Field studies were conducted in the northeastern part of the Republic of Croatia to determine the influence of the critical period of weed interference on sunflower (Helianthus annuus) yield, yield components, and oil content. For this purpose, different durations of competition were established, allowing weeds to infest the crop for increasing periods of time after planting or maintaining plots weed-free for increasing periods of time after planting. The beginning and the end of the critical period of weed control (CPWC), based on a 5% and 10% loss of sunflower yield, were determined by fitting the four-parameter log-logistic equations to the relative seed yield. The total weed biomass increased progressively in relation to the increase in the competition. The beginning of the CPWC period, based on a 5% acceptable yield loss, ranged from 141 to 234 growing degree days (GDD), which corresponded to the two-to-four true leaf development stage (the V2–V4 growth stages) across both sites and years. The crop had to be kept weed-free until a period when sunflower inflorescence began to open and flower (the R4–R5 growth stage) or from 1365 to 1932 GDD. The sunflower yield and yield components varied between the years and among locations. An increasing duration of weed interference negatively affected crop height, head diameter, and 1000-kernel weight, but not seed oil content.

Global warming and the associated climate change are imposing abiotic stress on plants. Abiotic factors are crucial for plant productivity, survival, and reproduction. Eight sunflower hybrids were tested in conditions of different water availability and with combinations of different temperatures and irradiation. The changes in the photosynthetic efficiency were measured in the morning (control conditions: 2013, 25.8 °C and 349.1 W m⁻²; 2014, 21.8 °C and 296.4 W m⁻²) and afternoon (the combination of increased temperatures and high irradiation: 2013, 34 °C and 837.9 W m⁻²; 2014, 29.4 °C and 888.9 W m⁻²) at a flowering stage in rainfed or irrigated conditions. The measurement time (morning and afternoon conditions) had a statistically significant effect on all the tested parameters. The performance index (PI_ABS) in 2013 and the maximum quantum yield of photosystem II (TR₀/ABS) in 2014 are the only parameters significantly affected by the irrigation. As a result of the combined effect of increased temperatures and high irradiation, PI_ABS values decreased by 73–92% in rainfed conditions and by 63–87% in irrigated conditions in 2013, depending on the hybrid, while in 2014, the decrease varied between 70 and 86%. The TR₀/ABS decrease was 7–17% in 2013, depending on the hybrid, and 6–12% in 2014, both in rainfed and irrigated conditions. The principal component analysis confirmed the effect of the combination of increased temperatures and high irradiation on hybrids, sorting them exclusively according to the time of measurement. All investigated parameters highly fluctuated between hybrids but without observable trends for the morning and afternoon conditions, as well as for irrigation. Plants’ reaction to the combination of increased temperatures and high irradiation manifested as a change in their photosynthetic efficiency, i.e., the photosynthetic apparatus’ functioning was impaired.

The reason for this examination is today’s wide usage of chlorophyll a fluorescence (ChlF) among researchers worldwide to measure photosynthetic efficiency. Although the instructions of the ChlF measuring device clearly emphasize the need for methodology adjustments, depending on the specific plant species, many researchers use the usual 30 min of dark adaptation before measurement. Namely, before any ChlF measurement, it is necessary to determine the specific duration of the leaf adaptation to the conditions of darkness of each plant tissue. Because of the numerous uses of the ChlF measurements, we decided to conduct this research to determine whether the appearance of the curves and parameter values depend on the time of sunflower leaf tissue adaptation to dark conditions. Therefore, this research aimed to examine the optimal adaptation time of sunflower tissue to dark conditions to obtain timely precise measurements and credible appearance of ChlF transient curves as well as accurate parameter values. The research was carried out on the sunflower hybrid Luka with 0, 15, 30, 45, 60, 75 and 90 min of dark adaptation in the vegetative, budding and flowering stages in the field conditions. According to the analyzed transient curves and parameters, it was determined that sunflower leaves should be kept in dark conditions for at least 15 min before the measurement of ChlF, which leads to the complete oxidation of PSII and the electron transport chain prior to a saturating pulse of light.

The objective of the investigation was testing of sunflower hybrids in different environmental conditions. Trials were set up at three locations (Osijek —Eastern Slavonia county, Karanac — Baranja county, and Feričanci — Western Slavonia county) during the 2002 and 2003 growing season. Fourteen sunflower hybrids were tested. Stability and adaptability analysis was performed using two parameters — regression coefficient (b\ud i\ud ) and deviations mean square (s\ud 2\ud \ud di\ud ). Determined were hybrids characterized by high and stable grain yield and wide general adaptability and hybrids for low and high yielding environments. The investigation showed no significant differences among single cross and three-way cross hybrids regarding grain yield stability

Sunflower is the most important oil crop in the Republic of Croatia. In the last two decades, it has been grown on average on 33,506 ha with a average grain yield of 2.66 t/ha. Weeds represent one of the most important causes of biotic stress, and their action is reflected in the competition for nutrients, water, light and space. The weed flora in sunflower crops in Croatia is extremely diverse, and we will most often meet: Polygonum persicaria L., Polygonum lapathifolium L., Ambrosia artemisiifolia L., Chenopodium polyspermum L., Amaranthus retroflexus L., Solanum nigrum L. emend Miller, Abutilon theophrasti Medik, Datura stramonium L., Xanthium strumarium L., Convolvulus arvensis L., Cirsium arvense (L.) Scop., Echinochloa crus-galli (L.) Pb., Setaria glauca (L.) Pb., Setaria viridis (L.) Pb., Digitaria sanguinalis (L.) Scop., Panicum spp. and Sorghum halepense (L.) Pers. For many years, weed control was based on the intensive use of chemical preparations, which were most often used after sowing and before emergence, in double or triple combinations. In addition to a number of advantages of using chemicals in sunflower production, the problem of broadleaf weeds was not satisfactorily solved until the advent of Clearfield, Clearfield plus and Express sun technology, i.e. hybrids tolerant to herbicides from the group of imidazolinone (imazamox) and sulfonyl-urea (tribenuron-methyl) at the beginning of this century. Integrated plant protection, including integrated weed management, with its holistic approach and greater application of direct and indirect non-chemical measures, is only environmental quality solution for controlling weeds in sunflowers.