Effects of Endogenous Salicylic Acid During Calcium Deficiency-Induced Tipburn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis).

By cultivating tipburn-susceptible plants in modified Hoagland's medium containing of gradient exogenous calcium (Ca(2+)), we have shown that Ca(2+) deficiency is one of the main causes of tipburn in Chinese cabbage (Brassica rapa L. ssp. pekinensis). The effect of endogenous plant Ca(2+) concentrations on tipburn was also studied in a doubled haploid (DH) population consisting of 100 individuals, but no correlation was found. We then examined the expression of 12 Ca(2+) transporter genes that function in cytosolic Ca(2+) homeostasis in both tipburn-susceptible and tipburn-resistant plants under normal and tipburn-inducing conditions. Expression patterns for most of these genes differed between the two types of plants. Salicylic acid (SA) accumulated in response to conditions of calcium deficiency in our study, and both total SA and SA β-glucoside (SAG) in tipburn-susceptible plants was ∼3-fold higher than it was in resistant plants following Ca(2+) deficiency treatment. Also, the changes observed in SA levels correlated well with cell death patterns revealed by trypan blue staining. Therefore, we speculate that the cytoplasmic Ca(2+) fluctuation-induced downstream signaling events, as well as SA signaling or other biological events, are involved in the plant defense response to tipburn in Chinese cabbage.