Apple Autotetraploids—Phenotypic Characterisation and Response to Drought Stress

Danuta Wójcik,Jacek Filipczak,Zbigniew Buler,Katarzyna Wójcik,Krzysztof Klamkowski,Monika Marat,Małgorzata Podwyszyńska,Waldemar Treder,Przemysław Piotr Tomczyk,Agnieszka Marasek-Ciołakowska

doi:10.3390/agronomy12010161

Abstract

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.

Highlights

The apple tree (Malus × domestica Borkh.) is one of the most important fruit crop plants, widely cultivated in the temperate climate zones of North America, Europe, and Asia
Compared to own-rooted diploid trees, all growth parameters were significantly reduced in Malling 9 (M9)-grafted diploids (Table 2)
In the case of tetraploids, M9-grafted plants had 80% higher total length of current season shoots, they had 60% more current season shoots, the current season shoots were longer by 14% and their trunk cross-section area was greater by 50% than in own-rooted tetraploid plants (Table 2)

Summary

Introduction

The apple tree (Malus × domestica Borkh.) is one of the most important fruit crop plants, widely cultivated in the temperate climate zones of North America, Europe, and Asia. The main factors that threaten apple cultivation and cause losses in plant productivity and fruit quality are unfavourable environmental conditions (including limited water resources) and the attack of pathogens; one of the main goals of apple breeding programs is to obtain cultivars with increased tolerance/resistance to biotic and abiotic stresses [1,2,3]. The most devastating bacterial pathogen of apple trees is Erwinia amylovora, the causative agent of fire blight disease. The most destructive is apple scab caused by the pathogen Venturia inaequalis (Cooke) Wint.; yield losses caused by this disease reach up to several dozen percent [6]

Methods

Results

Conclusion