Conditions and Indicators for Screening Cotton (Gossypium hirsutum L.) Varieties Tolerant to Low Potassium

To establish a screening system for evaluating the tolerance of cotton (Gossypium hirsutum L.) varieties to low potassium (K), 50 cotton varieties that covered more than 80% planting area in China in 2004 were used in a hydroponic sculture and a field experiment in 2006. The K concentrations were 0.02 (low K stress) and 2.50 mmol L−1 (high K), respectively, in the hydroponic culture, and the available K in soil was 59.88 mg kg−1 (K deficient). At 5-leaf stage, the total plant dry weight (DW) varied larger among cotton varieties than that at 3-leaf stage; thus, seedlings at 5-leaf stage were adapted in the comparison of tolerance to low K. Under the low K condition, DW was significantly correlated with the relative dry weight (RDW) (r = 0.7690, P < 0.01), K accumulation amount (KAA) (r = 0.9522, P < 0.01), and K utilization index (KUI) (r = 0.9791, P < 0.01). The KAA of whole plant that grew in low K solution significantly correlated with root length (r = 0.5201, P < 0.01) and root surface area (r = 0.3325, P < 0.05). The ratio of spotted area resulting from potassium deficiency to total cotyledon area (defined as S value) was determined as an indicator for screening low K tolerance due to its high variation among varieties with the coefficient of variation of 44.46% and significant correlation with DW (r = −0.4455, P < 0.01) in addition to its normal distribution characteristic. No significant correlations were observed between the K content in seeds and the S value, DW, KAA, and KUI. There was significant correlation between the DW of whole plant at 5-leaf stage under hydroponic culture and the dry weight of reproductive organs (RODW) in field (r = 0.5091, P < 0.01). Therefore, hydroponic culture is feasible for primary screening cotton varieties tolerant to low K stress, but some important varieties should be further identified in field experiment.