The survival of apple trees in old and abandoned orchards over 100 years indicates their adaptation to the local environment. These plants have passed an economic and ecological selection over the years. The structure of genetic diversity of apple germplasm in the old orchards of Lithuania is largely unknown. We applied a common set of 7 simple sequence repeat (SSR) markers to genotype 292 accessions across more than 50 geographic locations representing all Lithuanian geographic regions with the aim of analysing the distribution and structure of variation of the apple genetic resources. Eighty-one polymorphic alleles were identified. Of these, 5 alleles were unique, identified only in four individual genotypes. The COL locus was the most homozygous (44.7%) among the cultivars and clones of the apple; however, the part of the germplasm may be heterozygous for nullalleles and those individuals cannot be distinguished from the homozygous ones. The CH02c11 locus was the most heterozygous (90.0%) in the reference cultivars; meanwhile the CH04e05 locus was the most heterozygous (84.4%) among the analysed genotypes. Triploid genotypes constituted 12.5% among the analysed genotypes. A cluster analysis revealed 5 distinct clusters among the 182 diploid apple genotypes collected from the old orchards. According to molecular marker data, the names of 12 apple cultivars were attributed to 45 analysed unknown genotypes. It was found that traditional apple cultivars 'Popierinis', 'Paprastasis antaninis' and 'Pilkasis molinis' were the most prevalent cultivars in the old orchards. The apple genotypes that have survived in old orchards are valuable sources of traits for breeding as well as of high ecological, economic and heritage importance. Their use in breeding programs would significantly extend the diversity of adaptive genes of the domestic apple.

The heavy blooming of apple trees results in the inefficient usage of energy and nutritional material, and additional expenditure on fruitlet thinning is required to maintain fruit quality. A possible solution for controlling the fruit load on trees is the development of new cultivars that self-eliminate excess fruitlets, thus controlling yield. The aim of our study was to identify biological differences in apple cultivars in terms of blooming intensity and fruitlet load self-regulation. In total, 19 apple cultivars were studied in the years 2015–2017. The dynamics of fruitlet self-elimination, seed development in fruitlets and fruits, photosynthetic parameters, carbohydrates, and plant hormones were evaluated. We established that apple cultivars self-eliminating a small number of fruitlets need a lower number of well-developed seeds in fruit, and their number of leaves and area per fruit on a bearing branch are larger, compared to cultivars, self-eliminating large numbers of fruitlets. A higher carbohydrate amount in the leaves may be related to smaller fruitlet self-elimination. The amount of auxin and a high indole-3-acetic acid/zeatin ratio between leaves of cultivar groups with heavy blooming were higher than in cultivars with moderate blooming. A lower amount of abscisic acid was found in heavy-blooming cultivars during drought stress. All these parameters may be used as markers for the selection of different apple genotypes that self-eliminate fruitlets.

Apple has a tendency to bear huge amount of flowers or inflorescences. Plants have ability to eliminate excess number of fruitlets. Frequently fruit trees eliminate insufficient number of fruitlets to grow good quality fruits. Most processes related to yield self-regulation varies significantly between different apples varieties. Also meteorological conditions influenced flower buds differentiation and fruit growth during blooming and other development stages. The investigations were carried out in 2015 – 2016 at the Institute of Horticulture. The main aim was to evaluate yield self-regulation peculiarity in various apple varieties in different meteorological conditions. In our experiment we made fruit self – thinning evaluation of 15 different varieties. Meteorological conditions were varying. In 2015 there was lack of rainfall, while 2016 year was rainy. After each fruit-drop period the number of developed and undeveloped seeds were counted of self-sheded and normally growing fruits. Amount of developed seeds as auxins source is one of the most important factor for yield self-regulation. Varieties of apple which are blooming abundantly and eliminate high amount of fruitlets, grows fruits until maturity with stabile amount of developed seeds and produces permanent harvest annually, independently from meteorological conditions during vegetation. Such apple varieties need higher amount of developed seeds to inhibit abscission. Apple varieties tended to bloom abundantly and eliminate small amount of fruitlets depending on meteorological conditions. Demand of auxins to carry fruits depends on meteorological conditions therefore these apple-trees develop different amount of seeds, when weather conditions changes. We may presume, that different apple varieties need different amount of phytohormones or their ratio to hold fruitlets on the apple. Therefore apple trees with smaller amount of seeds in fruits grow more fruits to its normal size.