Effect of Potassium Deficiency on Root Growth of Cotton ( Gossypium hirsutum L.) Seedlings and Its Physiological Mechanisms Involved

Abstract

Premature senescence caused by potassium (K) deficiency has been an important limiting factor in cotton production in China, and K uptake ability of plant from media has a close correlation with root growth and development. In the present study, cotton cultivar NuCOTN99B was sowed in sand in growth chamber, uniform cotton seedlings were transferred to 1/2-strength modified Hoagland’s solution with low (0.05 mmol L -1) or moderate (0.50 mmol L -1) K + at 4 d after germination to investigate the effect of K levels on the root system indicators, endogenous free indole acetic acid (IAA) content and ethylene amount released from the cotton seedlings root. The results showed that the lateral root formation of seedlings grown in low K solution for four days was significantly inhibited by about 20%. After 10 d treatment, root elongation was also significantly reduced. Furthermore, we found that the reduction of lateral root mainly resulted from short length of branched root zone, and there was no change in the density of lateral root under lower K. In addition, root system was classified into fine roots (0.05-0.20 mm), middle roots (0.25-0.45 mm), and coarse roots (> 0.45 mm) according to root diameter. Total root length and total root surface area of cotton seedlings grown in low K medium for four days reduced by about 60%, and the fine root was inhibited severely, the coarse roots moderately, and the middle roots slightly. For example, the root length, root surface area and root volume of fine roots grown in low-K medium for 4 and 10 d were only approximately 10% and 25% of those in moderate-K medium, respectively. Additionally, only fine roots were constantly inhibited by low-K during treatment period in terms of the ratios of different diameter’s roots to total roots, which resulted in the fact that the magnitude of K deficiency in cotton seedling was higher than that of inhibition of root growth, because the uptake activity in fine roots is higher than that in middle and coarse roots. For example, the accumulated K of seedlings grown in low-K solution for 4 d and 10 d were 25% and 16% of that grown in moderate-K solution, respectively; whereas the total root length and total root surface area were 35.7-38.0% (4 d) and 47.7-50.6% (10 d). As expected, the endogenous free indole acetic acid (IAA) content in roots grown in low-K medium was reduced by 50%, whereas the amount of ethylene release had nearly six-fold increase, which might provide an explanation to some extent for the inhibition of lateral root formation and root elongation by potassium deficiency.