The present study was carried out to determine the response of some elite cotton cultivars to muriate of potash and sulphate of potash. Parallels were drawn between the resistance/susceptibility of cotton genotypes to cotton leaf curl virus (CLCuV) disease at different levels of two forms of potassium (K) fertilizer. The treatments consisted of (a) four cotton cultivars: [(NIAB-Karishma- moderately resistant to CLCuV), S-12 (highly susceptible to CLCuV), CIM-1100 and CIM-448 (both highly resistant to CLCuV)]; (b) four potassium fertilizer rates (0, 62.5, 125.0, 250.0 kg K ha-I), and (c) two potassium fertilizer sources [sulphate of potash (K2S04) and muriate of potash (KCl)]. These cultivars were selected on the basis of tolerance rating to CLCuV disease and detection of geminivirus by Polymerase Chain Reaction (PCR) technique.
A pot experiment was conducted to determine response of K -nutrition to the infestation of CLCuV. There were significant differences among cultivars in terms of incidence and intensity of CLCuV disease occurence. The cultivars CIM-448 and CIM-1100 showed complete resistance to CLCuV, whereas cv. S-12 was highly. susceptible and cv. NIAB-Karishma moderately tolerance to the disease. There was 12 to 38% reduction in the disease incidence due to application of 250 kg K ha-1. The incidence and intensity of CLCuV disease were little affected due to both sources of K-fertilizer. The intensity of CLCuV disease decreased with advancement in age of the plant and concurrent increase in K-fertilizer levels. The CLCuV susceptible cultivars (NIAB-Karishma and S-12) maintained lower K+ concentration in the leaf tissues during the flowering stage compared with resistant cultivars, CIM-448 and CIM-I100. Crop receiving K-fertilizer in the form of K2SO4 maintained higher K+ concentration in the leaf tissues than that receiving KCl.
The physiological parameters of cotton plant in response to K-nutrition were studied under field conditions. The various morpho-phenological events based on degree day (DO) and heat units (HU) differed significantly due to cultivars and K-fertilizer rates. The application of 250 kg K ha-I caused the crop to mature one week earlier than that of unfertilized crop. Degree-days had negative but significant correlation with varying levels of K-fertilizer. Various product quantity measures of earliness were greatly improved by the addition of K-fertilizer. The crop growth development parameters, viz., net assimilation rate (NAR), crop growth rate (CGR), relative growth rate (RGR), and leaf area ratio (LAR) showed positive relationship with varying levels of K rates.
Potassium nutrition had a profound influence on partitioning of dry weights and it caused greater accumulation of dry weights in the reproductive organs than in the vegetative ones. Crop fertilized with 250 kg K ha-1 maintained higher reproductive vegetative ratio (RVR) compared to zero K-rate plots. The accumulation of dry weights pattern formed a sigmoid curve. The main stem height increased significantly with concurrent increase in K-levels. There was a close association between main stem height and total dry weight, number of total fruiting positions and number of total retained fruit per unit land area.
Addition of K2S04 caused an increase in the length of sympodia thereby retaining more number of fruit on nodal positions along sympodia compared to KCl treated crop.
Net CO2 assimilation rate (PN) and transpiration rate (E) were markedly increased with concurrent increase in K-rates. Addition of 250 kg K ha-I increased PN by 75.9% compared to K-unfertilized plot. Stomatal resistance and sub-stomatal CO2/ambient CO2 ratio (ci/ca) values decreased with concomitant increasing levels of K nutrition. Leaf water potential (Ïˆw) and osmotic potential (Ïˆw) decreased significantly due to differential K-levels. There was a positive correlation between (Ïˆw) and number of intact fruit m-2. Turgor pressure (Ïˆp) increased linearly with increasing levels of K-fertilizer. Increase in Ïˆp resulted in significant improvement in seed cotton yield. Water use efficiency (PN/E) was greatly improved by K-fertilization.
The seed cotton yield, number of bolls plant-1 and boll weight were significantly increased by K-nutrition. Addition of 250 kg K ha-1 increased seed cotton yield by 37 percent than that of zero K-rate treatment. Cultivars responded differently at various K levels. Cultivar CIM-448 produced 64.5 percent higher seed cotton yield than that of cv. S-12 under zero K-rate treatment. The application of K2SO4 showed an edge of 5 percent over KCI in terms of seed cotton yield. Number of bolls plant-1 and boll weight were main determinants to increase seed cotton yield, which were markedly affected by K nutrition. An economic analysis showed that addition of K-fertilizer beyond 125 kg K ha-I was not cost effective.
Physical characteristics of cotton fibres, viz., fibre length, fibre fineness, fibre strength, fibre elongation, fibre maturity ratio, fibre uniformity ratio and colour grades [in terms of reflectance degree of fibre (Rd%) and yellowness degree of fibre (Hunter's +b)] in various cultivars were significantly improved by K-nutrition. The addition of 250 kg K ha-1 improved fibre length (4.9%), fibre fineness (18.7%), fibre elongation (11.8%) compared to K-unfertilized treatment. Potassium nutrition improved grades of cotton lint with respect of more whiteness in colour and reducing fibre yellowness. Potassium fertilization in the form of K2SO4 showed an edge over KCI in improving fibre quality parameters.
The interaction of potassium nutrient with other ions showed synergistic and antagonistic effects in the cotton plant. Addition of K-fertilizer caused a significant increase in K+ concentration in various organs, viz., leaves, stems, burs, seed, and lint compared to K-unfertilized treatment. Moreover, application of K-fertilizer caused increased absorption of N with simultaneous higher assimilation of K+ by various plant organs. Phosphorus content remained relatively constant under varying K levels. However, concentration of calcium (Ca2+), magnesium (Mg2+), and sodium (Na+) were reduced with concomitant increased concentration of K+ in the whole plant urlder differential K-rates. Addition of K-fertilizer in the form of KCl or K2SO4 increased the concentration of their accompanying anions, i.e., chloride (Cl-and sulphate-sulphur (S02-4-S) ions, respectively.
Analysis of the soil experimental site showed that there was a two fold increase in the exchangeable-K in the 0-30 soil profile by the addition of 250 kg K ha-1 compared to zero K-rate treatment. The exchangeable-K decreased in all soil depths in various proportions in relation to different K-rates beyond 30-60 cm depth. The decrease in exchangeable-K in the lower depths were attributed to uptake of 96.2 kg K ha-1 by crop receiving 250 kg K ha-1. Addition of 250 kg K ha-1 in the form of K2SO4 raised the level of SO2-4-S from 15.5 to 22.3 meq L-1 in 0-30 cm depth and its values increased with soil depth. Addition of 228 Cl kg ha-1 (coming from 250 kg K ha-1 as KCI) raised the level of cl- from 4.9 to 7.2 meq L-1 in the upper 0-30 cm profile. These studies show that continuous application of KCI may lead to a certain accumulation of cl- in the soils. This is more particularly in the arid regions, where rainfall is scanty and insufficient to leach down cl- to the lower horizons.
Application of K-fertilizer in the form of K2SO4 had an edge over KCI in terms of crop growth and development. The increase in seed cotton yield with concurrent improvement in fibre quality parameters resulted due to addition of K2SO4 compared to KCl. The significant interaction between K-doses and sources implies that PN was greatly affected by K-fertilizer in the form of K2SO4 compared to in the form of KCl. The increase in PN /E and Ïˆp due to addition of K2SO4 resulted in significant improvement in seed cotton yield. Addition of K-fertilizer in the form of K2SO4 and to form KCI caused an increase in the uptake of K+, N, Cl- SO42--S with concomitant decrease in K+, P, Ca2+, Mg2+, and Na+ nutrients. Addition of K2SO4 and /or KCI raised the level of their accompanying anions in the soil. The CLCuV disease was little influenced due to K sources.
Cultivars differed significantly amongst themselves in response to K-nutrition. Cultivar CIM-448 excelled among the cultivars in terms of seed cotton yield, better fibre quality parameters, efficient in utilization of thermal energy, maximal in attaining growth and development parameters, partitioning of higher dry weights from vegetative to reproductive organs, capability of higher production of fruiting bodies and retaining more number of bolls on the three first positions along sympodia. Moreover, cv. CIM-448 ''vas efficient in utilizing nutrients fr0111 natural reserves in higher quantity compared to other cultivars. Overall, K2SO4 proved to be a good source of potash fertilizer as compared to KCI for cotton crop under arid and semi-arid conditions.