Cold stress is one of the most common abiotic stresses experienced by plants and is caused by low temperature extremes and variations. Polyamines (PAs) have been reported to contribute in abiotic stress defense processes in plants. The present study investigates the survival and responses of PA-treated non-acclimated (N) and acclimated (A) winter oilseed rape to increasing cold conditions. The study was conducted under controlled conditions. Seedlings were foliarly sprayed with spermidine (Spd), spermine (Spm), and putrescine (Put) solutions (1 mM) and exposed to four days of cold acclimation (4 °C) and two days of increasing cold (from −1 to −3 °C). Two cultivars with different cold tolerance were used in this study. The recorded traits included the percentage of survival, H+-ATPase activity, proline accumulation, and ethylene emission. Exogenous PA application improved cold resistance, maintained the activity of plasma membrane H+-ATPase, increased content of free proline, and delayed stimulation of ethylene emission under increasing cold. The results of the current study on winter oilseed rape revealed that foliar application of PAs may activate a defensive response (act as elicitor to trigger physiological processes), which may compensate the negative impact of cold stress. Thus, cold tolerance of winter oilseed rape can be enhanced by PA treatment.

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The effect of exogenously applied 2,4-D (2,4-dichlorophenoxyacetic acid) on growth and antioxidant defence of pea plants, preliminary treated with two synthetic auxin compounds 1-[2-chloroethoxycarbonyl-methyl]-4-naphthalenesulfonic acid calcium salt (TA-12) and 1-[2-dimethylaminoethoxycarbonylmethyl]naphthalene chlormethylate (TA-14) was examined. All chemicals were applied by foliar spraying. Applied alone, TA-12 and TA-14 had no significant effects, but they modulated the 2,4-D induced changes on most investigated biochemical parameters. The shoot fresh weight reduction caused by 2,4-D was partially overcome by the use of TAs. The use of TAs partially overcame the shoot fresh weight reduction induced by 2,4-D. Apart from this, no significant changes were observed in the other biometric parameters. Treatment with 2,4-D did not enhance lipid peroxidation, and hydrogen peroxide content was slightly increased. These data indicate that treatment with 2,4-D did not cause severe oxidative stress, which is also confirmed by the results of the antioxidant defence system. The application of 2,4-D provoked mild accumulation of thiol-containing compounds, free proline and phenolic compounds and increased the antioxidant enzyme activities (GST, SOD, CAT, POD and GR) to a moderate degree. Pretreatment with TAs noticeably decreased the non-enzymatic antioxidants (free proline, total phenolics and total low-molecular thiols) compared to plants treated with 2,4-D only. Except for GR, TAs pretreatment returned the enzyme activities to levels close to the controls. Based on the results obtained, we suggest that the application of both synthetic auxins could modulate 2,4-D herbicide effects.

Cold is a major environmental key factor influencing plant growth, development, and productivity. Responses and adaption processes depend on plant physiological and biochemical modifications, first of all via the hormonal system. Indole-3-acetic acid (IAA) plays a critical role in the processes of plant functioning. To assess the influence of the auxin-like compounds 1-[2-chloroethoxycarbonylmethyl]-4-naphthalenesulfonic acid calcium salt (TA-12) and 1-[2-dimethylaminoethoxycarbonylmethyl]naphthalene chloromethylate (TA-14) in the process of cold acclimation, long-term field trials over four years were performed with two rapeseed (Brassica napus L.) plant cultivars with different wintering resistance in temperate-zone countries. In these two rapeseed cultivars, namely ‘Casino’ (less resistant) and ‘Valesca’ (more resistant), investigations were conducted in the terminal buds and root collars. The application of auxin-like compounds revealed a close interlinkage between the composition of dehydrins and the participation of the phytohormone IAA in the adaptation processes. By applying TA-12 and TA-14, the importance of the proteins, especially the composition of the dehydrins, the IAA amount, and the status of the oilseed rape cultivars at the end of the cold acclimation period were confirmed. Following on from this, when introducing oilseed rape cultivars from foreign countries, it may also be of value to assess their suitability for cultivation in temperate-zone countries.

In order to increase plants tolerance to drought, the idea of treating them with stress-protecting compounds exogenously is being considered. In this study, we aimed to evaluate and compare the impact of exogenous calcium, proline, and plant probiotics on the response of winter wheat to drought stress. The research was carried out under controlled conditions, simulating a prolonged drought from 6 to 18 days. Seedlings were treated with ProbioHumus 2 µL g−1 for seed priming, 1 mL 100 mL−1 for seedling spraying, and proline 1 mM according to the scheme. 70 g m−2 CaCO3 was added to the soil. All tested compounds improved the prolonged drought tolerance of winter wheat. ProbioHumus, ProbioHumus + Ca had the greatest effect on maintaining the relative leaf water content (RWC) and in maintaining growth parameters close to those of irrigated plants. They delayed and reduced the stimulation of ethylene emission in drought-stressed leaves. Seedlings treated with ProbioHumus and ProbioHumus + Ca had a significantly lower degree of membrane damage induced by ROS. Molecular studies of drought-responsive genes revealed substantially lower expression of Ca and Probiotics + Ca treated plants vs. drought control. The results of this study showed that the use of probiotics in combination with Ca can activate defense reactions that can compensate for the adverse effects of drought stress.

With the recent rapid development of the functional food sector, agriculture is looking for alternatives to improve the quality of food grown by limiting chemical fertilizers. This study evaluated the effects of two commercial plant probiotics, ProbioHumus and NaturGel, on the growth and quality of strawberry fruits. Strawberry plants were sprayed with microbial probiotics twice a year: after harvesting at the beginning of dormancy and at the stage of leaf development. Spray applications of ProbioHumus, NaturGel, and NaturGel + ProbioHumus in the organic farm fields significantly increased the fresh fruit weight up to 42%, 35%, and 37%, respectively, compared to the non-treated control. An increase in the weight of fresh strawberry fruits may be associated with an increase in dry matter accumulation. The probiotics had a positive effect on the total content of phenols, anthocyanins, and especially ascorbic acid in strawberry fruits. The increase in ascorbic acid in strawberry fruits was up to 97% compared to the non-treated control. The fruits from plants inoculated with probiotics showed significantly higher antioxidant activity. In summary, ProbioHumus and NaturGel are effective tools for improving the quality of strawberries and can be exploited in sustainable agriculture as a tool for adding value to functional food.

The berries of blackcurrants are commercially important and rich in antioxidants and have a high free radical scavenging capacity. Four controlled fields 1 ha in size were set up in an organic farm to test two commercial plant probiotic products, ProbioHumus and NaturGel, and their combined impact on blackcurrant berry yield and quality. Spray applications of ProbioHumus, NaturGel, and their combination significantly increased fruit yields 38, 25, and 16%, respectively, versus the nontreated control. The berries from plants inoculated with probiotics had significantly higher contents of anthocyanins versus the nontreated control and tended to exhibit increased levels of ascorbic acid. The blackcurrant berries treated with both probiotics exhibited a fair ability to scavenge DPPH free radicals. This is the first study of the improvement of both the yield and the quality of blackcurrant berries by the application of commercial plant probiotics under field conditions.