Similar results were found by Koch et al.. Nevertheless, the K content, even in leaves of +K plants, was below the range of 5–40 mg K indicating an adequate supply of K according to White and Karley. However, since the percentage of K in the DM decreased significantly during growth, this value may not be suitable for predicting K deficiency. Thus, a clear indication of an optimal K concentration should be defined for each growth stage and crop species. Chlorophyll, being an important pigment to absorb light energy,was not affected by K supply. In studies by Armengaud et al. on Arabidopsis and by Veazie et al. on Brassica rapa, the chlorophyll content was not affected by K deficit. Presumably, K was directed into the older leaves or side shoots to maintain chlorophyll production by activating specific enzymes. However, several studies revealed a decreased chlorophyll content in leaves of plants under low K supply,which was demonstrated by the decreased photosynthetic efficiency. Similarly, the effect of PEG-induced osmotic stress did not affect the chlorophyll content. However, both chlorophyll a and b content decrease because chlorophyll synthesis is inhibited by water deficit,hydroponic grow system which was obviously not severe in our study.
Generally, the chlorophyll content varies strongly in potato leaves, depending on the cultivar, time of drought stress induction and leaf development. The decrease in chlorophyll content was caused by reactive oxygen species that are formed as a response to drought stress damaging chloroplasts. This was not demonstrated in our study, perhaps because of the short osmotic stress period. It is also likely that the stress situation occurred at a time when the plants were already fully mature, and therefore less susceptible to abiotic stress. To protect cellular structures and maintain important physiological processes, plants can accumulate low-molecular-weight compounds,which are involved in osmotic adjustment during stress conditions. These include amino acids, measured as free amino acids in leaflets in this study, before, during, and after PEG-induced osmotic stress. In both cultivars at each sampling day, K deficiency increased the free amino acid concentration. This effect might be related to the substitution of K by organic solutes. An increased concentration of organic solutes, such as amino acids, lowers the osmotic potential of plant cells, improving cell hydration and therefore cell turgor to maintain physiological and metabolic processes, which enhance plant growth under stress conditions. Interestingly, the free amino acid content of leaves during PEG induction was not affected by PEG addition in +K plants but reduced in -K+PEG plants compared to -K plants in our study.
A sufficient K supply enhanced amino acid accumulation during PEG-induced osmotic stress compared to that at a low K supply. However, the K contents in the leaves increased during the same period. In accordance with Shabala and Shabala,under K deficiency and PEG-induced osmotic stress, K ions were distributed to developing tissues and accumulated in favour of osmotic compounds since production of these compounds is metabolically more expensive for plants.To characterise the metabolic responses of the plant during stress mitigation under different K supplies, metabolite profiling was used to investigate changes in the concentrations of primary and secondary metabolites. In the metabolomic pathway for Milva, single amino acids showed a higher concentration in -K plants compared to +K plants, which was also shown by higher concentrations of free amino acids in -K leaflets. This increase was even higher when comparing -K+PEG to +K+PEG plants. In some plant species, amino acids are known to increase during drought stress or K deficiency, which was already explained in the previous section. Proline increased under PEG-induced osmotic stress in -K+PEG plants compared to +K+PEG and -K plants. Proline concentrations are frequently considered a metabolic stress indicator for several stress factors and its accumulation is often associated with tolerance mechanisms against abiotic stress. Many studies have confirmed that proline accumulates in plant leaves for osmotic adjustment. In this regard, K deficiency alone did not increase the concentration of proline in the sample in our study, which indicates the importance of the proline accumulation under K deficiency when at the same time leaves are exposed to osmotic stress.
Furthermore, the amino acids isoleucine, leucine, and valine accumulated, which has been shown in Arabidopsis thaliana plants exposed to drought stress and was caused by abscisic-acid regulated protein degradation. These amino acids belong to branched chain amino acids formed by pyruvate or aspartate. They are involved in abiotic stress response and act as signaling molecules further regulating gene expression. The increase in concentration of these branched chain amino acids might therefore reflect a response mechanism that is triggered under K deficiency with more pronounced effects when K deficient leaves are exposed to osmotic stress. Another branched chain amino acid is alanine, which is also formed by pyruvate and its accumulation was reported to occur under PEG-stress. Furthermore, Lea et al. could show that asparagine and lysine accumulate in plant tissues under different stress conditions, revealing stress responses due to K deficiency and PEG, which was also shown in our study. However, the concentrations of amino acids vary strongly due to the stress effect,since individual amino acids react differently to various factors. Therefore, it is difficult to compare studies determining amino acids because of the differences in growth and environmental conditions. Responses of the mentioned free amino acids in potato leaves might provide information related to secondary metabolites of interest.
Accumulations were also observed due to -K for some organic acids involved in the TCA cycle, including α-ketoglutarate and succinate, whereas PEG led to a reduction in both K treatments. In rapeseed, Zhu et al. showed an increase in α-ketoglutarate and succinate under low K and drought stress conditions. Contrasting results were found by Das et al. in soybeans, where α-ketoglutarate and succinate decreased. The generally low accumulation of organic compounds and higher contents of TCA intermediates could indicate enhanced respiratory rates and energy metabolism. However, this supports the presumption that K accumulation as an osmotic adaptation against drought stress, is significantly more energy efficient than the formation of organic solutes. K is transported into young leaves via the phloem during stress to fulfill the osmotic function of other solutes. This could explain the lower sugar content in the youngest fully-developed leaves and the increased sucrose content in the -K+PEG plants on the whole plant level, at least for cultivar Milva. However, differences in metabolite profiles vary strongly depending on the type of stress, duration, severity, and cultivar.For potatoes, there is less information on K transporters and how they respond to nutrient deficiencies and abiotic stress. The analysed K transporter genes belong to the HAK/KUP/KT family, which have different functions in K uptake and transport. In addition, they play a role in stress tolerance and osmotic potential regulation. The expression of related genes is often triggered by a low K supply, enhancing the overall K uptake. The genes of this family are expressed in different plant parts, e.g., in the leaves, roots, or stems, which indicates a connection with the primary uptake of K from the soil, as well as with cellular K homeostasis.
The transporters studied—StPOT2, StPOT4, and StPOT12—correspond to homologous genes of Arabidopsis thaliana, where they are referred to as AtKUP2, AtKUP4, and AtKUP12, respectively. They belong to different clusters in which the various transporters can be subdivided according to their sequence homology. AtKUP2 and AtKUP4 are assigned to cluster II and described as low-affinity K transporters involved in cell expansion. The transporters of these clusters have many homologous transporters from other plant species and are described in more detail. However, AtKUP12 is assigned to cluster III and has rarely been studied. Our results showed different expressions of the three transporters depending on K supply, PEG-induced osmotic stress, and cultivar. StPOT2 and StPOT4 were more highly expressed in Milva and showed greater expression rates in -K plants at 55 dap. StPOT expression in Agria decreased at 69 dap. The expression rates of all K transporters increased again for all treatments and both cultivars during the recovery phase. Due to the fact, that the plants were already mature and had almost completed their growth cycle, the demand for K in young tissues was no longer as high as at the beginning of plant growth, which could explain the low expression levels of all the three StPOTs in Agria. Thus, Milva was not yet fully matured and was still taking up K. Accordingly, StPOT2 and StPOT4 were still active in Milva since both are related to growth processes. Compared to Agria, higher expression rates of the three StPOTs in Milva could indicate differences in the involved genes, which may vary genotypically. StPOT2 in potato and the homolog gene AtKUP2 in Arabidopsis, as well as AtKUP6 and AtKUP8, play a role in lateral root development by enhancing abscisic acid and responding to osmotic stress. In contrast, AtKUP4 is involved in growth processes by participating in auxin transport, indoor garden which in turn contributes to root hair development. These two transporters are also known to increase their expression more in roots than in leaves, which may explain the lower expression rates in our study.
Increases in the three StPOTs at 82 dap could be related to the increased water uptake of plants in the recovery phase, because more K was available, and therefore, the expression of the three StPOTs increased again. Our results showed that StPOT12 reacted similarly to StPOT2 and StPOT4, but showed higher expression rates in -K Milva plants at 55 dap and up to eight-fold higher expression at 82 dap. Therefore, protein synthesis for StPOT12 may be enhanced, participating more in K uptake compared to proteins of StPOT2 and StPOT4. Zhang et al. also showed a connection between AtKUP12 and increasing K uptake in addition to improving the salt tolerance of Arabidopsis. Therefore, StPOT12 is also involved in abiotic stress. However, the detailed physiological functions remain unclear, and further investigations are needed.Rice, a staple food for more than three billion people worlds over belongs to the genus Oryza of the family Poaceae. Consisting of two cultivated species, O. sativa and O. glaberrima, and 22 wild species, Oryza is a large genus of predominantly tropical aquatic or semi-aquatic grasses. O. sativa is grown worldwide, while O. glaberrima is mostly confined to West Africa. Domesticated since early Holocene,rice cultivation has led to cultivar diversity through phenotypic selection for desirable characters such as grain yield and grain quality. Being widely cultivated, adaptive evolution of rice cultivars over thousands of years has distributed them over wide range of environs, such as deep water to montane ecologies. Being one of the primary centers of origin, India is bestowed with wide diversity of rice landraces, wild congeners and modern cultivars.Estimated between 75000 to 100000, the number of indigenous landraces in India has shrunk over time, although a considerable portion is still conserved for a variety of reasons. In areas of conservation, farmers cherish profound knowledge of landraces such as their uses, properties and peculiarities. In Tamil Nadu, popularly called as the ‘rice granary of South India’, there were about 400 traditional landraces in vogue since olden times, which are still extant and used in households. These landraces brandish several features such as pest and disease resistance and tolerance to flood,drought and salt. Some others are suitable for special uses such as fodder and thatching,stamina boosters and for medicinal uses. Understanding their importance, most of these landraces are now conserved in gene banks across the India, so that they are not destroyed due to climatic vagaries and catastrophes. The practicality of conservation of specialty rice is particularly realized during disasters such as tsunamis, floods, cyclones, etc. when conventional cultivars fail to save the situation. For instance, when the coastal Tamil Nadu was devastated by the Indian Ocean tsunami on 26th December 2004, the salt water ingression from the bay has destroyed most of the standing crops in the affected region.However, soil salinization continues to happen even without disasters along the coastal as well as inlands of India, due to poor quality of irrigation water. Globally, rising salinization limits rice production significantly in those areas, which according to an estimate, accounts for about 33% of irrigated land.