Corn is one of the high-yielding cereal crops of the universal purpose which dominates many crops by the yield level with sufficient water supply. It has a number of fodder and food properties and is used in various branches of agriculture and processing industry. Corn is a crop which drastically reduces its productivity under inappropriate care.

As a result of weediness, crop losses can reach 80 % or more. The losses caused by weeds exceed losses from pests and diseases. The main element in solving the problem of weediness of corn agrophytocenoses is the development of effective measures to regulate the presence of the weed component.

In view of the lack of effectiveness of mechanical means of weeds control, most highly developed foreign countries, including the United States, Spain and France started using soil and insurance herbicides – their cost is recovered with the yields increased by up to ten times.

Experiments were carried out in field and laboratory conditions of Department of microbiology, biochemistry and plant physiology at Uman National University of Horticulture in corn crops of Porumben 359 MV hybrid in 2016–2017. Herbicide Stellar, c.e. in norms 1.0; 1.1; 1.2 and 1.3 l/ha were applied in the phase of 3–5 leaves. The number of replications is 3.

Herbicide was applied by SHH–600 sprayer using working solution 200 l/ha. Chlorophyll amount content (a+b) and formation of pure photosynthesis productivity were determined according to generally accepted techniques.

While determining chlorophyll amount content (a+b) in corn leaves in the phase of 8–10 leaves in 2016 it was found out that applying 1.0 liters per hectare of herbicide it increased by 0.15 mg/g of raw matter compared to the control variant and after applying 1.1 and 1.2 l/ha it increased by 0.19 and 0.28 mg/g of raw matter, respectively, with SSD₀₅ 0.07 mg/g of raw matter. At the highest dose of this drug the content of pigments exceeded the check variant by 0.07 mg/g of raw matter. However, it declined compared to the norm by 1.2 l/ha which, apparently, was caused by some phytotoxicity of high doses of the drug.

During ear emergence absolute indicators of photosynthetic pigment content exceeded the value of previous records. However, the dependence between the application rate of the herbicide and change in chlorophyll content was the same. In case of weeding out it was the highest (by 0,42 mg/g of raw matter more than the check variant I). In other variants applying herbicide at the rate of 1,2 l/ha it was by 0.65 mg/ g of raw matter more than the check variant I at SSD₀₅ 0,13 mg/g of raw matter.

Determining the chlorophyll content in 2017, it was found that their total content was slightly lower than in 2016 which is explained by hotter weather. However, it also depended on the application rate of the herbicide. Thus, in particular, as in the previous year, the highest rates among the experimental variants with the application of only herbicide were noted at the rate of 1.2 l/ha (by 0.21 and 0.35 mg/g of raw matter more than the check variant I in the phase of 8-10 leaves and ear emergence, respectively, at SSD₀₅ 0.06 and 0.10 mg/g of raw matter).

Determination of the value of the indicator of net productivity of photosynthesis (PNP) showed that it varied in direct dependence on the content of chlorophylls in corn leaves and depended on the rate of herbicide application.

Thus, during ear emergence in 2016, applying 1.0 liter per hectare of the herbicide the indicator of photosynthesis net productivity increased by 5 % and applying 1.1 l/ha it increased by 12 %. The indicator of photosynthesis net productivity was the highest among experimental variants, applying 1.2 l/ha of the herbicide (by 17 % more than the check variant I). Further increase in the application rate of the herbicide to 1.3 l/ha also contributed to some increase in the photosynthetic yield of corn plants, although to lesser extent than the norm at 1.2 l/ha. In this case, PNP indicator exceeded the check variant by 14 %.

Determining PNP indicator in 2017 showed that it also depended on the application rate of the herbicide. So, applying 1.0 and 1.2 l/ha of the herbicide, the photosynthetic yield of corn plants also exceeded the check variant by 0.5 and 1.2 %, respectively. As in the previous year, it was the highest applying 1.2 l/ha of the herbicide (20 % more than the check variant I). Increase in Stellar, c.e. application rate to 1.3 l/ha somewhat depressed photosynthetic productivity of corn plants, although it exceeded the check variant by 13 %.

The research on the influence of Stellar, c.e. herbicide application rates on the basic physiological processes of corn plants showed that application of this herbicide contributes to the growth of photosynthetic pigment content in corn leaves and photosynthetic productivity of the crop by reducing competition with weeds for nutrients and moisture. The rate of 1.2 liters per hectare was the most effective applying different rates of the herbicide in experimental variants. Increase in chlorophyll content ranged from 0.28 to 0.35 mg/g of raw matter compared to the check variant I. PNP indicator exceeded the check variant by 17–20% depending on the phase of crop development and the research year.

Key words: corn, herbicide, stellar, с.e., the sum of chlorophylls (a+b), the net productivity of photosynthesis, efficiency.