Qualitative and Quantitative Plant Screening Measurements for Yield and Quality Enhancement

Climate variability limits resources, and the growing demand for high-yielding, high-quality crops is increasingly challenging global agricultural production [1]. In this context, plant phenotyping has provided a key link between genotype and phenotype. Phenotyping processes enable researchers, especially breeders, to identify plant characteristics associated with productivity, resistance, and quality, which are the metrics of successful production [2].

Technological progress is evident in the introduction of noninvasive, rapid phenotyping methods that enable the continuous monitoring of plant physiological responses without damaging plant tissue. Recent research has incorporated techniques such as chlorophyll fluorescence, infrared thermal imaging, and visible/near-infrared (VIS/NIR) reflectance.

At the photosynthetic level, the JIP test and other analyses based on chlorophyll fluorescence have become essential tools in assessing the performance of the photosynthetic apparatus under stress conditions (including water content, temperature extremes, and nutrient deficiency) [3]. These techniques have transformed the way in which researchers assess plant vitality, photosynthetic activity, and stress responses in real time, enabling traditional plant screening methods to evolve into multidimensional, data-rich processes. However, there is a significant lack of knowledge about the association between these physiological measurements and agronomic performance, particularly yield and product quality. While numerous studies have demonstrated the sensitivity of fluorescence parameters to stress [4,5,6,7,8], fewer have quantitatively linked these parameters to crop productivity and the economic relevance of stress tolerance in breeding programmes. Such approaches have become essential tools in germplasm screening and improving selection efficiency in breeding programmes.

The work reported on in this Special Issue has made significant contributions to our understanding of this topic.

Contribution 1: The study by Šrajer Gajdošik et al., “Zinc and Selenium Biofortification Modulates Photosynthetic Performance: A Screening of Four Brassica Microgreens”, provides insights into how micronutrient supplementation affects photosynthetic mechanisms and stress tolerance across four Brassica species. Analysis of chlorophyll fluorescence showed that selenium application increases photosynthetic efficiency (on average by 8–26%), especially in pak choi (+62%). At the same time, it improves primary photochemistry and energy transfer between photosystems. Additionally, high photosystem connectivity and efficient cyclic electron flow were observed in both kale and kohlrabi, indicating a greater acclimatisation potential under selenium treatment. In contrast, exposure to zinc led to more variable and often inhibitory effects on photosynthetic performance, especially at lower and moderate concentrations, when broccoli proved the most sensitive species. These results highlight the dual role of zinc as an essential but potentially toxic element and confirm its stimulatory effect on photosynthetic resistance.

Contribution 2: The study by Vukadinović et al., “Comparing Chlorophyll Fluorescence and Hyperspectral Indices in Drought-Stressed Young Plants in a Maize Diversity Panel”, provides insights into the responses of 165 inbred maize lines to drought stress. Using chlorophyll fluorescence and hyperspectral imaging at three consecutive time points, the authors showed that hyperspectral imaging indices were consistently associated with overall plant vigour throughout the experiment. In contrast, during the more intense phases of drought, chlorophyll fluorescence parameters became especially useful for distinguishing how different genotypes regulated photosynthesis and reacted to stress at an early stage of development. The research established that chlorophyll fluorescence and hyperspectral imaging indices are complementary and not redundant. Together, they enable more precise detection of physiological responses and stress tolerance traits.

Contribution 3: The study by Mihaljević et al., “Characterisation of Heat Tolerance in Two Apple Rootstocks Using Chlorophyll Fluorescence as a Screening Method”, offers valuable insight into the physiological mechanisms that underlie heat tolerance in apple rootstocks. By combining chlorophyll fluorescence induction kinetics (the JIP test) with biochemical analyses such as lipid peroxidation, hydrogen peroxide accumulation, and the assessment of phenolic and flavonoid compounds, this study revealed clear differences in how the two genotypes responded to heat stress. The M.9 rootstock showed greater stability of photosystem II. It maintained higher photosynthetic efficiency, accompanied by elevated levels of protective pigments and osmolytes, suggesting a stronger ability to acclimate to elevated temperatures. In contrast, the G.210 rootstock exhibited higher oxidative stress, pointing to its lower tolerance to heat.

Contribution 4: The study by Varga et al., “Industrial Hemp Finola Variety Photosynthetic, Morphometric, Biomechanical, and Yield Responses to K Fertilization Across Different Growth Stages”, examined how two potassium fertilisers, KCl and K₂SO₄, influence the photosynthesis, stem traits and yield of Finola hemp. Fertilisers were applied before sowing. Plants were monitored during flowering and ripening using chlorophyll fluorescence (the JIP test) along with morphometric and biomechanical measurements. The type of fertiliser did not significantly affect stem height, diameter, or yield, but fluorescence parameters varied noticeably between growth stages. K₂SO₄-treated plants had slightly stronger stems than those treated with KCl. Overall, both fertilisers proved suitable for dual-purpose hemp production, and KCl appears to be a cost-effective option for crops tolerant to chloride. Additional studies using higher potassium doses could help clarify how K availability impacts photosynthetic efficiency in hemp.

Contribution 5: The study by Matoša Kočar et al., “Soybean Genotype-Specific Cold Stress and Priming Responses: Chlorophyll a Fluorescence and Pigment-Related Spectral Reflectance Indices as Tools for Breeding”, investigated the cold tolerance of 12 soybean cultivars at the early stage of soybean development under controlled conditions. The effects of low temperatures on biomass reduction and photosynthetic efficiency have been proven. On the other hand, the application of priming during cold stress did not have a positive effect. Several parameters of chlorophyll fluorescence and spectral reflectance (ET₀/TR₀, RE₀/RC, TR₀/DI₀, F_m, F_v, ARI₁, and ARI₂) helpful in detecting cold stress and variability in soybean cultivars were determined. The strong correlations found between reflectance and fluorescence properties indicate that reflectance could serve as a simple, noninvasive alternative for stress detection. The discovery of varietal differences in cold tolerance in this research provides practical guidelines for breeding more cold-tolerant strains.

Contribution 6: The study by Abdelrady et al., “Evaluating Physiological and Yield Indices of Egyptian Barley Cultivars Under Drought Stress Conditions”, examined the responses of ten barley cultivars to a water deficit over two growing seasons. Drought stress significantly reduced most of the measured traits, although the extent of the reduction differed among cultivars. More tolerant cultivars achieved higher yields, chlorophyll content, and physiological stability under stress. In contrast, susceptible cultivars showed a noticeable decline in yield and growth traits. These results highlight the potential of tolerant cultivars as valuable breeding material for the development of drought-tolerant barley cultivars suitable for arid and semi-arid conditions.

Contribution 7: The study by Imran et al., “Exogenously Applied Sodium Nitroprusside Alleviated Cadmium Toxicity in Different Aromatic Rice Cultivars by Improving Nitric Oxide Accumulation and Modulating Oxidative Metabolism”, provides important insights into the physiological mechanisms that enable plants to better adapt to heavy metal stress. Using sodium nitroprusside (SNP) as a source of nitric oxide, the research shows that the negative effects of cadmium can be mitigated by stimulating antioxidant enzymes, more stable photosynthetic activity, and reduced metal accumulation in plant tissues. A moderate dose of SNP proved to be particularly effective, contributing to growth restoration, preservation of photosynthetic function, and maintenance of grain yield and quality, especially in the Guixiangzhan cultivar. These results link the physiological and biochemical responses of plants to stress with practical approaches in agriculture, highlighting the role of nitric oxide signalling in creating cultivars more resistant to stress in polluted soil conditions.

Contribution 8: The study by Mohamed et al., “Utilising Infrared Thermometry to Assess the Crop Water Stress Index of Wheat Genotypes in Arid Regions under Varying Irrigation Regimes”, explores the application of infrared thermometry as a simple and reliable tool for monitoring wheat water status and for more efficient irrigation management. By analysing the relationship between the canopy–air temperature difference and the vapour pressure deficit, the authors developed reliable models for calculating the crop water stress index (CWSI) under different water supply conditions. The results showed a strong inverse relationship between CWSI and yield. Genotypes with lower CWSI values achieve higher yield and better water status. Additionally, more sensitive genotypes respond more strongly to increasing vapour pressure deficit. The results of this study confirm that CWSI can be used as a practical tool for assessing irrigation needs and predicting yield, especially in arid regions where efficient water management is crucial.

Contribution 9: The study by Duvnjak et al., “Effects of Drought at Anthesis on Flag Leaf Physiology and Gene Expression in Diverse Wheat (Triticum aestivum L.) Genotypes”, analyses how drought during flowering affects physiological and molecular responses in wheat. By comparing genotypes with different resistance, clear differences were found in the ability to maintain photosynthetic activity and activate antioxidant defence mechanisms. Sensitive genotypes showed greater chlorophyll loss and more pronounced oxidative stress, while tolerant genotypes maintained higher levels of carotenoids and earlier activated enzymes involved in the neutralisation of reactive oxygen species. Gene expression analysis confirmed that tolerant genotypes up-regulated genes such as DHN5 and WZY2, which are associated with yield stability under stress. The results indicate that drought resistance in wheat largely depends on the speed and efficiency of antioxidant and protective reactions in the flag leaf.

Contribution 10: The study by Colpo et al., “Drought-Stressed Apple Tree Grafted onto Different Rootstocks in a Coastal Sandy Soil: Link between Fast Chlorophyll a Fluorescence and Production Yield”, investigated how different apple rootstocks respond to drought and how these responses are reflected in yield. By combining field measurements of fast chlorophyll fluorescence and yield analysis, it was shown that even a moderate water deficit can reduce the efficiency of photosystem II and the energy conservation capacity of leaves. Parameters such as F_v/F_m and PI_ABS showed a strong correlation with the number and weight of fruits, indicating their value as early indicators of drought-induced productivity reduction. Rootstocks differed in sensitivity; those with stronger growth maintained better photosynthetic activity and fruit quality under stress. These results confirm the importance of using chlorophyll fluorescence in assessing rootstock resistance and optimising apple cultivation in water-limited soils.

The review of papers published in this Special Issue demonstrates the importance of combining physiological, biochemical, and molecular measurements to understand crop tolerance to stresses such as drought, low temperatures, and metal toxicity. Noninvasive methods such as chlorophyll fluorescence and water stress indices allow the early detection of stress and yield prediction, while molecular and antioxidant markers help distinguish tolerant genotypes from sensitive ones. Applying physiological insights in practice, through treatments such as foliar SNP or the selection of tolerant rootstocks, together with genotype selection and irrigation optimisation, can preserve yields and fruit quality under stress. These approaches highlight how linking physiological, agronomic, and molecular data supports the development of more stress-resistant crops and more efficient resource management under climate change [9,10].