The osmotic potential of the nutrient solutions with and without PEG was calculated according to Bündig et al. from measured values determined by using an osmometer. For nutrient solutions without PEG, an osmotic potential of -0.013 MPa was calculated, while the solutions with PEG revealed a potential of -0.16 MPa.From 74 to 89 dap, nutrient solutions were changed without the addition of PEG for both cultivars and K supplies. The plants grew for a total of 89 days in a greenhouse with 12 h light and 12 h darkness at an average temperature of 20.6 ± 7.5 ◦C in a completely randomised design.The phenotype was influenced by K supply, as shown by the example of cultivar Milva in Fig. 3. The +K plants produced more biomass and showed no K deficiency symptoms. Even after the addition of PEG, the plants remained vigorous and only a few chlorosis and necrosis were observed on the oldest leaves. In comparison, the -K plants produced less biomass and both necrosis and chlorosis were observed on the oldest leaves. Under PEG addition, this effect was further enhanced, so that the oldest leaves were almost completely necrotic. Plant height and biomass were significantly affected by K supply with higher rates for +K plants compared to -K plants in both cultivars. The differences between K supplies were higher in Agria starting from the beginning of the measuring period. With PEG addition, plant height and biomass increased for both K supplies compared to plants without PEG addition for ‘Agria’ and ‘Milva’. However, the effect was not significant. In -K plants, water consumption was significantly reduced compared to +K plants in both cultivars ; but, differences between the K levels were again greater in Agria.
The addition of PEG decreased water consumption in Agria at both K levels but increased again after 74 dap. In contrast, water consumption in Milva initially increased due to PEG addition in -K and +K plants,and a delayed decrease in water consumption was observed at 74 dap. However, PEG-induced osmotic stress did not significantly affect water consumption. The number of leaves and internodes was higher in +K plants than in -K plants for both cultivars. Differences were only significant in Agria, flower pot which also produced more leaves compared to Milva.More biomass was produced in +K compared to -K plants of both cultivars, but differences were only significant in Agria. In Milva, +K+PEG and -K+PEG plants produced 13 and 43% more biomass, respectively, than plants of the same K supply without PEG. Side shoots made up the largest proportion of the biomass, followed by leaves. In comparison to the +K-treated plants, the -K plants produced fewer stems, whereas the root biomass developed equally. A similar distribution was observed in Agria, although the +K+PEG plants did not produce more biomass compared to +K plants. The +K plants of Milva accumulated up to three-fold higher K on the whole plant level compared to -K plants. A higher K content was found in +K plants compared to +K+PEG Milva plants, although the biomass was higher in +K+PEG plants. The highest K content was measured in side shoots and leaves and thus correspond to the biomass and K distribution of individual plant parts. Compared to +K plants, less K was translocated into the leaves and stems of -K plants, reflecting the biomass and K distribution for both cultivars. However, the K content in Agria was almost equal between +K and +K+PEG plants, although the biomass of +K +PEG plants was 20 g lower based on DM. The results for stolons and tubers are shown in Supplementary Table S7.
Reducing sugars accumulated more in -K plants compared to +K plants of both cultivars. However, almost twice the content of reducing sugars was detected in +K and +K+PEG plants of Milva than in Agria. Within plant parts, reducing sugars were accumulated mostly in side shoots and leaves at both K levels and in both cultivars. PEGinduced osmotic stress did not affect the accumulation of reducing sugars in -K and +K plants. In contrast to reducing sugars, the sucrose content was not affected by K treatments. However, the sucrose content was two to three-fold lower than the reducing sugar content and was highest in side shoots and leaves. In Milva more sucrose was measured in +PEG treated plants at both K supplies, while the opposite was observed in Agria. The results for stolons and tubers are shown in Supplementary Table S8.For adaptations to changing environmental conditions, it is important to understand the morphophysiological and metabolic processes of plants to provide specific stress-mitigation strategies. In this study,osmotic stress was induced by adding PEG—with an osmotic pressure in the nutrient solution of -0.16 MPa—to simulate drought stress under hydroponic conditions. Two different K rates were applied to investigate the effect of K on plants tolerance to osmotic stress. Phenotypic observations revealed more biomass and almost no osmotic stress symptoms due to +K fertilisation. In comparison -K plants showed typical chloroses and necrosis on older leaves, which were even more severe with PEG addition. PEG symptoms on older leaves were also described by Büssis et al.,which might be related to a water deficit. Furthermore, the results of this study showed that K supply has a wide range of effects on plant physiological parameters. Weekly determined plant height and biomass decreased under -K conditions compared to +K for both cultivars. This is in accordance with a study on potatoes and other plant species, including tomato and wheat.
A reduction in plant growth is known to be a physiological adaption to insufficient K supply for maintaining the tissue K concentration sufficient for several cell functions. During PEG-induced osmotic stress, plants were still able to grow under both K treatments, which was also observed by Büssis et al.. However, in contrast to their study where PEG-treated plants were inhibited in growth, in our experiment, +PEG plants showed higher growth rates within the osmotic stress period. An increase in growth due to the influence of PEG has been rarely described,and several studies show contrasting results,primarily when investigated under in vitro conditions. According to Ahmad et al.,who investigated the in vitro growth processes of Stevia rebaudiana, it was presumed that water deficiency induced by PEG at a critical level led to the manipulation of plant physiology and biochemistry. Changes in the cellular environment can result in stress stimuli affecting cellular receptors and further triggering signal cascades involved in physiological, and therefore growth processes. Another explanation by Khalid et al. refers to an increased carbohydrate and mineral content including K, N, and P under PEG addition, which enhanced the growth parameters of Pelargonium odoratissimum. This could also explain the results of our study, as increased concentrations of carbohydrates were found in the +PEG treated plants for both cultivars and K levels, except for sucrose content in Agria. Similar to plant height and biomass, K deficiency negatively affected water consumption, as root growth is frequently reduced under deficient K supply. For Milva, water consumption increased in the first week after PEG addition in +K+PEG and -K+PEG plants compared to plants without PEG. This was not observed for Agria, where water consumption decreased in PEG-treated plants in both K supplies when osmotic stress was induced. In a study by Dorneles et al.,the stress responses of potato plants exposed to water deficit under osmotic and matric induction were investigated.
They observed a more negative osmotic potential in plants than in nutrient solutions containing PEG, suggesting that plants osmotic potential may promote an osmotic force and thus water uptake under PEG conditions. This could explain the briefly higher water uptake rates of Milva immediately after PEG addition for both K supplies, indicating cultivar-specific responses to osmotic stress. Maintaining water consumption and general growth processes, despite stress situations, could therefore reflect a possible tolerance mechanism to osmotic and drought stress in Milva. At final harvest, the total biomass was lowest in -K-treated plants than +K plants for both cultivars. Sufficient K supply enhances photosynthetic processes, leading to increased leaf area expansion, which results in elevated biomass production Milva produced more biomass during PEG-induced osmotic stress for both K treatments, whereas +PEG Agria plants produced a higher biomass only under -K, probably related to increased carbohydrate and mineral content, enhancing growth parameters. Overall, the biomass of side shoots was higher in -K plants compared to +K plants, in which the proportion of leaves was greater. Presumably, the plants under K deficit attempt to maintain the photosynthetic capacity by developing more side shoots to increase leaf biomass. Schittenhelm et al. showed that potatoes can produce a large above-ground biomass as a strategy against soil water deficit. Accordingly,berry pots the increased biomass of side shoots in the -K+PEG plants could be the result of a similar mechanism to maintain biomass production under osmotic stress. This may as well be an adaptation to K deficiency, such that -K plants increased the leaf area by producing additional shoots, thus maximising the potential for additional photosynthetic activity. However, berry pots the cultivars used in this study could be divided according to their shoot morphology, which is based on genotypic characteristics. Cultivar Agria, used in our study, belongs to the stem type,supporting the results of stem biomass for +K plants, which was higher than that in Milva.
In contrast, Milva belongs to the intermediate type and equally forms both stems and leaves. Therefore, the increased biomass production of side shoots could be an adaptation to the prevailing conditions. Another reason for the high production of side shoots could be due to the experimental arrangement, as under greenhouse conditions light was available from all sides and the plants had enough space to spread out, which is usually not the case in the field due to narrow plant spacing. Reducing sugars accumulated more in -K plants than in +K plants.Since K is involved in phloem loading,impaired sucrose transport from source leaves to sink organs is a result of K deficiency. Compared to +K plants, reducing sugars accumulated more in the roots of -K plants. This is in accordance with Sung et al.,who found high concentrations of sugars in the roots of tomatoes grown under K deficient conditions. However, compared to the concentration of sugar in the roots, more reducing sugars were accumulated in the stems of the -K plants, which, in turn, could indicate an inhibited sugar transportation. Overall, the influence of PEG-induced osmotic stress also tended to affect the sugar concentration. Thus, +K+PEG and -K+PEG plants produced more sugars compared to plants without PEG addition. However, +K+PEG-treated Agria plants were an exception, suggesting that Agria is less susceptible to osmotic stress and may also be less susceptible to drought stress under optimal K supply. This also confirms the classification from different studies, where Agria was considered a more tolerant cultivar to drought stress under pot and field conditions. However, other studies classified Agria as sensitive to drought stress in a pot experiment or as susceptible to water deficit under field conditions. Thus, there is no clear classification for Agria. In contrast, Milva was described as sensitive to drought stress in pot experiments under greenhouse and under field conditions, which could not be confirmed in our study. The literature review showed that due to very different experimental conditions, an exact classification and a respective comparison is not easily possible. However, our investigation has shown that Milva and Agria can adapt to PEG-induced osmotic stress under hydroponic conditions. These strategies could also be used when adapting to drought stress under field conditions. Comparing matric and osmotic stress, Dorneles et al. found that stress responses to both forms of stress were similar. Accordingly, the results from the hydroponic system may be applied to field conditions.Leaf samples at three different growth stages during the experiment provided detailed information on the physiological and physiochemical processes under altered conditions induced by sufficient and deficient K supply, and by PEG-induced osmotic stress. The K content in the leaf was positively influenced by the K supply, so that three-fold more K was detected in the leaves of +K plants compared to -K plants.