Effects of Potassium Deficiency on Root Growth of Cotton Seedlings and Its Physiological Mechanisms

For the purpose of dissecting the mechanism of root growth in response to potassium (K) deficiency in cotton (Gossypium hirsutum L.), young seedlings of NuCOTN99B grown in half-strength modified Hoagland's solution with low K nutrient (0.05 mmol L−1) were investigated for the root configuration, content of endogenous free indole acetic acid (IAA), and amount of ethylene released from the roots 4 d after treatment. Compared with the treatment with moderate K nutrient (0.50 mmol L−1, control), the K deficient treatment significantly inhibited root length and the formation of lateral roots. The reduced lateral roots mainly resulted from the shortened branched root zone, and there was no change in the lateral root density. Under K deficient condition, the greatest reductions for root length, total root surface area, and root volume occurred in fine roots (0.05 mm ≤ diameter < 0.20 mm), followed by the coarse roots (diameter [.tau] 0.45 mm) and the middle roots (0.25 mm ≤ diameter < 0.45 mm). The fine roots were more important in nutrient uptake than the middle and the coarse roots. Thus, the K starving damage was greater in cotton seedlings than the growth inhibition of roots. When the cotton seedlings exposed to K deficient media for 4 d and 10 d, the total root length and the total root surface area were 35.7–38.0% and 47.7–50.6% of the values of the control plants; whereas the K accumulation was approximately 25% and 16% to the control values, respectively. As expected, the endogenous free IAA content in the roots grown in K deficient media reduced by 50%, whereas the amount of ethylene released from roots increased by nearly 6-fold, which partially explained the inhibition of lateral root formation and root elongation by K deficiency.