Furthermore, the reduced need for filters in the DFP system compared to DAPS also indicates that FLOCponics might bring economic advantages for the producers. From an environmental point of view, the dependence on feeds is an aquaculture issue. Reducing the dietary CP level may mitigate the negative impact of feed on aquaculture sustainability due to the lower need for protein-rich ingredients and lower concentration of N excreted into the natural environment. With respect to the question of whether DFP might enable lettuce production in comparable yields to DAPS and traditional hydroponics,this study found no differences among the treatments for the growth parameters in the seedling and final production phase. Interestingly, to achieve these similar yields, less commercial fertilizer was required in the DFP-32 compared to the other treatments. In Cycle 1, the volume of fertilizer added to a DFP-32 plant tank was approximately 51.9% and 6.4% lower than in HP and DAPS, respectively. In Cycle 2, these differences between DFP-32 to HP and DAPS dropped to 16.3% and 1.3%, respectively. Another important finding was that reducing the amount of N in the fish diet in DFP systems did not affect lettuce growth in either cycle. The higher volume of fertilizer added to the DFP-24 and DFP-28 compared to DPF-32 seems to have compensated for the reduction in the amount of N. Nevertheless, in both treatments, the volumes of fertilizer were lower or similar to those added to the DAPS plant tanks. The use of conventional dosages of a commercial fertilizer in the hydroponics subsystem could have hindered lettuce growth, as a result of nutrient imbalances in the water. However, the nutritional management employed in this study seemed to have facilitated lettuce production in both cycles and in all treatments. In spite of this, knowing the specific nutrients that need to be supplemented in the hydroponics subsystems, based on the profile of nutrients in the BFT effluents and on the requirements of the crop, might result in less fertilization dependence,vertical grow rack and possibly even greater plant production.
For the purpose of developing a specific fertilizer scheme for DFP systems, efforts to constantly characterize the profile of macro- and micro-nutrients in BFT water and to adjust the formulations of the fertilizer according to the dynamics of the BFT will be needed. The curves for lettuce growth presented in Figs. 5 and 6 may be used to predict production in the hydroponic subsystem according to the experimental conditions employed. A notable finding from these curves is the tendency of the seedlings’ growth rate in Cycle 1 to decrease. Possible explanations for this decrease might be that the nutritional management employed in Cycle 1 was sub-optimal, leading to growth limitations. Optimal nutritional management could have been adopted.The dataset presented in this article provides details on the growth of 18 acclimatized and 11 non-acclimatized rice varieties grown hydroponically. The data from each variety were collected under three conditions: control, salinity, and salinity after acclimation. Growth measurements were taken at the end of 36 days of germination. The first dataset provides the shoot dry weight and length of acclimatized and non-acclimatized rice varieties, whereas the second dataset represents the root dry weight and length. In both cases, the first column is a variety name, and the next six columns are the measured traits of plants grown under the three different treatment conditions. Each column indicates the average of four samples along with the standard error of the measured trait. The same letter indicates no significant differences among the three treatments at p < 0.05. Table 1 presents the list of rice varieties with their taxonomic classification and country of origin. Fig.1 represents the average shoot and root dry weight, and average shoot and root lengths of both acclimatized and non-acclimatized varieties. Tables 2 and 3 represent the original data sets of dry weight and length of shoots and roots of both acclimatized and non-acclimatized varieties.The experiment was conducted in a glasshouse at Hiroshima University, Japan.
The conditions of the glass house were 55% humidity, 19–27 °C day/15–20 °C night temperature and natural sunlight. Seeds of twenty-nine rice varieties selected from the World Rice Core Collection were initially heat-sterilized at 60 °C for 10 min in a water bath, then surfacesterilized using 5% sodium hypochlorite solution for 30 min, and finally rinsed thoroughly with distilled water. The seed germination process, seed transfer to Kimura-B nutrient solution, and the composition of the Kimura-B solution are provided in a related research article. The nutrient solution was changed every 3 days, and the pH was maintained daily between 5.0–5.5. Three sets of four seedlings from each variety were maintained throughout the experiment. One set received only the Kimura-B nutrient solution. In the second set, 1-week-old seedlings grown in the Kimura-B nutrient solution were acclimated with 1 mM NaCl for 2 weeks and then exposed to 50 mM NaCl for the next 2 weeks. In the third set, hydroponically grown plants were directly subjected to 50 mM NaCl during the third week of growth and maintained for the next 2 weeks. The seedlings were harvested at the end of the salinity treatment. After harvest, the roots were thoroughly rinsed with distilled water and then gently dried with a paper towel.For dry weight determination, each seedling was divided into leaves, sheaths, and roots, which were then oven-dried at 70 °C for 3 days before being weighed. Shoot dry weight was calculated by combining the dry weight of the leaves and sheaths.Climate change influences crop production worldwide due to longer and more unpredictable periods of drought stress. Drought stress occurs because of reduced water availability in the soil and enhanced water loss through evapotranspiration processes caused by atmospheric conditions. It is one of the most severe factors affecting plant growth and yield in agricultural crops. During periods of stress, plants are influenced at different scales ranging from phenological to morphological and molecular levels.
In such cases, overall plant growth is reduced, where shoots are more inhibited in growth compared to roots. Moreover, photosynthetic processes are affected by impaired assimilate transport of sugars and amino acids to the plant parts where they are needed,leading to an accumulation of these osmotically active solutes in plant tissues. An increase in organic or inorganic solutes lowers the osmotic potential, improving cell hydration to maintain several metabolic processes under drought stress conditions. However, the severity of drought stress depends on the intensity and duration of the stress event and which plant species are affected. Potatoes are known to be sensitive to drought stress and water deficit due to their poorly developed root system. According to Hijmans,the global yield potential of potatoes could decrease by 18–32% between 2040 and 2069 due to drought stress. However, drought stress effects differ in developmental stages during plant growth. Tuber initiation and bulking are the most sensitive stages, while plants at maturity level are more tolerant to drought stress. During tuber formation, optimal temperatures between 15–20 ◦C and soil moisture above 65% favour tuber initiation while higher temperatures and lower soil moisture can reduce tuber formation and yield. Consequently, regulating plants’ water supply is particularly important, which can be improved especially by an adequate supply of potassium. K is the most abundant inorganic cation in plants and is well described as an osmotic substance for maintaining cell turgor during drought stress. It regulates stomatal opening and closing, and therefore, water loss through transpiration. In addition, sufficient K supply positively affects phloem loading and consequently assimilate transport, e.g., sugars, from source to sink. For most crops, tissue concentrations of K ranging from 5 to 40 mg g− 1 DM are considered adequate. According to Sharma and Arora,the critical K content in leaves of potatoes varies between 3.69 and 5.15%, which is necessary to achieve 95–100% of the yield maximum and refers to the fourth leaf from the top. However, this range depends on the leaves harvested and the maturity of the plants.
The availability of K in plant cells strongly influences a wide range of enzymes for primary metabolism which are related to protein and carbohydrate synthesis. Furthermore, K is important for plant growth and improves crop productivity by increasing yield. However, the functions of K in osmotic regulation can be replaced by organic acids, amino acids or sugars, whose concentrations increase under K deficiency. Nevertheless, such production of compatible solutes is more energy-intensive compared to the accumulation of K in plant tissues. Adaption mechanisms to low K supply include a wide set of mechanisms, e.g., the reduction of plant growth for maintaining cell functions due to adequate tissue K concentrations or the redistribution of K to developing plant part. However, even under low K supply, plants can survive because of specialised high-affinity K transporters which were first identified and described for plants in the model plant Arabidopsis. Maintenance of cellular K homeostasis is the most important function of the high affinity K+/K+ uptake/K+ transporter family,and they are triggered under low K supply. Several transporters are described in Arabidopsis, but only limited information is available for potato plants. In particular, vertical grow tables the expression level of these genes under drought stress or K supply is still scarce and needs to be further explored. The positive effect of K on plants exposed to drought stress affects many metabolic processes in plant development. It has been shown on different plant species that K addition increased biomass partitioning to roots, and enhances leaf membrane stability. Moreover, K regulates the photo-assimilation and translocation processes of carbohydrates together with related enzyme activities. In addition, K increases the abscisic acid concentration in the leaves, thus reducing the transpiration of plants. Measuring the effects of drought stress on plants with differing fertilisation regimes on a field scale is challenging. For this reason, controlled systems offer a suitable approach to exclude external factors and provide detailed insights into physiological and metabolic processes within a plant.
To simulate drought stress, in hydroponic systems, polyethylene glycol can be used to induce osmotic stress conditions in plants by promoting water deficiency and imitating soil drying. The hypothesis that sufficient K supply to the plant under drought stress conditions induced by PEG mitigates the adverse effects by triggering various adaption mechanisms on several plant levels such as on phenological, morphological and molecular level was the basis for this study. The specific objectives were to investigate the effects of osmotic stress, induced by PEG, on biomass production, water consumption, mineral and sugar allocation in potato plants under low and sufficient K supply, to evaluate changes in metabolites and gene expression levels in leaflets before, during, and after drought stress simulated by PEG-induced osmotic stress under low and sufficient K supply, and to determine indicators of adaption strategies by assessed parameters. The investigations were carried out using a hydroponic experiment with PEG 6000 to induce osmotic stress, with the two cultivars, Agria as tolerant and Milva as susceptible to drought stress. Leaf and whole plant analyses were performed to obtain insights into the stress responses of the two cultivars and to gain further information on adaptation mechanisms.Medium-early potato cultivars Milva and Agria were selected according to their drought stress tolerance. Agria is specified as a tolerant cultivar,whereas Milva is described to be more sensitive to drought stress. At first, seedlings of potato tubers were grown in 3 L pots filled with quartz sand at a sufficient K supply and low K supply. All other nutrients were added in adequate amounts. At 28 days after planting,the plants were transferred to 5.5 L pots filled with nutrient solutions,according to Koch et al.. After transferring the plants to the hydroponic system, the nutrient concentration was gradually increased every 3 days until the desired nutrient level was reached at 35 dap. The nutrient solution was constantly aerated. In addition, K was applied at low levels and sufficient levels. Twenty biological replicates of each cultivar were grown, using 10 plants for each of the two different K supply regimes. Nutrient solutions were changed twice a week until 53 dap. During the last weeks of the experiment,nutrient solutions were changed once a week. At 60 dap, osmotic stress was induced by adding 8% polyethylene glycol 6000 to half of the pots for each cultivar and K supply to simulate drought stress.