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.

Multi-year studies are crucial for the evaluation of sunflower hybrids and quantifying the environmental effect in the expression of genetic potential. In order to assess the adaptation of eight hybrids and test the impact of water availability on plant height (PH), head diameter (HD), 1000-grain weight (TWG), hectolitre mass (HM), seed yield (SY), oil content (OC) and oil yield (OY), a two-year study was conducted with irrigation as treatment. Analysis of variance (ANOVA) and principal component analysis (PCA) confirmed the year influence on all tested agronomic traits. According to the three-way ANOVA, irrigation significantly affected all tested traits except HD, OC and OY. All agronomic traits had lower values in both rainfed and irrigated treatment in 2014, confirming the influence of the environment. The hybrid was a significant source of variation for all traits. ANOVA and PCA grouped hybrids 1, 7, and 8 in one group and 2, 3, 4, 5, and 6 in another. The first group had lower PH, HD, TGW and SY values and higher HM, OC and OY values, and the second group had reversed traits values. Furthermore, the PCA biplot indicates SY was positively correlated with PH, HD, TGW and OY and HM was positively correlated with OC. This facilitates the breeding process because it enables indirect breeding for economically important traits such as seed yield, oil content and oil yield. As treatments were significant sources of variation for PH, HM, TWG and SY, sunflower irrigating is considered justified and can be used as an additional agrotechnical measure to target the agronomic traits. Understanding the expression of traits under rainfed and irrigation conditions will greatly help design effective breeding programs by creating hybrids suitable for cultivation in semi-arid environments.

Sunflower (Helianthus annuus L.) has high needs for water but can tolerate drought very well because, under stress conditions, its well developed root system can supply water and mineral nutrients from deeper soil layers. Reduced water content in soil affects plant growth and development, photosynthetic rate and causes rapid leaf senescence. In this study, we measured maximum quantum yield of photosystem II (Fv/Fm), photosynthetic performance index (PIABS) and leaf temperature (LT) on 13 sunflower genotypes at different soil water contents. By calculating stress tolerance indices (STI) of Fv/Fm and PIABS parameters we evaluated drought tolerance for every tested sunflower genotype at given soil water contents. The experiment was set up in vegetation pots in two treatments with different soil water contents (60% and 80% of field water capacity) in three replications. Based on the obtained results for Fv/Fm and PIABS and STI values of Fv/Fm and PIABS parameters, we concluded that genotypes 5 and 12 had higher tolerance at both treatments, as opposed to genotypes 2 and 13 which were less tolerant. These analyses will help breeders to select genotypes adapted to different farming areas which is, along with the use of recommended production practices, the background for profitable sunflower production.