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In Russia's North Caucasus where drought is frequent the apple tree is one of the most important garden crops. Physiological and biochemical studies are necessary to assess adaptiveness of apple varieties to stressors during summer, in particular, to summer drought. The purpose of this work is to study physiological, biochemical and anatomical parameters of leaves to assess the water regime, photosynthetic activity of apple trees in summer conditions, and to identify the most drought tolerant varieties for cultivation in the North Caucasus region. Research was carried out in 20112013 in fruit-bearing plantations (Central’noe Farm, Krasnodar) on apple varieties of different ecogeographical origin and ploidy: Idared, Earle Mack, Dayton (United States), Ligol (Poland), Prikubanskoe, Rassvet, Fortuna, Soyuz, Rodnichok (Russia). Varieties Soyuz and Rodnichok are triploids, the rest ones are diploids. Monthly, fully formed leaves were collected from (from the middle part of annual shoots of three trees in 3 replicates for each variety, 10 leaves per replicate. Indicators of water regime (total, free and bound water contents) were analyzed gravimetrically. The total water was determined after drying samples at 105 С to a constant weight. For anatomical examinations, leaf blade transverse sections (temporary preparations) were used. It was shown that the leaf tissue water content, as well as the ratio of the bound and free water depend on both the variety specificity and the meteorological conditions of the year. Leaf water content in Prikubanskoe, Fortuna, Soyuz, and Rodnichok trees during July and August decreased by an average of 1-4 % compared to June, and the bound-to-free water ratio was the highest. Also, direct correlation between the leaf area and water availability (r = +0.98), and negative correlation between the leaf area and air temperature (r = 0.99, (p  0.05) were characteristic of these varieties. Pair correlation coefficients between (а + b) chlorophylls and fruit yield (r = +0.87), and between water content of tissues and fruit bud initiation (r = +0.97) (p  0.05) indicate that water and temperature regimes influence the yield and fruit bud formation. In the varieties of Prikubanskoe, Fortuna, Soyuz, Rodnichok, the chlorophyll content was more constant during the summer, and the ratio of the sum of chlorophylls to carotenoids is the highest. A positive correlation was found between the carotenoids and the air temperature (r = +0.91) (p  0.05). Morpho-anatomical structure of the leaf has varietal characteristics and depends on temperature and water availability. In 2012, the varieties exhibited xeromorphic features of leaves to varying degrees, which determined the resistance to drought, and the highest palisade index (1.471.49) was characteristic of the varieties Prikubanskoe, Fortuna, Soyuz, Rodnichok. The obtained results are in line with the field data obtained in the gardens without irrigation. The varieties showed different responses to summer stress factors, i.e. high temperatures and drought. Idared, Earley Mack, Dayton, Ligol plants were “passive” with a reduced water content, high solids, and smaller leaves. The rest varieties maintained high water and pigments in leaves and showed sustainable growth. Thus, Russian apple varieties Prikubanskoe, Fortuna, Soyuz, Rodnichok possess greater ecological plasticity and adaptive reserves compared to the studied introduced foreign varieties. The revealed adaptive features make it possible to involve these varieties in breeding for drought resistance. The applied tests provide accurate assessments of apple drought resistance and can be

PHYSIO-BIOCHEMICAL CRITERIA FOR APPLE TREE TOLERANCE TO SUMMER ABIOTIC STRESSES