Dimitrios Savvas

Recycling drainage solutions (DS) in closed-loop soilless culture systems reduces fertilizer and water consumption in greenhouse production while preventing environmental pollution caused by nitrogen and phosphorus emissions. However, recycling DS increases the risk of spreading root- infective pathogens through the irrigation system, making disinfection necessary prior to reuse. In this study, two oxygen-containing oxidizing agents, hydrogen peroxide (H₂O₂) and chlorine dioxide (ClO₂) were evaluated as nutrient solution (NS) disinfectants against Verticillium dahliae. The experiment was conducted in a closed-loop soilless pepper crop grown in a greenhouse during the spring–summer period. All treatments received nutrient solutions (NS) of identical composition. Three treatments were applied with three replicates each: (i) non-treated NS (control), (ii) NS supplemented with 2 ppm ClO₂, and (iii) NS supplemented with 2 ppm H₂O₂. Six-week-old pepper plants were inoculated with V. dahlia strain 402V. Both disinfectants significantly improved pepper fruit yield and quality compared with the non-disinfected control. ClO₂ resulted in the highest number of marketable fruits and total fruit yield, while both ClO₂ and H₂O₂ reduced unmarketable fruits and increased average fruit weight. Pathogen biomass quantification by qPCR demonstrated that ClO₂ significantly reduced pathogen presence in the hydroponic system, achieving approximately 63% lower biomass compared with the untreated control. In contrast, H₂O₂ showed limited efficacy and pathogen levels comparable to the control treatment. Application of H₂O₂ and ClO₂ significantly enhanced mineral accumulation in pepper tissues, increasing macronutrient (P, K, Mg, Ca) and micronutrient (Fe, Zn, Cu) concentrations compared with the control treatment.

Tropospheric ozone (O ₃ ) is an air pollutant and phytotoxic stressor that impairs plant physiology and causes chlorotic and necrotic lesions on leaves. While shoot responses are well characterized, the role of root in plant tolerance to O ₃ is not elucidated yet. Here, the O ₃ -sensitive Pinto common bean cultivar (PI) was reciprocally grafted onto a common bean O ₃ -tolerant genotype (R) or the Greek landrace Zargana Kavalas, (ZK) to investigate rootstock effects on plant O ₃ responses. Self-grafted (PI/PI, R/R, ZK/ZK), non-grafted (PI, R, ZK), and hetero-grafted (PI/R, PI/ZK, R/PI, ZK/PI) plants were grown in separate walk-in chambers and subjected to either charcoal-filtered air (CF-air) or O ₃ -enriched air (eO ₃ ; 150 ppb × 7 days × 8 h day ⁻¹ ). Under eO ₃ treatment, both self-grafted and non-grafted PI plants exhibited ~ 33% visible injury, decreased photosynthetic assimilation rate ( A ), stomatal conductance (g _s ), and CO ₂ use efficiency (CUE), reduced photosynthetic pigment content, and elevated oxidative stress indicators (malondialdehyde content, proline, total phenolics, chlorophyll fluorescence). In contrast, grafted sensitive plants onto tolerant rootstocks showed limited visible injuries (< 2%) and maintained more stable physiological and biochemical traits. These findings suggest that plant sensitivity/tolerance to O ₃ is root-mediated, and support grafting as a promising strategy to improve plant tolerance to O ₃ stress.

Golden thistle (Scolymus hispanicus L.) is a wild edible green of high nutritional value, used in the traditional Mediterranean diet. Nowadays, there is an increasing demand from consumers for golden thistle and concomitantly an increasing interest in integrating it into modern cultivation systems. Soilless culture is a promising cultivation option that can maximize yield and quality of golden thistle. The aim of this study was to examine the combined effect of electrical conductivity (EC) and nitrogen (N) supply level on growth and nutritional quality of golden thistle grown on a substrate in a soilless cropping system. The two experimental factors were examined in a 2-factorial experiment with two EC levels, a low (2.2 dS m⁻¹) and high (2.8 dS m⁻¹), combined with two total-N (NO₃⁻ + NH₄⁺) supply levels, low (13.30 mmol L⁻¹) and high (17.30 mmoL L⁻¹), in the supplied nutrient solution. Root fresh and dry weight (commercial yield) were unaffected by treatments; however, high EC significantly reduced shoot fresh and dry biomass by 21 and 28% compared to low EC. High EC increased K⁺ concentrations in shoots and roots but decreased shoot Ca²⁺ level. Nitrate concentration in the drainage solution and plant tissues was primarily driven by N supply, with high N increasing leaf NO₃⁻ by up to 45% without surpassing the regulatory safety limit. Water productivity did not differ among treatments, but low EC improved agronomic efficiency of K⁺, Ca²⁺, Mg²⁺, and S, while low N enhanced N agronomic efficiency by 44%. Overall, low EC promoted vegetative growth and nutrient use efficiency, while increasing N above 13.3 mmol L⁻¹ offered no yield benefit and raised tissue nitrate levels. For optimal yield and quality, a nutrient solution with low EC and N supply is recommended for the soilless cultivation of golden thistle.

This study examined the effects of both nitrogen (N) rate and form on the growth, nutrient uptake, and quality parameters of hydroponically grown purslane (Portulaca oleracea L.) during a spring cultivation cycle. Purslane was cultivated in a floating hydroponic system under either adequate or limiting N conditions. More specifically, under adequate N conditions, plants were supplied with NS where ammonium nitrogen (NH₄-N) accounted for either 7% (Nr7) or 14% (Nr14) of the total-N. The limiting N conditions were achieved through the application of either an NS where 30% of N inputs were compensated with Cl (N30), or an NS where 50% of N inputs were balanced by elevating Cl and S by 30% and 20%, respectively (N50). The results demonstrated that mild N stress enhanced the quality characteristics of purslane without significant yield losses. However, further and more severe N restrictions in the NS resulted in significant yield losses without improving product quality. The highest yield reduction (20%) occurred under high NH₄-N supply (Nr14), compared to Nr7-treated plants, which was strongly associated with impaired N assimilation and reduced biomass production. Both N-limiting treatments (N30 and N50) effectively reduced nitrate accumulation in edible tissues by 10% compared to plants grown under adequate N supply (Nr7 and Nr14); however, nitrate levels remained relatively high across all treatments, even though the environmental conditions of the experiment favored nitrate reduction. All applied N regimes and compensation strategies improved the antioxidant and flavonoid content, with the highest antioxidant activity observed in plants grown under high NH₄-N application, indirectly revealing the susceptibility of purslane to NH₄-N-rich conditions. Overall, the form and rate of N supply significantly influenced both plant performance and biochemical quality. Partial replacement of N with Cl (N30) emerged as the most promising strategy, benefiting quality traits and effectively reducing nitrate content without significantly compromising yield.

Drought stress is a critical abiotic constraint on agricultural productivity, particularly affecting crops like pepper (Capsicum annuum L.), which are highly susceptible to water deficits due to their physiological characteristics. The present study investigated the impact of a 40% reduction in irrigation on yield, macronutrient concentrations, and fruit quality across several pepper genotypes. The cultivars evaluated included two landraces, namely ‘JO109’ and ‘JO204’ (Capsicum annuum var. grossum), as well as the California cultivar ‘Yolo Wonder’ and the commercial F1 hybrid ‘Sammy RZ’, which served as controls. The experiment was conducted at the greenhouse facilities of the Laboratory of Vegetable Production, Agricultural University of Athens. Under reduced irrigation, most of the cultivars studied exhibited a decline in yield, which was attributed to a decrease in fruit number in ‘Yolo Wonder’ and a reduction in fruit weight in both ‘JO204’ and ‘Sammy’. In contrast, the landrace ‘JO109’ exhibited consistent yields under both growing conditions, a response likely attributed to elevated K concentration in the leaves and lower Na accumulation in the fruit, indicating enhanced tolerance to water deficit. A decline in leaf K concentration was observed in response to drought stress, while concomitantly increased concentrations of Na, Ca and Mg were recorded. Among fruit macronutrients, only Ca showed a significant decrease under reduced irrigation. Furthermore, fruit firmness (FF), titratable acidity (TA) and total soluble solids content (TSSC) exhibited higher levels under drought stress, particularly in ‘JO109’, while TA remained unaltered. These findings highlight the potential of landraces such as ‘JO109’ to be utilized in breeding programs aimed at enhancing resilience, while maintaining pepper fruit quality under limited water availability.

This study investigates the effects of nitrogen fertilization, irrigation, and biostimulant application on the growth and nutrient composition of Cichorium spinosum L. The experimental design included two nitrogen rates (NR100 and NR30, 100% and 30% of plant requirements), two irrigation levels (WA100 and WA50, 100% and 50% of water availability,), and foliar application of a nitrogen-rich biostimulant (BS and NoBS, biostimulated or not biostimulated). In comparison to NR100, NR30 reduced agronomical parameters leaf number, leaf area, leaf fresh, and dry weight by 13.53%, 24.93%, 20.76%, and 15.00%, respectively, whereas dry matter content was increased by 7.64%. WA50 also resulted in reduction in the agronomical characteristics by 8.62%, 7.19%, 5.53%, and 5.26, respectively, whereas the dry matter content was not affected. BS positively affected the agronomical characteristics by 7.49%, 8.01%, 7.18%, and 5.56, respectively, whereas the dry matter content was not affected. The effects of nitrogen rates and water availability suggest the more pronounced impact of nitrogen compared to water stress on the agronomical characteristics. Biostimulant application partially mitigated the effects of NR30 but was ineffective against WA50. The nutrient content of the leaves was also affected. NR30 reduced leaf nitrate, calcium, and zinc content, but increased iron, manganese, and copper concentrations. WA50 altered magnesium and zinc levels: it increased the former and decreased the latter. The interaction between nitrogen and water stress notably affected the plants’ calcium content, which was higher under the NR100 x WA50 treatment. These findings provide significant insights into the perlite-based cultivation of C. spinosum L., and its resilience against drought stress. Moreover, the beneficial effects of sufficient nitrogen rates on leaf fresh weight of Cichorium spinosum L. outline the importance for improving nutrient solution management schemes. Biostimulant application demonstrated promising results and could, after further research, become a viable solution for maintaining optimal yields under nitrogen stress.