Among the plant nutrients potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of plant structure but also plays regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, enzyme activation. There are several physiological processes like stomatal regulation and photosynthesis are dependent on K. In the recent decades K was found to provide abiotic stress tolerance. Under salt stress, K helps in maintaining ion homeostasis and regulation of osmotic balance. Under drought stress condition K regulates the stomatal opening and makes the plants adaptive to water deficit. Many reports provided the notion that K enhances the antioxidant defense in plants and therefore, protects the plants from oxidative stress under various environmental adversities. Also, it provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although a considerable progress in understanding K-induced abiotic stress tolerance in plants has been achieved the exact molecular mechanisms of such protections are still under research. In this review, we summarized the recent literature on the biological functions of K, its uptake, and translocation and its role in plant abiotic stress tolerance.

Abstract:The eddy covariance technique was used to measure surface energy balance and evapotranspiration in a double-cropping paddy field in Mymensingh, northern Bangladesh (24Ð73°N, 90Ð42°E, 18 m above sea level). Because irrigation in dry season and frequent rainfall in the summer monsoon season kept the field flooded most of the year, most of the radiant energy was converted to latent heat flux except for the drained fallow period in winter. The ratio of the latent heat flux to the available energy, or evaporative fraction (EF ), was 71-74% on average for two growing seasons and the flooded summer fallow period in rainy season. The evapotranspiration rate (ET ) ranged from 0Ð2 to 6Ð6 mm d 1 . The seasonal accumulated ET from transplanting to harvest in dry-season rice (Boro rice) and wet-season rice (Aman rice) was 370 and 307 mm with seasonal average of 3Ð3 and 2Ð9 mm d 1 , respectively. In addition, 283 mm was lost as ET during the flooded summer fallow period, which characterized the seasonal variation of ET in this study site. The canopy conductance (G c ) showed a wide range from 1Ð0 to 20Ð2 mm s 1 , but larger G c values were found at higher leaf area index (LAI) periods for both Boro and Aman rice. G c was closely related to the ratio of ET to equilibrium evaporation (E eq ), suggesting seasonal variability of ET is control by E eq and G c . Partitioning of ET by applying an empirical equation indicated that about 70 and 64% of seasonal ET came from transpiration in the Boro rice and the Aman rice, respectively.

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Incorporation of polyunsaturated fatty acids (PUFAs) as biofunctional compounds with feed is an effective way for gonadal maturation without any hazardous effects on animal health, and thus it is possible to save the vulnerable species from the danger of extinction. In the present study sperm quality, level of Ca(2+) concentration in serum, histological structure of the liver and developmental stages of ovary of an endangered fish species, Nandus nandus were investigated for the confirmation of the positive effects of PUFAs in reproduction and gonadal maturation. Fishes were collected from Brahmaputra River, Mymensingh, Bangladesh. Treated group was fed 1% squid extracted phospholipid supplemented diet that was mixed with silver carp fish muscle where as controlled group was fed the same except phospholipid. For histology of liver and gonads, samples were dehydrated, cleaned and infiltrated, embedded in paraffin wax and sectioned. After that, the samples were stained with hematoxylin and eosin. The photomicrographs of the stained samples were taken by using light microscope. In comparison with the control group, treated group exhibited higher gonadal maturation which resulted in spontaneous spawning. Treated female demonstrated advanced gonadal developmental stages in comparison with the controlled female during different months. During spawning season, lipid granules and normal morphological alteration were observed in case of treated fish liver, whereas less lipid granules with more histological alteration of liver were observed in control group. Serum Ca(2+) concentration in treated female was found significantly higher (P < 0.01) in contrast to the controlled female during the breeding season which was an indicator of the augment of estrogen secretion during ovarian maturation. Better sperm quality, early maturation of oocytes, less histological alteration of liver hepatocytes and spontaneous spawning performances of PUFA-treated fish were as a result of the efficiency of PUFAs in enhancing maturation. The experiment suggests that supplementation of dietary PUFAs improve the spawning performances of fish.