Juan Gabriel Perez-Perez

Rootstock choice can shape orchard establishment and fruit production, but integrated evidence for young ‘Verna’ lemon trees under semi-arid Mediterranean conditions remains limited. Four rootstocks were evaluated for ‘Verna 51’ lemon: Citrus macrophylla (CM), sour orange (NA), Rangpur lime (LR), and Forner-Alcaide No. 5 (FA-5). The trial covered four orchard-establishment seasons (2016–2017 to 2019–2020) in Murcia, south-eastern Spain. Irrigation was scheduled to replace 100% of crop evapotranspiration (ET_c). Vegetative growth, yield, fruit quality, midday stem water potential (Ψ_stem), leaf gas exchange, and foliar mineral composition were assessed in a randomised complete block design. Rootstock affected canopy development, cumulative yield, fruit size, crop water-use efficiency (CWUE), several fruit-quality attributes, and leaf mineral composition. Rootstock effects on Ψ_stem and gas exchange were detected in 2019, whereas neither intrinsic nor instantaneous leaf water-use efficiency showed a significant overall rootstock effect in either year. CM produced the greatest canopy development and the highest numerical cumulative yield and CWUE; for the latter two variables, CM exceeded LR and FA-5 but did not differ significantly from NA. Overall mean fruit weight was also greater in CM than in NA and FA-5, with LR showing an intermediate value. Fruits from trees grafted onto CM had thicker peel than those from LR and FA-5, lower total soluble solids than those from the other rootstocks, and lower titratable acidity than those from NA and FA-5. FA-5 had the lowest numerical final canopy volume and a lower cumulative yield than CM, whereas NA did not differ significantly from CM in final-season yield and had higher soluble solids and acidity than CM. LR was generally intermediate in growth and production and had a less negative mean Ψ_stem than FA-5 in 2019. In this young orchard, CM favoured rapid canopy establishment, cumulative production, and fruit size, although these gains were accompanied by less favourable values for some internal fruit-quality attributes.

Hydrogen peroxide (H2O2) is increasingly considered a potential tool for improving plant performance, although its effects may depend strongly on root-zone conditions and application regime. This study evaluated the physiological, biochemical and hormonal responses of Citrus macrophylla seedlings to H2O2-amended fertigation under high-frequency irrigation (HF; daily) and low-frequency irrigation (LF; every two days). Plants received 0 or 2,000 mg L-1 H2O2 (Control and H2O2, respectively) immediately before each irrigation event for 43 days. A separate assay showed that H2O2 caused a rapid but transient increase in dissolved oxygen in the irrigation solution. Plant responses were most evident under HF, where H2O2 increased net CO2 assimilation (ACO2) without altering stomatal conductance (gs) or transpiration (Eleaf) and reduced intercellular CO2 concentration (Ci). It also increased instantaneous carbon gain relative to gs and Eleaf across irrigation frequencies. Under HF, these photosynthetic responses coincided with lower malondialdehyde (MDA) and proline concentrations, indicating reduced lipid peroxidation and a lower requirement for osmotic and redox adjustment. However, H2O2 did not increase biomass and reduced total leaf area under HF. It also decreased leaf indole-3-acetic acid (IAA) and salicylic acid (SA) concentrations, whereas abscisic (ABA) and jasmonic (JA) acids remained unchanged. Under LF, plants displayed a stronger non-enzymatic antioxidant and photoprotective profile, with limited response to H2O2. These findings show that H2O2-amended fertigation can enhance leaf-level photosynthetic and redox performance, but its effects depend on root-zone conditions and exposure pattern.

Boron (B) is an essential but narrow-range micronutrient for citrus, with toxicity risks heightened in dry regions due to potentially high-B irrigation water and limited soil leaching. ‘Forner-Alcaide 5’ (FA5) is a promising rootstock for enhancing B-tolerance of sweet orange, but it had not been sufficiently tested before this study, specifically considering soil texture. Therefore, this greenhouse study investigated the effects on B absorption and biomass buildup of irrigating navel orange seedlings (cv. Navelina) grafted onto ‘Carrizo’ citrange (CC) and FA5 rootstocks, with 0.11, 2, or 5 mg B L⁻¹ waters and grown in clay loam or sandy loam soils. The results of this complete three-factor trial revealed that leaves are the primary sink for B (24–1300 mg kg⁻¹), indicating passive, transpiration-driven uptake and limited phloem redistribution. The presumed absence of sugar alcohols, and the weak binding affinity of B to the abundant sucrose, may account for the restricted phloem mobility of B in citrus, consistent with the mechanistic interpretation proposed in this study. FA5 rootstock showed greater B tolerance, sustaining 28% higher biomass than CC at 2 mg L⁻¹ B. Plant B uptake was found to be more related to soil soluble B than adsorbed B. Interestingly, the relationship followed a diminishing-returns pattern, thereby suggesting a balancing feedback mechanism, potentially based on B-induced stomatal closure. This analytical link between irrigation B and plant accumulation offers a framework for managing B toxicity, pending field validation.

Water scarcity and climate variability threaten citrus production in semi-arid regions, requiring strategies to improve drought resilience. This study evaluated the physiological and hormonal responses of two citrus cultivars, alemow (Citrus macrophylla Wester) and ‘Cleopatra’ mandarin (Citrus reshni Hort. Ex Tanaka), inoculated with arbuscular mycorrhizal (AM) fungi (Rhizophagus irregularis + Funneliformis mosseae) and subjected to drought stress imposed by progressive soil drying (water withholding) and quantified by volumetric soil water content (θ_v) classes: >0.20 cm³ cm⁻³ (well-watered), 0.05–0.20 cm³ cm⁻³ (moderate drought), and 3 cm⁻³ (severe drought). Gas exchange, plant water status, and abscisic acid (ABA) dynamics were monitored to assess cultivar-specific effects of AM symbiosis. Under well-watered conditions, +AM plants exhibited higher photosynthetic rates than non-inoculated plants, with a stronger response in Macrophylla. During drought, contrasting patterns emerged: +AM Macrophylla maintained higher stomatal conductance and photosynthesis, with foliar ABA increasing only under severe stress, suggesting that non-hormonal mechanisms support gas exchange. In Cleopatra, AM inoculation was associated with higher root-derived ABA and earlier stomatal closure, suggesting a more conservative water-use strategy under soil drying conditions; however, the benefits were limited to moderate stress and decreased beyond a stomatal conductance threshold. These findings reveal that AM symbiosis enhances drought resilience through contrasting mechanisms: hydraulic stabilization predominates in Macrophylla, whereas hormonal (ABA-mediated) regulation drives the response in Cleopatra. This cultivar-dependent modulation highlights the importance of developing AM-based strategies adapted to each cultivar for effective citrus drought management. Combining AM inoculation with irrigation-saving practices could improve water productivity and support climate-smart citrus production.

Oxyfertigation with hydrogen peroxide (H₂O₂) has been successfully applied in several crops and production systems, but its use in mature citrus orchards under no-tillage conditions and semi-arid Mediterranean environments remains scarcely studied. This study aimed to evaluate the physiological responses of adult citrus trees and the agronomic performance of a mature citrus orchard subjected to chemical oxyfertigation based on the application of H₂O₂ in irrigation water as an oxygen source for the root zone. The experiment was conducted over four consecutive seasons (2018–2021) on adult ‘Ortanique’ hybrid mandarin trees grown in an orchard located in Torre Pacheco (Murcia, Spain). Two treatments were established: a ‘Control’ (0 mg L⁻¹ of H₂O₂) and an ‘OXY’ treatment (50–100 mg L⁻¹ of H₂O₂ applied throughout the growing season). Oxyfertigation significantly increased the dissolved oxygen in irrigation water and soil oxygen diffusion rate, with treatment and treatment × time effects showing greater oxygenation under conditions favoring transient root-zone hypoxia. Soil CO₂ and H₂O vapor fluxes exhibited marked seasonal dynamics but no consistent treatment effect, and soil salinity and macro- and micronutrient contents were not significantly altered. At the plant level, oxyfertigation episodically enhanced leaf gas exchange and transiently improved the water status, but did not produce a sustained increase in leaf-level water use efficiency. In contrast, OXY trees showed greater pruning biomass, more fruits (+18%), higher cumulative yield (+13%), and significantly higher crop water use efficiency (YWUE) while the mean fruit weight and most quality attributes were governed by interannual climatic variability. In summary, oxyfertigation acted as a complementary and safe agronomic practice that improved rhizosphere oxygenation and supported modest gains in fruit load and YWUE in mature citrus orchards.

Avocado trees (Persea americana Mill.) grown in Mediterranean conditions are exposed to high temperatures and intense solar radiation during summer, factors that can severely compromise plant water status and key physiological processes. To minimize these stressful conditions, the use of shade nets is an agronomical technique that permits the creation of an optimal microclimate for crop development. Thus, the aim was to evaluate the effects of shade netting on the physiological response of young avocado trees commercially grown under Mediterranean climatic conditions. The main results showed similar circadian rhythms of plant water status under both crop systems (open-air and shaded) in both seasons. However, the use of shading nets altered the circadian rhythm of leaf gas exchange. In summer, stomatal conductance (g_s) remained significantly more open after midday in shaded trees, allowing higher leaf transpiration (E_leaf) and cooler leaf temperature (T_leaf). A similar daily pattern was observed in chlorophyll a fluorescence parameters, including the effective quantum yield of photosystem II (ΦPSII) and the electron transport rate (ETR), with the lowest values occurring at midday. In shaded plants, ΦPSII and ETR remained higher after midday than in open-air, suggesting a lower photochemical inhibition of photosynthesis caused by heat stress and photoinhibition. Thus, the use of shade nets represents an agronomic alternative technique for cultivating avocados in Mediterranean climate conditions.

Drought stress, which is one of the most critical environmental constraints affecting global crop productivity, is exacerbated by climate change and increased atmospheric water demand. This review comprehensively examines plant responses to drought, integrating physiological, morphological, biochemical, and genetic adaptations that contribute to water‐use efficiency and stress tolerance. Key mechanisms such as osmotic adjustment, stomatal regulation, antioxidant defense, and hormonal signaling are analyzed, highlighting their role in mitigating drought‐induced cellular damage. Advances in plant breeding and biotechnological approaches, including transgenic strategies, genome editing, and marker‐assisted selection, are discussed in the context of improving drought resilience. The importance of root system architecture, leaf anatomical modifications, and stress‐responsive transcription factors is underscored as essential components of drought adaptation. Additionally, agronomic innovations such as precision irrigation, soil management techniques, and plant‐microbe interactions are reviewed due to their potential to enhance sustainable water use in agriculture. The role of epigenetic modifications and long‐distance signaling mechanisms in drought acclimation is explored, shedding light on emerging strategies for engineering multi‐stress tolerant crops. Furthermore, we assess the impact of drought on crop nutritional quality, the trade‐offs between drought tolerance and pest resistance, and the socio‐economic implications of water scarcity on global food security. This review provides a roadmap for integrating cutting‐edge scientific knowledge with practical agricultural applications, aiming to develop resilient cropping systems capable of sustaining productivity under increasingly unpredictable climatic conditions.