Journal of Experimental Botany

Plant encounters with saline conditions trigger a phase of osmotic stress, followed by ionic stress as salt accumulates in plant tissue. Jasmonate (JA) signalling plays a vital role in both phases, but its role for adaptation depends on timing. Using wild-type rice (Oryza sativa L. cv. Kitaake) and a JA-deficient allene oxide cyclase (aoc) mutant, we followed whole-plant physiology (growth, biomass, photosynthesis, ion distribution) with expression analysis for genes of jasmonate signalling, ion transporters and redox balance to separate adaptive from maladaptive JA effects. The aoc mutant sustained shoot and root growth more effectively than the wild type, maintaining cell expansion in the shoot and cell proliferation in the root under saline conditions. Likewise, stomatal conductance in the mutant was more persistent under salinity, accompanied by improved shoot K/Na homeostasis and reduced sodium in young leaves. In the wild-type, prolonged saline conditions were linked with induction of OsJAZ13 and OsNHX1, stomatal closure and chlorophyll degradation. We discuss these findings in the context of a model, where the function of JA signalling does not merely depend on its accumulation, but on its temporal dynamics. Here, OsHAK16 emerges as a marker of early adaptive responses, whereas OsNHX1 and OsJAZ13 characterize later maladaptive stress states, emphasising the potential of modulating JA dynamics to improve salt resilience in rice breeding.

Gene regulatory networks (GRNs) establish the spatio-temporal gene expression patterns necessary for reproducible development of organisms. MicroRNAs have emerged as important regulators of gene expression controlling either target gene levels, spatial patterns or timing. These molecular functions may vary depending on how they are integrated into GRNs. Here we used the Arabidopsis leaf as a model to study the genetic and molecular interactions between the CUP-SHAPED COTYLEDON 2 (CUC2) transcription factor and its negative regulator, miR164A. Quantitative morphometric analyses showed that miR164A acts from early stages during leaf development regulating CUC2 expression dynamic rather than the spatial distribution of CUC2. At the molecular level, CUC2 directly activates MIR164A expression, forming a single negative feedback loop. Further, using a ratiometric approach, we identified a cis-regulatory element (CRE) sufficient to drive CUC2-dependent MIR164A expression at the leaf sinus. Ordinary differential equation (ODE) modelling suggests that CUC2-CRE may control the timely accumulation of CUC2. Accordingly, quantification of CUC2 protein accumulation dynamics using a spatio-temporal mean cartography strategy in CRISPR cis-regulatory variants at the MIR164A locus revealed that CUC2-dependent MIRA164A expression is required to maintain both the precise timing of CUC2 expression and CUC2 levels. Altogether, our work provides experimental evidence for a specific role of the CUC2-MIR164A module in shaping leaf development, highlighting the contribution of network topology to GRN functions.

Reproductive stage drought disrupts flag leaf metabolism in rice (Oryza sativa), limiting photoassimilate translocation to developing grains and reducing grain yield (GY). How natural variation in this response relates to GY remains poorly understood. This study combines untargeted metabolomics of flag leaves from 271 indica rice accessions grown under well-watered (WW) and reproductive-stage water-limited (WL) field conditions with genome-wide association studies of 89 annotated polar metabolites. A total of 180 metabolite quantitative trait loci (mQTLs) were identified: 93 detected only under WL, 68 only under WW, and 19 under both conditions, indicating a predominant stress-dependent genetic control; mapping of GY itself detected only four marginal QTLs, none overlapping the mQTLs. Four high-density regions (HDRs) of mQTLs were identified: HDR1-1, detected only under WW conditions and associated mainly with raffinose family oligosaccharides; HDR1-2, HDR4, and HDR11, detected only under WL conditions and associated mainly with tricarboxylic acid cycle intermediates and amino acids. After correcting for genetic relatedness, accessions carrying the favourable haplotype at both HDR1-2 and HDR4 yielded 21.5% more under WL conditions than accessions carrying the unfavourable haplotype at both loci and lost 11.4 percentage points less GY to stress. Accessions carrying the favourable haplotype at only one locus showed a comparable reduction in yield loss. Haplotype-guided variant detection against the 3,000 Rice Genomes Project panel identified major-impact variants in candidate genes, including a polyphenol oxidase at HDR1-2 and tandem cytochrome P450s at HDR4. These hubs are promising, metabolically defined targets for breeding drought-resilient rice.

Plants frequently experience recurrent drought events separated by periods of rehydration. Although drought imposes strong constraints on plant physiology, prior exposure may alter subsequent stress responsiveness through memory-based mechanisms. Here, we investigated whether recurrent drought at distinct developmental stages establishes stress memory in sugarcane and whether this response persists across vegetative propagation. Two genotypes contrasting in drought tolerance and productivity (IACCTC07-8008 and IACSP95-5000, respectively) were grown under greenhouse conditions and subjected to three drought cycles imposed either at tillering or maturation stage. Gas exchange, photochemical performance, leaf water status, primary metabolite profile, and growth traits were assessed across cycles, and vegetative propagules were subsequently evaluated under renewed drought. The first drought cycle imposed strong limitations on carbon assimilation and photochemistry in both genotypes and developmental stages. However, subsequent cycles resulted in attenuated reductions in A and g, improved intrinsic water use efficiency, and partial stabilization of PSII performance, indicating a modified stress-response trajectory. Young plants displayed earlier improvements (from the second cycle), whereas in mature plants this shift was evident mainly during the third cycle. Recurrent drought promoted sustained reorganization of amino acid, carbohydrate, organic acid and polyol metabolism alongside increased root biomass and higher relative water content during later cycles. Importantly, propagules derived from drought-conditioned plants exhibited faster recovery of photosynthetic performance and reduced cumulative physiological impairment under renewed drought, despite showing similar stress sensitivity at maximum water deficit. This persistence of enhanced recovery capacity across vegetative propagation indicates that drought-induced memory was maintained beyond the initially stressed plants. Together, our findings demonstrate that recurrent drought establishes a metabolically imprinted state in clonal sugarcane, integrating physiological adjustment, metabolic reprogramming, and whole-plant acclimation. These results highlight the potential of stress memory as a mechanism supporting resilience in perennial crops exposed to increasingly recurrent drought events.

Plastidial phosphorylase (Pho1) regulates starch metabolism and also modulates photosystem I (PSI) activity. In rice, Pho1 contains an 80 amino acid intrinsically disordered region (IDR), L80, that acts as a negative regulator. While total removal of the IDR boosts seedling growth, flowering time, overall biomass, and grain yield and alters PSI redox properties, the precise underlying regulatory basis remains unclear. To pinpoint IDR regulatory elements, four Pho1 partial-IDR deletion variants ({delta}N, {delta}C, {delta}M, and {delta}N{delta}C) were evaluated. Lines carrying {delta}M and {delta}C exhibited enhanced starch metabolism, yielding larger, heavier grains, and flowered 5 to10 days earlier than wild type. Both d{delta}M and {delta}C share an acidic motif, (E/V)SEEV, identifying it as a primary negative regulator of starch synthesis and floral timing. By contrast, no partial deletion fully mirrored the PSI redox modifications of Pho1{delta}L80. These results demonstrate that starch biosynthesis and flowering are governed by one or more discrete sequence motifs within the middle and C-terminal regions, whereas PSI modulation likely requires the structural integrity of the intact IDR.

Understanding the genetic architecture of ecophysiological traits related to carbon gain and water loss is crucial to develop cultivars tolerant to abiotic stresses and that maintain productivity in the context of climate change. This can be achieved both through experiments in the greenhouse on potted plants under controlled conditions and in the field. However, the stability of genetic determinism of ecophysiological traits across environmental conditions has rarely been evaluated. We studied a grapevine diversity panel grown both in a greenhouse and in the field, to investigate the impact of growing conditions on leaf ecophysiological traits related to leaf morphology, fluorescence, gas exchange and water use efficiency. The correlation structure between traits was conserved within each environment even though, for most traits, cultivars ranking was not conserved between both environments. However, a set of cultivars, sharing the same functioning pattern in terms of gas exchange and fluorescence in both environments, was identified. Single- and multi-environment Genome-Wide Association Studies (GWAS), based on a dense genotyping matrix, were used to dissect the genetic architecture of leaf traits. The Quantitative Trait Loci (QTLs) identified through single-environment analyses differed substantially from those detected using multi-environment ones. Multi-environment approaches highlighted shared QTLs across environments for gas exchange and fluorescence traits. Candidate genes associated with carbon- and water-related functions were further identified in these genomic regions. Despite the weak correlation between greenhouse and field trait values, combining both environments enabled the identification of robust genetic regions associated with ecophysiological traits of interest.

Photosynthesis is a key determinant of crop productivity, yet conventional gas-exchange measurements are labor-intensive and unsuitable for high-throughput assessment of dynamic photosynthetic acclimation across large genotype panels. Consequently, genetic variation in photosynthetic acclimation remains poorly characterized and difficult to exploit in breeding programs. Here we present a computational pipeline that integrates rapid optical sensing (chlorophyll meters and hyperspectral reflectance) with a mechanistic model of photosynthetic protein-turnover to estimate dynamic nitrogen allocation among photosynthetic components. The pipeline was demonstrated using 60 winter wheat (Triticum aestivum L.) cultivars sampled at 10 time points spanning leaf emergence to senescence. Uncertainties associated with each pipeline component were quantified and benchmarked against gas-exchange ground-truth measurements under three controlled-environment light and temperature regimes. Dynamic nitrogen allocation to light harvesting, electron transport, and carboxylation was characterized using three biologically interpretable parameters: maximum synthesis rate, degradation rate, and age-dependent decline in synthesis. Although hyperspectral models showed moderate predictive accuracy for photosynthetic capacity, prediction errors were predominantly random, allowing robust parameter estimation when observations across the leaf lifespan were integrated. The pipeline successfully resolved genotype-by-environment interactions in photosynthetic acclimation, with environmental and interaction effects contributing more strongly than genotype main effects to nitrogen dynamics. These results demonstrate a scalable, uncertainty-aware sensor-to-trait architecture for dynamic physiological phenotyping that enables high-throughput germplasm screening across diverse environments and provides novel physiological selection targets for improving crop adaptation under variable climates.

Water availability in drying soils is most often spatially heterogeneous, requiring plants to adjust their root system architecture to sustain water uptake. This acclimation response to water deficit (WD) relies on the integration of local and systemic signals. The later and their mode of action remain poorly understood. Using hydroponic split-root systems in maize and Arabidopsis, we show that, with respect to control well-watered plants, a local WD does not affect shoot biomass but suppresses lateral root (LR) growth in the water-deprived compartment while promoting LR elongation in the well-watered compartment. Spatiotemporal analyses in maize revealed an early and transient (<2h) transcriptomic response of LRs triggered by both local and systemic WD signals. This response involved activation of jasmonate (JA) biosynthesis and signaling components, including MYC2 like regulators and JAZ repressors. It occurred in both the water-deprived and well-watered root halves of split roots, with no evidence of long-distance transport of JA. Furthermore, exogenous methyl jasmonate locally inhibited LR elongation, whereas the Arabidopsis JA insensitive jar1-1 mutant showed an enhanced systemic LR growth response to a local WD. Together, these findings demonstrate that JA acts as an early negative regulator that counterbalances the systemic enhancement of LR growth induced by a local WD, thereby fine-tuning root system architecture and water uptake during local soil drying.

Although iron (Fe) is abundant in soils, its low solubility, particularly under alkaline conditions, often limits plant Fe acquisition. Under these conditions, coumarin-mediated Fe mobilization becomes increasingly important. Arabidopsis thaliana FE DEFICIENCY-INDUCED KELCH REPEAT PROTEIN 1 (FDK1, AT3G07720) is strongly induced by Fe deficiency and regulated by the central transcription factor FIT, but its function is unknown. We investigated FDK1 using genetic, physiological, metabolite, and transcriptomic analyses. Loss of FDK1 altered root growth, whereas this phenotype was restored in the FDK1-Ox overexpression line. Under alkaline Fe deficiency, growth medium from FDK1-Ox contained strongly increased levels of the Fe-mobilizing coumarins fraxetin and esculetin compared with wild type, whereas their induction was attenuated in fdk1-1. RNA-seq analysis linked FDK1 abundance to coumarin biosynthesis and iron-homeostasis gene expression. FIT and many FIT-associated genes were highly expressed in FDK1-Ox, whereas several upstream iron-homeostasis regulators, including bHLH Ib factors, were highest in fdk1-1. These findings identify FDK1 as a component of the alkaline Fe-deficiency response that promotes iron-mobilizing coumarin accumulation and is associated with coordinated regulation of iron-homeostasis genes.

Despite the recent cloning of RLK-encoding resistance genes and the identification of a common point of hyphal arrest at the stomatal level in incompatible interactions, the molecular mechanisms leading to the establishment of wheat resistance against Septoria tritici blotch (STB) remain poorly characterized. Here, we use long-read RNA-sequencing data to compare the early transcriptomic responses of two wheat cultivars carrying major, isolate-specific resistances to Zymoseptoria tritici via differential expression, differential transcript usage (DTU), functional- and pathway enrichment analyses. We show that both magnitude and composition of the responses depend strongly on the combination of host cultivar and pathogen isolate and that DTU appears to be uncommon at this infection stage. We do not detect a set of genes exclusively differentially expressed across incompatible interactions, but genes coding for proteins involved in several defense-related processes consistently show higher expression levels in incompatible than compatible interactions, including receptors and ROS homeostatic, antifungal hydrolytic, phenylpropanoid and diterpenoid biosynthetic enzymes. Our results suggest that whilst interaction-specific differences in magnitude and composition may also shape resistance, different major STB resistances may quantitatively enhance overlapping defense machinery, providing a basis for its functional investigation.

Groundnut (Arachis hypogaea) is an important oilseed crop, and its production is severely affected by drought worldwide. Improving the drought-tolerance of groundnut without compromising its agronomic traits is a major challenge for researchers. Here, a meta-transcriptome analysis identified tandemly duplicated G{beta} subunits (AhG{beta}1 and AhG{beta}3) as early signaling genes under water deficit conditions. Heterotrimeric G-protein {beta}-subunit is a major signaling element that can regulate diverse responses in plants. Heterologous complementation studies in Arabidopsis G{beta} mutant (agb1-2) demonstrated evolutionary conservation of AhG{beta} function, while the transgenic overexpression and RNAi knockdown in groundnut established AhG{beta}1 as a positive regulator of plant growth, yield, and drought tolerance. Integrated transcriptomic, transcription factor regulatory network, and metabolomic analyses demonstrated that AhG{beta}1 coordinates drought-responsive transcriptional and metabolic reprogramming characterized by suppression of cell wall-associated processes and activation of phenylpropanoid, flavonoid, and isoflavonoid biosynthesis. This reprogramming resulted in enhanced accumulation of antioxidant flavonoids, including quercetin and rutin, reduced ROS accumulation, and improved drought tolerance. Furthermore, exogenous application of these flavonoids enhanced drought recovery, highlighting their role as protective downstream metabolites. Collectively, our findings establish AhG{beta}1 as a conserved regulator linking early G-protein signaling with transcriptional and metabolic reprogramming to maintain redox homeostasis during drought stress and identify promising molecular and metabolic targets for improving drought resilience in groundnut.

- C2 photosynthesis reduces photorespiratory carbon loss by shuttling glycine from mesophyll (M) to bundle sheath (BS) tissues, where glycine decarboxylase (GDC) releases CO2 that accumulates around rubisco. C2 photosynthesis has been considered a constitutive character, but as a metabolic trait, it may acclimate to the rate of photorespiratory glycine production. - We grew eleven species from two eudicot and two grass lineages spanning C3 to C2 phenotypes at 32 {degrees}C days and 200 or 800 {micro}mol mol-1 CO2. Acclimation was assessed using leaf gas exchange, GDC P-protein immunolocalization, M and BS ultrastructure, photosynthetic enzyme activity, and photosynthesis modelling. - Low relative to high growth CO2 enhanced C2-related phenotypes in sub-C2 and C2 species but not C3 or proto-Kranz species. Sub-C2 species positioned chloroplasts closer to mitochondria and had lower modelled BS conductance. C2 species increased rubisco capacity and GDC allocation to the BS. Aspartate aminotransferase activity increased in both sub-C2 and C2 species. - Acclimation to greater photorespiratory conditions strengthens C2 photosynthesis, but only in species whose BS has crossed a threshold of organelle and GDC investment. We hypothesize that acclimation could expose C2-related traits to selection and facilitate the origin of C4 photosynthesis.

Background and aims: Perennial root systems develop over decades and exhibit continuous turnover of structural and absorptive components, yet most root architecture models treat woody roots as permanent and neglect root age structure. We developed an empirically constrained modelling framework that explicitly represents age-structured demography, growth regulation and dormancy in perennial root systems. Methods: We extended the functional-structural plant model CPlantBox to simulate perennial root development over decades by integrating age-dependent survival, secondary radial growth and climate-driven dormancy. The model was parameterised using unique three-dimensional root system data from young and mature grapevines, together with dendrochronological root age measurements, and coupled to root hydraulic properties. Key results: Empirical analyses revealed multi-cohort age distributions and substantial turnover of lignified structural roots, contradicting the assumption of a static woody scaffold. The model reproduced age distributions, order-specific survival probabilities and fine-to-structural root length ratios, and generated realistic, genotype-specific three-dimensional architectures. Simulated uptake profiles and hydraulic scenarios showed strong age- and architecture-driven effects. Conclusions: Explicitly representing woody root age distributions and age-dependent processes may fundamentally change predictions of perennial root system architecture and function. The framework advances root modelling beyond annual paradigms and provides a transferable tool for analysing long-term architectural and hydraulic consequences of genotype x climate interactions in perennial crops.

Potassium (K) deficiency is a widespread constraint on maize production, yet how it influences the suberized apoplastic barriers that control radial K transport in roots, and whether the endodermis and the constitutively suberized exodermis respond differently, has remained unclear. We grew maize under K deficiency in soil and across a hydroponic K gradient, with abscisic acid (ABA) and fluridone treatments, and analysed seminal roots by Fluorol Yellow 088 staining, tissue-resolved suberin chemistry of endodermis and exodermis, rubidium (Rb) flux, and RNA-sequencing with weighted gene co-expression network analysis (WGCNA). K deficiency selectively increased endodermal aliphatic suberin and eliminated endodermal passage cells in soil-grown roots, whereas exodermal suberin was unchanged. Although K influx remained high, root-to-shoot Rb translocation fell sharply, consistent with K retention. A single co-expression module linked the suberin-biosynthetic genes with several K transport genes, and the suberin programme responded to exogenous ABA, indicating ABA-dependent co-regulation. Soil and hydroponic systems converged on the same endodermis-specific anatomical response. Endodermal, but not exodermal, barrier reinforcement limits K leakage from the stele in maize roots. This tissue-specific plasticity identifies the endodermal suberin biosynthesis programme, and its coordination with K uptake, as a target for improving K use efficiency.

Boron toxicity constrains grapevine (Vitis) production in irrigated arid and semi-arid regions, yet most available rootstocks are sensitive to excessive boron. Although underutilized Vitis species may harbor sources of adaptation, the genetic basis of natural variation in boron exclusion remains poorly understood, limiting the development of boron-tolerant rootstocks. We screened 267 wild Vitis accessions representing 16 species for their ability to limit leaf boron accumulation under excess boron conditions. Leaf boron concentration varied 190-fold among accessions and was highly heritable (entry-mean H^2 = 0.71). Its correlation with stomatal conductance was modest (r = 0.34), indicating that transpiration rate explained only a small proportion of the variation in leaf boron accumulation. Leaf boron was correlated with leaf chloride (r = 0.36), but their largely non-overlapping association profiles supported distinct genetic control. Genome-wide association analysis identified a suggestive association on chromosome 11. Independent quantitative trait locus mapping in a full-sib F1 population (n = 158) derived from Longii-9018 (V. acerifolia) x GRN3 (commercial rootstock) identified qBEx11.1 in the same syntenic region on chromosome 11, which explained 19.5% of the phenotypic variance. Positional candidate genes near the association peak included a pectinesterase and a MATE efflux transporter, suggesting roles for cell wall modification and membrane transport. Leaf spectral reflectance was associated with leaf boron concentration in both populations, particularly at visible and red-edge wavelengths, indicating potential for high-throughput phenotyping. Together, these findings identify qBEx11.1 as a priority locus for boron exclusion and support integrated genetic and spectral approaches for breeding boron-tolerant grapevine rootstocks.

High temperatures during grain filling stage are becoming increasingly frequent, compromising both grain and eating quality and thereby driving demand for heat-resilient cultivars. Such conditions are known to reduce the expression of granule-bound starch synthase I (GBSSI) and starch branching enzyme IIb (BEIIb), which are involved in starch biosynthesis, resulting in a decrease in amylose content and an increase in long-chain amylopectin. Thus, the present study introduced functional mutation in starch synthase IIa (SSIIa) that increases the proportion of short amylopectin chains to genetically compensate for the high-temperature-induced increase in amylopectin long chain. Rice lines carrying the ss2a mutation were grown at two locations with cooler (Akita) and warmer (Okayama) temperatures. Their grain traits, starch structure, and eating quality were compared. The ss2a mutant lines showed an increased proportion of short amylopectin chains as well as an increased apparent amylose content. Furthermore, these alterations in starch structure varied with the grain-filling temperature of the cultivation sites, ultimately affected eating quality. These results suggest that enriching short amylopectin chain via the ss2a mutation can counteract the increase in long amylopectin chain caused by high temperatures during grain filling, thereby maintaining a desirable starch structure and eating quality.

Understanding how grafts form has fascinated scientists for over a century, and numerous transcription regulators involved in wound healing, callus formation, and grafting have been identified in model plants. However, the genetic architecture underlying grafting traits remains poorly characterized. To address this gap, we aimed to identify quantitative trait loci (QTLs) associated with grafting success rates and grafting-related traits (including callus formation, mechanical strength of the graft union, and post-grafting growth) in a grapevine rootstock mapping population, Vitis berlandieri x V. rupestris. Although no QTLs were identified for grafting success rate, QTLs were identified for component traits revealing a genetic architecture that would have remained hidden when considering grafting success as a single composite phenotype. By integrating QTL and transcriptome data, we identified regulators potentially mediating grafting-related traits. We then showed that one transcription factor that regulates callus formation and grafting in Arabidopsis potentially has a similar role in grapevine by silencing with exogenous double-stranded RNAs and that exogenous application of methyl jasmonate increases callus development at the graft interface. Candidate transcriptional regulators located within QTLs and evidence from gene silencing and hormone treatments suggest the conservation of regulatory mechanisms underlying wound-induced callus formation across plant species.

In common wheat (Triticum aestivum L.), variation at the Vrn1 and Ppd1 loci changes plant phenology, resulting in differential adaptation useful to breeders. Because the effects of phenology on grain yield are conditional on environmental factors, the incorporation of markers for Vrn1 and Ppd1 alleles into breeding approaches has proven difficult. Historic phenotypic, genotypic, and environmental data can provide insight into the relationship between major phenology alleles and grain yield as mediated by environmental variables. Analyses of eight years of breeding trials (1,038 lines at 219 site-years) determined that weak vrn1 alleles and Ppd1 insensitivity alleles have variable effects. These effects change depending on an environment's winter temperature and latitude, respectively, but the environmentally-driven changes in effect of vrn1 alleles are much more variable than effects of Ppd1 alleles. Mediation analyses showed phenologically-dependent environmental variables are one mechanism through which phenological changes created by vrn1 and Ppd1 variants alter yield across multiple developmental stages. Environmentally-driven relationships between flowering time and yield were modeled to estimate yield allele effects as a function of effects on phenology and environmental variables. Weak winter alleles at vrn1 may play a stabilizing role on yield, due to the correlation of winter conditions that drive larger effects with later-season warm temperatures that penalize later maturity. A better mechanistic understanding of how these loci drive environment-specific yield effects may facilitate utilization of markers for these alleles and improve predictive modeling of yield.

Bulk phloem flow is driven by pressure gradients generated by water drawn from the xylem, coupling source-sink transport with water status. Huanglongbing (HLB), a phloem-affecting bacterial disease of citrus, disrupts phloem transport, and causes canopy water deficit. Although improved water management can enhance HLB-affected tree health, the effects of HLB on water-driven source-sink relations are unknown. Fruits function as both carbon sink and water reservoir, but the role of fruit in regulating whole-tree water status remain poorly understood. We examined leaf-fruit water relations in HLB-affected citrus and evaluated how HLB exposure duration, fruit developmental stage, and source-sink manipulation shape these dynamics. Source-sink relations were manipulated in branches of symptomatic trees with long-term (four years) and recent (six months) HLB exposure. We girdled branches with a single fruit as the sink and defoliated at (0%, 50%, or 75%), and an ungirdled, non-defoliated branch served as control. We measured stomatal conductance, leaf hydraulic traits, leaf and fruit soluble sugars, diurnal fruit shrinkage (to estimate xylem backflow) and fruit water loss. Source-sink manipulation did not affect shoot total soluble sugar. HLB-exposure resulted in less negative osmotic potential, more negative turgor potential and reduced modulus of elasticity and relative water content at turgor loss point. HLB exposure also increased diurnal fruit shrinkage, with shrinkage responses to source-sink manipulation varying across fruit development stages. Collectively, these findings suggest that long-term HLB exposure promotes leaf acclimation to water deficits, however, high sink demand may diminish the leaf water status regulation at Stage II, when fruit sink carbon demand is highest.

Climate change is increasing the frequency and intensity of heatwaves and droughts, threatening medicinal and aromatic plants (MAP) production. Temporary shading during the hottest part of the day may reduce heat and radiative stress while limiting the reduction in light availability. We evaluated this strategy in a field experiment using three MAP species with contrasting ecological requirements and harvested organs: lavandin (Lavandula x intermedia), curly parsley (Petroselinum crispum) and valerian (Valeriana officinalis). Vertical shade nets provided midday shading and effects on crop microclimate, leaf temperature, water status, PSII photochemical efficiency, plant growth and biomass and essential oil production were assessed over two growing seasons. Midday shading reduced daily photosynthetically active radiation by 19.9-55.9 %, with weak effects on air temperature and vapor pressure deficit. In contrast, leaf temperature at solar noon decreased by 6.9 degrees C, 5.6 degrees C and 3.0 degrees C in lavandin, valerian and parsley, respectively. Midday shading improved light-adapted and maximum PSII photochemical efficiencies in all three species and reduced branch dehydration in lavandin during heatwave periods. However, agronomic responses were species-dependent. Floral biomass decreased in lavandin, whereas parsley leaf biomass and valerian root biomass increased. Essential oil concentration was unaffected in lavandin and valerian. Overall, our results show that midday shading primarily reduced leaf-level thermal and radiative stress rather than modifying the surrounding air microclimate, and suggest that its agronomic effects may reflect a balance between stress alleviation and reduced carbon acquisition, that varies with species ecology and production objectives.