Regul. Mech. Biosyst., 2024, 15(1) Regulatory Mechanisms in Biosystems ISSN 2519-8521 (Print) ISSN 2520-2588 (Online) Regul. Mech. Biosyst., 2024, 15(1), 3–9 doi: 10.15421/022401 Photosynthetic potential of Malus domestica columnar group O. S. Havryliuk*, T. Y. Kondratenko**, V. M. Mezhenskyj*, L. M. Shevchuk* , **, O. D. Baranovska*, O. L. Tonkha*, D. V. Litvinov*, B. M. Mazur* *National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine **Institute of Horticulture of the National Academy of Agrarian Sciences, Kyiv Region, Ukraine Article info Received 14.12.2023 Received in revised form 25.01.2024 Accepted 17.02.2024 The National University of Life and Environmental Sciences of Ukraine, Heroiv Oborony st., 15, Kyiv, 03041 Ukraine. Tel.: +380-67-800-10-23. E-mail: o.havryliuk@nubip.edu.ua Institute of Horticulture of the National Academy of Agrarian Sciences, Sadova st., 23, Novosilky, Kyiv Region, 03027 Ukraine. Tel.: +380-67-326-03-26. E-mail: kondratenko.pv@gmail.com Havryliuk, O. S., Kondratenko, T. Y., Mezhenskyj, V. M., Shevchuk, L. M., Baranovska, O. D., Tonkha, O. L., Litvinov, D. V., & Mazur, B. M. (2024). Photosynthetic potential of Malus domestica columnar group. Regulatory Mechanisms in Biosystems, 15(1), 3–9. doi:10.15421/022401 Productivity is one of the primary economic and biological characteristics of an apple tree variety; it is this characteristic that de- termines the value and economic feasibility of the variety. The photosynthetic activity of the leaf surface of plants plays a leading role in forming the potential productivity of the apple tree. By "potential productivity," we mean the productivity of plants under condi- tions of ideal agroenvironment, optimal nutrition, and absence of diseases. To diagnose the potential productivity of cultivars we used an indicator of chlorophyll fluorescence induction–induction coefficient (К і ). According to our data, the К і of leaves of columnar cultivars ranged from 0.720 to 0.740, indicating a high level of efficiency of photophysical processes near the photosystem II (PS II) reaction centers. For plants of columnar cultivars, the viability index ranged from 1.78 to 2.19. It has been established that individual age sections of tree trunks form different productivity potentials. Thus, based on the chlorophyll a (Chl a) fluorescence induction index, a higher intensity of photosynthesis was observed in the leaves of the cultivars ‘Valuta’, ‘Favoryt’, and ‘Bilosnizhka’ on seven to nine-year-old trunks. In contrast, for ‘Tantsivnytsia’, the highest intensity was observed on the oldest spur formation (14–19 years old). In traditional apple cultivars, the intensity of photosynthesis, as indicated by К і , decreased with the age of spur formation; how- ever, such a decline was not observed in columnar cultivars. The leaves of plants from the traditional cultivar ‘Idared’, situated on a homogeneous shoot, exhibited the highest photosynthetic intensity based on the F 680γt / F 680βt light intensity. Conversely, in the leaves of three-year-old spur formations, a 25.3% decrease in photosynthetic intensity was observed, falling to a 45.7% decrease in six-year- olds. With the optimal combination of agroecological factors for columnar cultivars, stability in the photosynthetic potential across various complex spur formations is observed. In typical apple cultivars, the age of spur formation leads to a suppression of photosyn- thetic intensity. The cultivar ‘Bilosnizhka’ is characterized by the highest Chl a/ Chl b content, indicating lower adaptability. In the leaves of this cultivar, the amount of Chl a exceeded Chl b by three times; in contrast, in the cultivars ‘Sparta’ and ‘Tantsivnytsia’, the difference was 2.5 times higher. This value (2.5x) correlates with increased adaptability to the conditions in which they were studied. The highest level of leaf net productivity of photosynthesis (NPP) is observed in plants of the ‘Valuta’ cultivar (13.9 g/m² for day); in the cultivars ‘Bilosnizhka’, ‘Favoryt’, ‘Bolero’, ‘Sparta’, ‘Tantsivnytsia’, and ‘Papirovka’, the accumulation of dry matter is lowered by 32.6–40.6%. Keywords: leaf photosynthesis; chlorophyll fluorescence; cultivar; induction coefficient; chlorophyll; adaptability. Introduction Entirely new perspectives in the evolution of the apple orchard have opened up with the discovery of a spontaneous mutant ‘Wijcik’ (‘Mcin- tosh Wijcik’), which has become the basis for the selection of columnar cultivars around the world (Tobutt, 1984, 1994; Jacob, 2007). The muta- tion is characterized by an unusual type of crown and plant habit, as well as the enhanced establishment of generative organs (Blažek, 1990; Blažek & Křelinová, 2011; Wada et al., 2018; Sun et al., 2020, 2021). The identi- fication of the columnar gene (Co) in ‘Wijcik’ has opened up fundamen- tally new opportunities in the breeding of apple trees, specifically in terms of plant structure and an increased potential yield of more than 200 t/ha (Okada et al., 2020), which has been observed in cultivars with a colum- nar-type of growth. According to International Code of Nomenclature for Cultivated Plants, these cultivars belong to Malus domestica Сolumnar Group (Mezhenskyj, 2008). The biological feature of these columns is the almost complete absence of lateral branching, crop formation on both sim- ple and complex spur formations located on the trunk of the tree, dwarf- type growth, early fruiting, and high yields. Ukrainian breeders have crea- ted over 20 columnar apple cultivars (Havryliuk & Kondratenko, 2019, 2020). Some of them have successfully passed the primary and state culti- var studies and are consequently undergoing technical evaluation in small industrial plantations of various types at present. These studies show the ability of “columns” to form high yields of one-dimensional fruits, consis- tent in shape and size. These yields are located on complex spur formati- ons, ranging in age from 2–20 years, which is atypical in conventional ap- ple cultivars (Havryliuk et al., 2022a, 2022b). We investigated the unique features of some of these columnar apple cultivars by assessing the func- tional state of their photosynthetic apparatus, utilizing the fluorescence in- duction method. Currently, there are a large number of methods for diagnosing the functional state and potential productivity of plants (Vasylenko et al., 2021). Scientists point to the high susceptibility of photosynthesis to oxi- dative damage (Melis, 1991; Hsu, 1992). It has been proposed that the work of the photosynthetic apparatus is an indicator of a plant's response to the ecological pressures of growth conditions specifically (Allen, 1992). Modern biophysical methods of diagnosing the state of photosynthe- tic systems make it possible to assess not only the result of plant reactions to stress but also to obtain data on the processes occurring during the direct action of a factor (Shikhov et al., 2011; Ptushenkom et al., 2013). The dia- 3