaf and [cis-OPDA]leaf, with stomatal closure of 50%ET PRD plants occurring earlier than 50%ET DI plants. While stomatal closure at 50%ET correlated with foliar accumulation of multiple plant hormones, foliar ABA dynamics best explained transient stomatal closure at 100%ET PRD but not stomatal re-opening with prolonged soil drying. Thus, stomatal sensitivity to drying soil (and putative regulatory signals such as ABA) depended on irrigation volume and placement

Distinctive phytohormonal and metabolic profiles of Arabidopsis thaliana and Eutrema salsugineum under similar soil drying

Main conclusions: Arabidopsis and Eutrema show similar stomatal sensitivity to drying soil. In Arabidopsis, larger metabolic adjustments than in Eutrema occurred, with considerable differences in the phytohormonal responses of the two species. Although plants respond to soil drying via a series of concurrent physiological and molecular events, drought tolerance differs greatly within the plant kingdom. While Eutrema salsugineum (formerly Thellungiella salsuginea) is regarded as more stress tolerant than its close relative Arabidopsis thaliana, their responses to soil water deficit have not previously been directly compared. To ensure a similar rate of soil drying for the two species, daily soil water depletion was controlled to 5–10% of the soil water content. While partial stomatal closure occurred earlier in Arabidopsis (Day 4) than Eutrema (from Day 6 onwards), thereafter both species showed similar stomatal sensitivity to drying soil. However, both targeted and untargeted metabolite analysis revealed greater response to drought in Arabidopsis than Eutrema. Early peaks in foliar phytohormone concentrations and different sugar profiles between species were accompanied by opposing patterns in the bioactive cytokinin profiles. Untargeted analysis showed greater metabolic adjustment in Arabidopsis with more statistically significant changes in both early and severe drought stress. The distinct metabolic responses of each species during early drought, which occurred prior to leaf water status declining, seemed independent of later stomatal closure in response to drought. The two species also showed distinct water usage, with earlier reduction in water consumption in Eutrema (Day 3) than Arabidopsis (Day 6), likely reflecting temporal differences in growth responses. We propose Arabidopsis as a promising model to evaluate the mechanisms responsible for stress-induced growth inhibition under the mild/moderate soil drying that crop plants are typically exposed to

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 (Lp(Root)) 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. WTplants were capable of maintaining leaf water potential (psi(L)) that was likely due to increased Lp(Root) enabling higher water flow from the roots, which increased in response to air warming. The increased Lp(Root) 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 psi(L) during air warming may be due to lower Lp(Root) than WT plants, and an inability to respond to changes in air temperature. The correlation between root ABA content and Lp(Root) 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

Mangrove species found in contrasting environments show differing phytohormonal responses to variation in soil bulk density

Background and aims Mangrove species respond to variation in soil bulk density (BD). However, very little is known about the regulatory mechanisms that trigger these responses.Methods Endogenous concentrations of different phytohormones were measured in the roots of two mangrove species (Avicennia marina and Rhizophora stylosa) grown in low and high BD soils. The potential involvement of ethylene in regulating plant growth responses was tested by applying the ethylene biosynthesis inhibitors cobalt chloride (CoCl2) and aminoisobutyric acid (AIB).Results The two mangrove species responded differently to variation in soil BD. High BD decreased root growth of R. stylosa, but not A. marina. Soil BD had no effect on root phytohormone levels in R. stylosa, but loose soils increased 1-aminocyclopropane-1-carboxylic acid whilst decreasing salicylic acid and gibberellin in A. marina. Applying ethylene inhibitors enhanced R. stylosa root growth, while increasing indole-3-acetic acid but decreasing isopentenyl adenine levels. In contrast, AIB inhibited A. marina root growth, while increasing trans-zeatin levels. Ethylene inhibitors affected salicylic acid levels in both species.Conclusion Salicylic acid is central to root growth responses to variation in BD in A. marina. Conversely, the interaction of ethylene and gibberellin drives responses in R. stylosa. Hormonal interactions involving ethylene potentially reflect the adaptations of the two species to differing conditions within the intertidal zone, with A. marina behaving like an aquatic species and R. stylosa behaving like a terrestrial species