Polyploidization is an important source of variability for plant breeding. Polyploids are often characterised by increased resistance to biotic and abiotic stresses. Since drought and pathogen attack are the main threats to apple cultivation, obtaining new sources of resistance is an important issue for apple breeding. The newly obtained autotetraploid clones of apple cv. ‘Redchief’ showed superior resistance to fire blight. The aim of the presented research was the in-depth phenotypic characterisation of ‘Redchief’ tetraploids and assessment of their response to drought at the physiological and genetic level. The growth of own-rooted five-year-old trees of ‘Redchief’ tetraploids was poor compared with diploids; all growth parameters—the number and length of current season shoots, the total length of current season shoots per tree and the cross-section area of the trunk—were reduced in tetraploid clones. Grafting on M9 rootstock improved the growth characteristics of ‘Redchief’ tetraploids. Compared with diploid plants, the leaves of tetraploids were thicker, with altered shape, higher chlorophyll content, and larger stomata, but the stomatal density decreased. The leaf anatomical structure of tetraploids was changed, the adaxial and abaxial epidermis and both types of mesophyll were significantly thicker than in diploids. Moreover, the pollen grains of tetraploids were larger, but their viability and germination were reduced. Under conditions of limited water supply, the reduction in growth parameters was smaller and the physiological parameters were higher in the ‘Redchief’ tetraploid clone 4x-25 than in diploid plants. The expression of APX gene was higher in tetraploids than in diploids 15 days after drought stress induction. The results suggest the enhanced drought tolerance of the studied ‘Redchief’ autotetraploid clone compared with its diploid counterpart.

Gooseberry (Ribes grossularia L.) is a small fruit crop producing valuable fruits, which is constantly gaining importance. In vitro propagation of this species can significantly support the production of virus-free planting material and accelerate the introduction of new cultivars to the market. The aim of presented study was to assess field performance and genetic stability of micropropagated plants (MPs) of four gooseberry cultivars, “Captivator”, “Hinnonmaki Rot”, “Invicta”, and “Resika”. The growth vigor and yield of MPs and plants propagated by standard methods from softwood cuttings (ST) were evaluated in a field experiment. Microscopic observations of the number and length of the stomata of MP and ST plants were carried out. Two DNA-based techniques, amplified fragment length polymorphism (AFLP) and inter simple sequence repeat (ISSR), were used to assess genetic stability of MP plants. For analysis of genetic stability of ST plants, the ISSR technique was applied. For three cultivars, Captivator, Hinnonmaki Rot, and Invicta, the plants’ growth vigor and fruit yield were greater in MP plants than in ST plants. In the case of Resika, most of these parameters were higher in ST plants. Microscopic observations of the stomata indicated a lack of differences in the length between MP and ST plants, while the stomata frequency on leaves of MP plants was higher than that of ST plants. The genetic variability of MP plants, assessed by AFLP, ranged from 0.35% for Hinnonmaki Rot to 2.12% for Resika. The results of ISSR analysis of MP plants showed variability from 0% in the case of Hinnonmaki Rot and Resika to 4% and 8.69% for Captivator and Invicta, respectively. No polymorphism was detected among ST plants of all analyzed gooseberry cultivars.

The possibility of inducing resistance to the two-spotted spider mite, Tetranychus urticae Koch, in ‘Gala’ apple trees growing under optimal fertilization or nitrogen-deficiency conditions was investigated. The effects of jasmonic acid (JA) at 1.5 and 2.5 mM, and acibenzolar-S-methyl (benzothiadiazole, BTH) at 0.5 and 1.5 mM, applied separately or together, on the fecundity of T. urticae females in a laboratory test as well as on the population growth of the pest in a greenhouse experiment were determined. The influence of both elicitors on the induction of LOX and PAL gene expression was assessed in a parallel experiment using real-time PCR. Jasmonic acid showed significantly higher effectiveness in inducing apple tree resistance to T. urticae, as compared to BTH. This was particularly evident in the reduction in pest numbers that was observed in the greenhouse experiment and was also confirmed by increased LOX gene expression after treatment with JA. BTH induced the expression of the PAL gene more strongly than jasmonic acid; however, this was not reflected in the performance of the two-spotted spider mite in the laboratory and greenhouse experiments. It was also found that the antagonistic effect of BTH on JA might lead to decreased effectiveness of the jasmonic acid used to induce apple tree resistance to the two-spotted spider mite. Although nitrogen fertilization stimulated the development of spider mite populations, the resistance induction mechanism was more effective in N-fertilized plants, which was especially evident at the higher jasmonic acid concentration.

It is generally believed that plant polyploids exhibit greater tolerance to abiotic stress conditions than their diploid counterparts. The aim of the present research was to investigate the mechanisms underlying enhanced drought tolerance in the autotetraploid apple ‘Redchief’ as compared to its diploid counterpart. The study was conducted on potted plants over two growing seasons, and simulated drought conditions were induced by limiting or withholding irrigation. Under drought stress, the responses of the clone ‘Redchief’ 4x-25 and its diploid counterpart were compared at physiological, biochemical, and molecular levels. In addition, changes in leaf anatomical structure, stomatal characteristics, and parameters related to growth dynamics were examined in drought-challenged plants. The results indicate that apple tetraploids have a greater ability to adapt to water-deficit conditions than diploids. Under drought stress, apple tetraploids exhibited better physiological and biochemical parameters and maintained a greater capacity for continued growth than diploids. We propose that the primary mechanism underlying the increased drought tolerance in apple tetraploids is a faster and more efficient activation of antioxidant defenses and proline accumulation compared to diploids. The high plasticity of anatomical traits in apple tetraploids in response to adverse environmental conditions was also demonstrated.