Potassium use efficiency in barley (Hordeum vulgare L.) genotypes under salt stress conditions

The growing global human population and the expansion of agriculture into less productive areas result in an increased demand for fertilisers. At the same time, sustainability of agricultural practices and increasing cost of fertilizers both call for reduced input and a use of genotypes with increased fertiliser use efficiency. This trend is applicable to all major fertilisers including potassium (K\\(^+\\)). Potassium is an essential macronutrient for plants and plays a crucial role in growth, development, yield, quality, quantity and stress resistance of all plant crops. The use of K\\(^+\\)-efficient genotypes along with optimised soil fertilisation would be important to develop optimal nutrient management strategies for sustainable farming systems. High potassium-use efficiency (KUE) genotypes can tolerate nutrient deficiency because they have adapted physiological mechanisms that enable access to adequate levels of specific nutrients (uptake efficiency) and use that nutrient effectively (utilisation efficiency). The issue of KUE is further exacerbated by the fact that K\\(^+\\) biological availability is severely affected by hostile environmental conditions. One of them is soil salinity. Salinity affects 10% of the earth's land surface and around 50% of all irrigated land, leading to lost agricultural production in excess of $27.3Bln per annum. At the same time, K\\(^+\\) nutrition, both directly and indirectly, affects plant resistance to a broad range of abiotic and biotic stress. The causal relationship between KUE and salinity stress tolerance, nonetheless, remains elusive. In this project, we used a wide range of barley (Hordeum vulgare L.) genotypes contrasting in salinity stress tolerance to understand mechanisms conferring KUE and understand how these are affected by the presence of high NaCl concentrations in the rhizosphere. Thirty barley genotypes, originating from Australia, China, USA and Japan were evaluated in a randomized block design with four levels of K\\(^+\\) (0.002 mM, 0.2 mM, 2 mM, and 20 mM) in three replicates, for phenotypic and physiological variation in K\\(^+\\) efficiency in shoot growth and grain yield. A subset of genotypes with contrasting KUE identified in the first experiment (highly efficient varieties YF374, Skiff and Yan 89110; and genotypes with low KUE - Dayton, Dysyh and Franklin) was used in the second experiment. Here, plants were grown at two potassium levels (low, 0.002 mM; and high, 20 mM) under two sodium levels (no salt and in the presence of 300 mM NaCl) for five weeks. The results showed that the availability of K\\(^+\\) in the soil had a major effect on yield components, i.e. spike number, grain number and grain weight, in most of the studied genotypes and the increase in grain weight in the response to K\\(^+\\) application was correlated with an increase in numbers of spikes and grains. Although an increase in K\\(^+\\) supply led to an increase in plant height in all genotypes, the extent of response differed significantly between genotypes. K\\(^+\\) availability did not increase tiller number in barley and 0.02 mM K\\(^+\\) would be the threshold of deficiency for tiller number for most genotypes. The positive effect of K\\(^+\\) on yield components, plant height and plant dry weight might be due to the role of K\\(^+\\) in activating protein synthesis and improved enzymatic and photosynthetic activity, which shifts assimilates to sink and produce more grains with heavier weight. K\\(^+\\) supplementation caused only a small increase in plant dry shoot, which indicated that shoot dry weight alone cannot be used as a suitable selection criterion to detect tolerance of K\\(^+\\) deficiency. It might be due to the fact that K\\(^+\\) utilization influenced the translocation of dry assimilates into grains and caused a reduction in dry biomass. This variable response to low K\\(^+\\) exhibited by these genotypes could be because of the difference in their ability to absorb K\\(^+\\) and translocate it by K\\(^+\\) transporter channels in the high and low-affinity uptake systems. K\\(^+\\) availability led to an increase in K\\(^+\\) uptake and leaf K\\(^+\\) content. Elevated leaf K\\(^+\\) content correlated with higher xylem K\\(^+\\) content, pointing out at essentiality of xylem K\\(^+\\) loading as a key trait conferring KUE. The leaf Na\\(^+\\) has an opposite trend to leaf K\\(^+\\). The high performing plants showed the lowest leaf Na\\(^+\\) and the lowest yielding plants showed high leaf Na\\(^+\\). This indicates that plants that are not capable of accumulating enough K\\(^+\\) instead rely on Na\\(^+\\) to partially replace Na\\(^+\\) with K\\(^+\\) for opening and closing of stomata, which helps to regulate internal water balance. The result of the second experiment showed that the application of K\\(^+\\) alleviates the adverse effect of salinity, significantly improved the yield, and yield components, fresh and dry weight. Under salinity stress, genotypes Dayton, Dysyh and Franklin did not produce any grain regardless of the amount of K\\(^+\\) applied (except Dysyh that produced 0.02 g/plant in high K\\(^+\\)), but genotypes with high KUE were doing well under saline conditions. Salinity stress caused an increase in leaf osmolality and a decrease in stomatal conductance. Leaf chlorophyll content (SPAD readings) was adversely affected by salinity; increased K\\(^+\\) availability reverted this effect. The overall outcomes of this work are three-fold. First, varieties with high KUE (Gebeina, Skiff, YF374, and Flagship) may be recommended to growers. Second, contrasting varieties selected in phenotyping experiments can be used to create DH lines to understand the genetic basis of KUE. Finally, this project provided further insights into mechanisms underlying KUE and plant responses to salinity. While a competitive interaction between the K\\(^+\\) and Na\\(^+\\) and their transport into plant parts exist, Na\\(^+\\) uptake showed a strong dependence of KUE, with K\\(^+\\)- efficient cultivars being more responsive to reduce Na\\(^+\\) uptake under K\\(^+\\) supply along with greater increases in K\\(^+\\) uptake compared with K\\(^+\\) inefficient cultivars. In the current study, application of salt stress promoted the uptake of Na\\(^+\\) in plant leaves and xylem cells; however, the application of K\\(^+\\) reduced its uptake in plants and promoted the accumulation of K\\(^+\\) contents in plant leaves and xylem cells. This study also confirmed that application of supplemental K\\(^+\\) could significantly ameliorate the toxicity of Na\\(^+\\) to promote plant growth in soil-affected soils.