Varied tolerance and different responses of five citrus rootstocks to acid stress by principle component analysis and orthogonal analysis

Soil acidification is a worldwide problem and adoption of acid-tolerant rootstocks is an essential strategy to face this challenge in order to maintain the production of citrus. In the present study, seedlings of five genotypes of citrus rootstocks including KPJ-020 (Citrus depressa Hayata 'Shiikuwasha'), KPJ-007 (C. reticulata Blanco), KPJ-010 (C. daoxinensis Blanco), Carrizo (C. sinensis x Poncirus trifoliata) and ZYXC (C. junos Siebold Tanaka) were used as testing materials and subjected to 10, 30 and 60 days treatment with nutrient solutions at pH2.5, 3.5, 4.5 and 6.0 (control), respectively, to evaluate their tolerance to acidic stress. Symptoms of leaf damage, relative electrical conductivity (REC), the content of chlorophyll, soluble sugar (SS), soluble protein, proline, malon-dialdehyde (MDA), and the activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in the leaves were examined at different period time of treatment. Acidic stress caused symptoms of chlorosis on leaves due to reduced content of chlorophyll pigments. Analysis of physiological and biochemical parameters indicated that both physiological and biochemical processes of plants were significantly affected by acidic stress. Lower pH decreased the accumulation of N, P, K, Ca, Mg and B, but increased the accumulation of 5, Fe, Zn, Mn and Cu. Principal component analysis based on the nine parameters was performed in order to comprehensively evaluate the acidic tolerance of rootstocks. The degree of acid tolerance significant varied within the rootstocks. ZYXC and KPJ-020 showed better tolerance than other rootstocks. Analysis of orthogonal experiment results revealed that the influence of the factors in the treatment to acidic tolerance varied among different rootstocks. In KPJ-020, the duration of treatment was the main factor affecting the tolerance, but the strength of acidic stress was the main factor determining the tolerance of ZYXC, as well as other three rootstocks. This suggested that the mechanisms of acidic tolerance were probably different between the two tolerant genotypes of rootstocks of ZYXC and KJP-020. The former possesses the tolerance by maintaining the capability to uptake mineral nutrients under acidic stress, while the latter has better ability to adapt to lower pH conditions.