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Waterlogging and drought have become major challenges in many regions worldwide. Under water stress, plants exhibit a range of physiological and electrical responses, including changes measurable by electrical impedance spectroscopy (EIS). Monitoring these parameters can provide valuable insights into plant growth status under adverse conditions. This study investigated changes in relative chlorophyll content (SPAD), maximum photochemical efficiency (Fv/Fm), relative water content (RWC), non-structural carbohydrates (NSC), and EIS parameters in apple rootstocks subjected to different water stress treatments. Results indicated that all physiological indicators, except NSC, showed a declining trend under two water stress episodes. Critically, the initial water stress episode elicited significantly greater physiological disruption than its subsequent counterpart. This suggests that plants developed a degree of physiological adaptation—such as osmotic adjustment and enhanced antioxidant activity—reducing their sensitivity to subsequent stress. Correlation analysis revealed that high-frequency resistivity (r) and intracellular resistivity (ri) were strongly associated with key physiological parameters. Thus, r and ri may serve as effective indicators for assessing plant water stress status. Furthermore, classification algorithms—Fuzzy K-Nearest Neighbors (FKNN) and sparse Linear Discriminant Analysis (sLDA)—were applied to distinguish water status in apple rootstocks, achieving high classification accuracy. These findings provide a theoretical basis for improved water management in apple cultivation.

As a result of the continuous global warming in recent years, the average annual number of rain days in China has been on the decline, while the number of rainstorm days has gradually increased. These conditions make it extremely easy to form a waterlogging environment, which has an adverse impact on plant growth and development. In many apple-producing areas in China, apples are subject to severe flooding during planting. In this study, two-year-old apple rootstock M9T337 was used to explore the effects of interval water stress on the morphological and physiological parameters of apple leaves. The purpose was to determine the plant’s adaptability to waterlogged environments and provide theoretical reference for management and maintenance after waterlogging. The results showed that the effect on flooded (T2) on apple stock was greater than that of waterlogged (T1), Short-term (7 d) waterlogging (T1) did not affect the growth of seedlings but was conducive to the accumulation of dry matter. Furthermore, the initial stress was be imprinted on the plants, which could directly affect their response to later stress. The results of principal component analysis (PCA) revealed that PC1, PC2, and PC3 explained 26.92%, 17.46%, and 13.03% of the physiological changes under water stress, respectively. By calculating the weight of each indicator, we concluded that high-frequency resistance r, relative chlorophyll content (SPAD) and maximum photochemical efficiency Fv/Fm are important parameters for apple rootstocks affected by water stress.

Apple trees are frequently subjected to varying degrees of salt stress. HYL1, a key protein involved in miRNA biosynthesis, has been shown to play critical roles in plant responses to cold, drought, and pathogen infection. However, the specific function of MdHYL1 in mediating salt-alkali stress tolerance in apple remains unknown. In this study, we demonstrated that overexpression of MdHYL1 in M9-T337 rootstocks significantly enhanced salt-alkali stress tolerance, including improved growth performance, reduced Na⁺/K⁺ ratio, decreased membrane damage, enhanced photosynthetic, and antioxidant capacity, which significantly impairs their growth, fruit quality, and yield. Moreover, scions grafted onto MdHYL1 OE rootstocks displayed superior saline-alkali stress tolerance compared to those grafted onto M9-T337 rootstocks. Taken together, our findings highlight MdHYL1 as a promising candidate gene for improving saline-alkali stress tolerance in fruit trees through biotechnological approaches.

Cadmium (Cd) is a nonessential and highly toxic element causing agricultural problems. However, little information is available about the variation in Cd tolerance among apple rootstocks and its underlying physiological regulation mechanisms. This study investigated Cd accumulation, subcellular distribution, and chemical forms as well as physiological changes among four apple rootstocks exposed to either 0 or 300 μM CdCl . The results showed that variations in Cd tolerance existed among these rootstocks. Cd exposure caused decline in photosynthesis, chlorophyll and biomass in four apple rootstocks, which was less pronounced in , indicating its higher Cd tolerance. This finding was corroborated with higher Cd tolerance indexes (TIs) of the whole plant in than those in the other three apple rootstocks. Among the four apple rootstocks, displayed the lowest Cd concentrations in roots, wood, and leaves, the smallest total Cd amounts as well as the lowest BCF. In apple rootstocks, it was found that to immobilize Cd in cell wall and soluble fraction (most likely in vacuole) and to convert it into pectate- or protein- integrated forms and undissolved Cd phosphate forms may be the primary strategies to reduce Cd mobility and toxicity. The physiological changes including ROS, carbohydrates and antioxidants were in line with the variations of Cd tolerance among four apple rootstocks. In comparison with the other three apple rootstocks, had lower concentrations of ROS in roots and bark, H O in roots and leaves and MDA in roots, wood and bark, but higher concentrations of soluble sugars in bark and starch in roots and leaves, and enhanced antioxidants. These results indicate that are more tolerant to Cd stress than the other three apple rootstocks under the current experiment conditions, which is probably related to Cd accumulation, subcellular partitioning and chemical forms of Cd and well-coordinated antioxidant defense mechanisms.

Background Drought is considered the main environmental factor restricting apple production and thus the development of the apple industry. Rootstocks play an important role in enhancing the drought tolerance of apple plants. Studies of the physiology have demonstrated that ‘ZC9-3’ is a strong drought-resistant rootstock, whereas ‘Jizhen-2’ is a weak drought-resistant rootstock. However, the metabolites in these two apple rootstock varieties that respond to drought stress have not yet been characterized, and the molecular mechanisms underlying their responses to drought stress remain unclear. Results In this study, the physiological and molecular mechanisms underlying differences in the drought resistance of ‘Jizhen-2’ (drought-sensitive) and ‘ZC9-3’ (drought-resistant) apple rootstocks were explored. Under drought stress, the relative water content of the leaves was maintained at higher levels in ‘ZC9-3’ than in ‘Jizhen-2’, and the photosynthetic, antioxidant, and osmoregulatory capacities of ‘ZC9-3’ were stronger than those of ‘Jizhen-2’. Metabolome analysis revealed a total of 95 and 156 differentially accumulated metabolites in ‘Jizhen-2’ and ‘ZC9-3’ under drought stress, respectively. The up-regulated metabolites in the two cultivars were mainly amino acids and derivatives. Transcriptome analysis revealed that there were more differentially expressed genes and transcription factors in ‘ZC9-3’ than in ‘Jizhen-2’ throughout the drought treatment. Metabolomic and transcriptomic analysis revealed that amino acid biosynthesis pathways play key roles in mediating drought resistance in apple rootstocks. A total of 13 metabolites, including L-α-aminoadipate, L-homoserine, L-threonine, L-isoleucine, L-valine, L-leucine, (2 S )-2-isopropylmalate, anthranilate, L-tryptophan, L-phenylalanine, L-tyrosine, L-glutamate, and L-proline, play an important role in the difference in drought resistance between ‘ZC9-3’ and ‘Jizhen-2’. In addition, 13 genes encoding O -acetylserine-(thiol)-lyase, S -adenosylmethionine synthetase, ketol-acid isomeroreductase, dihydroxyacid dehydratase, isopropylmalate isomerase, branched-chain aminotransferase, pyruvate kinase, 3-dehydroquinate dehydratase/shikimate 5-dehydrogenase, N -acetylglutamate-5-P-reductase, and pyrroline-5-carboxylate synthetase positively regulate the response of ‘ZC9-3’ to drought stress. Conclusions This study enhances our understanding of the response of apple rootstocks to drought stress at the physiological, metabolic, and transcriptional levels and provides key insights that will aid the cultivation of drought-resistant apple rootstock cultivars. Especially, it identifies key metabolites and genes underlying the drought resistance of apple rootstocks.

Rootstocks largely determine the tree architecture of the grafted scions, significantly affects yield, suitability for mechanical harvesting, and planting pattern of apple orchards. It is thus important to reveal the mechanisms behind the rootstocks influence on the tree architecture of scions in apple trees. This study analyzed the grafting survival rate, the physiological parameters including plant growth, photosynthesis and nutrient accumulation in the apple variety ‘Harlikar’ with eight apple rootstocks. We also explored the mechanism of scion architecture formation using transcriptomics based on different scion/rootstock combinations. The results indicated that ‘Harlikar’ had the lowest grafting survival rate with rootstock ‘M26’, with less callus formed at the graft interface, foliage etiolation, and weak photosynthetic capacity. While ‘Harlikar’ had better affinities with ‘M9-T337’, ‘M9-Nic29’, ‘M9-Pajam2’, ‘B9’, ‘71-3-150’, ‘Qingzhen 2’, and ‘Malus baccata’. Among these, the highest plant height and the highest number of lateral branches were observed in ‘Harlikar’ with rootstock ‘Qingzhen 2’, they were 1.12-times and 2.0-times higher than ‘Harlikar’ with vigorous rootstock ‘M. baccata’, respectively. The highest accumulations of total nitrogen, total phosphorus, and total potassium in scions were observed in ‘Harlikar’/‘Qingzhen 2’, they were 2.22-times, 2.10-times, and 11.80-times higher than that in ‘Harlikar’/‘M. baccata’. The lowest plant height was observed in ‘Harlikar’/‘71-3-150’, only 50.47% of ‘Harlikar’/‘Qingzhen 2’ and 56.51% of ‘Harlikar’/‘M. baccata’, and the lowest internode length was observed in ‘Harlikar’/‘M9-Nic29’, only 60.76% of ‘Harlikar’/‘Qingzhen 2’ and 79.11% of ‘Harlikar’/‘M. baccata’. The transcriptome, weighted gene co-expression network and KEGG enrichment analyses revealed that, compared to ‘Harlikar’/‘M. baccata’, most differentially expressed genes screened from ‘Harlikar’/‘Qingzhen 2’, ‘Harlikar’/‘71-3-150’, and ‘Harlikar’/‘M9-Nic29’ were enriched in hormone signal transduction pathways. Specifically, auxin-repressed protein gene ARP, cytokinin synthesis related genes CKXs and CYP92A6, and brassinosteroid synthesis related gene CYP87A3 were involved in the dwarfing of ‘Harlikar’/‘71-3-150’ and ‘Harlikar’/‘M9-Nic29’. Cytokinin synthesis related gene ARR-A and abscisic acid-responsive element binding factor gene ABF were the key to increased branching in ‘Harlikar’/‘Qingzhen 2’. In addition, acid phosphatase genes ACPs, and serine/threonine-protein kinase genes PBLs were involved in the vegetative growth of scions in ‘Harlikar’/‘Qingzhen 2’ by affecting the absorption and utilization of nutrients. These results provide theoretical guidance for cultivating high-quality ‘Harlikar’ apple trees and elucidate the molecular mechanisms regulating plant height and lateral branch formation in apple.

Soil salinization is a major abiotic stress in apple production; consequently, it is important to improve the salt tolerance of apple trees. Here, a physiological analysis and isobaric tags for relative and absolute quantitation-based proteomics were combined, with a particular interest in proteomic changes in seedling leaves of the apple rootstock Malus halliana induced by a mixed salt–alkali (100 mM NaCl + NaHCO3) stress. The salt–alkali stress produced significant stress symptoms with time, including decreased chlorophyll and leaf water contents. Using proteomics, 179 differentially expressed salt–alkali-responsive proteins were identified. These proteins were involved in the TCA cycle, glycolysis, oxidative phosphorylation, pentose phosphate pathway, starch and sucrose metabolism, and amino sugar and nucleotide sugar metabolism, revealing that M. halliana likely regulated carbohydrate metabolism to satisfy energy and substrate needs. Additionally, Photosystem I and II proteins were up-regulated, enhancing the photosynthetic capacity. The related catalase, superoxide dismutase and peroxidase proteins facilitated the detoxification of reactive oxygen species and maintained redox homeostasis. Auxin, salicylic acid and abscisic acid promoted cell enlargement, resistance and stomatal closure. Additionally, the plants were better adapted to stress owing to enhanced protein processing and synthesis, as well as accelerated protein replacement. This study provides new information on the molecular networks of energy metabolism and stress regulation that can be used to develop salt–alkali-tolerant apple rootstocks.

In apple (Malus) cultivation, the adaptability of rootstocks has a fundamental influence on multiple abiotic-stress resistance properties of their scions. Here, we investigated the high-salt resistance of apple rootstocks, by treating 2-year-old apple trees consisting of the ‘Fuji’ cultivar grafted onto three different dwarfing rootstocks (SH6, M9-T337 and G935) with a high-salt solution for 30 days. We found that the high-salt stress caused leaf yellowing and growth inhibition to all apple plants, and that the most serious reduction in relative chlorophyll content, plant height and biomass accumulation was associated with the SH6 rootstock. In addition, based on the salt injury index, the high-salt resistance of the ‘Fuji’ scion on the three dwarfing rootstocks was ranked as G935 > M9-T337 > SH6. In addition, analysis of mineral elemental composition showed that the Na+ content in ‘Fuji’ leaves on the G935 rootstock increased less than twofold compared to the control, whereas those on the SH6 rootstock increased nearly 20-fold, accompanied with reduced K and Fe element absorption under high-salt treatments. Expression analysis of nine Na+ transport-related genes indicated that in the salt overly sensitive (SOS) pathway, the SOS1, SOS2 and SOS3 genes showed considerably greater induction by high-salt stress in G935 roots than in SH6 roots. Given the Na+ content detected in roots of different rootstocks, we propose that more efficient Na+ extracellular efflux in G935 roots, rather than intracellular Na+ compartmentalization, contributed to the improved characteristics of the G935 rootstock under high-salt conditions.

Apple (Malus × domestica Borkh.) is one of the most cultivated fruit crops in China. Apple trees frequently encounter waterlogging stress, mainly due to excess rainfall, soil compaction, or poor soil drainage, results in yellowing leaves and declined fruit quality and yield in some regions. However, the mechanism underlying the response to waterlogging has not been well elucidated. Therefore, we performed a physiological and transcriptomic analysis to examine the differential responses of two apple rootstocks (waterlogging-tolerant M. hupehensis and waterlogging-sensitive M. toringoides) to waterlogging stress. The results showed that M. toringoides displayed more severe leaf chlorosis during the waterlogging treatment than M. hupehensis. Compared with M. hupehensis, the more severe leaf chlorosis induced by waterlogging stress in M. toringoides was highly correlated with increased electrolyte leakage and superoxide radicals, hydrogen peroxide accumulation, and increased stomata closure. Interestingly, M. toringoides also conveyed a higher ethylene production under waterlogging stress. Furthermore, RNA-seq revealed that a total of 13,913 common differentially expressed genes (DEGs) were differentially regulated between M. hupehensis and M. toringoides under waterlogging stress, especially those DEGs involved in the biosynthesis of flavonoids and hormone signaling. This suggests a possible link of flavonoids and hormone signaling to waterlogging tolerance. Taken together, our data provide the targeted genes for further investigation of the functions, as well as for future molecular breeding of waterlogging-tolerant apple rootstocks.