Tolerance and physiological response of citrus rootstock cultivars to manganese toxicity

Objectives Soil pH affects the availability of soil manganese (Mn), and excess Mn is prone to occur in acidic soils. Citrus are mainly distributed in the red or yellow soil areas in south China, where the proportion of acid or strongly acid soil area in citrus orchards is high, and excess Mn stress is common in citrus production. For this reason, we studied the tolerance and physiological response of four different citrus rootstocks in Mn excessive stress, in order to provide a basis for the selection of suitable rootstock cultivars in Mn excessive soil.

Methods In a pot experiment, vermiculite and pearlite in ratio of 1∶1 were used as substrates and Hoagland nutrition solution as nutrient source, and the selected four popular citrus rootstock cultivars included trifoliate orange Poncirus trifoliate (L.) Raf, Ziyang Xiangcheng (Citrus junos ‘Ziyang Xiangcheng’, ZyXc), red tangerine (Citrus reticulata Hort. ex Tanaka, Tanaka) and Shatian pomelo Citrus grandis (L.) Osb. ‘Shatianyou’, StY. Seedlings were subjected to 0.01 (CK), 0.05, 0.25, 1.25 and 6.25 mmol/L of Mn in the nutrition solution treatments in this study. The chlorophyll and photosynthetic parameters were determined after treatment for 60 days. The treatment was terminated after 67 days, the physiological and biochemical indexes of citrus rootstock seedling such as growth amount, biomass, peroxidase and element content were measured, and the tolerance of the four rootstocks to excessive Mn was comprehensively evaluated by membership function.

Results The citrus rootstock seedlings suffered from Mn toxicity had common symptoms such as chlorosis and brown necrotic spots in leaves, less mass and brown spots in roots, etc. Under Mn 0.25 mmol/L treatment, all the four citrus rootstock seedlings showed symptoms of Mn toxicity. Trifoliate orange showed the earliest and most serious toxic symptoms, while Ziyang Xiangcheng showed the latest and lightest ones. The excessive Mn stress was observed in all the four rootstock seedling under Mn 0.05 mmol/L treatment, including the significant decrease in the dry and fresh weight of above and underground parts of the seedlings, the decreased chlorophyll content, declined net photosynthetic rate and stomatal opening but the increased intercellular CO₂ concentration in leaves. In terms of cell membrane damage, the relative electrical conductivity and MDA values in leaves of citrus rootstocks seedlings increased, indicating that the cell membrane of the rootstock seedlings suffered serious damage under excessive Mn stress (> 0.05 mmol/L). The activity of SOD and POD for scavenging reactive oxygen species increased, while the activity of CAT decreased. Mn content in leaves and roots increased, but the translocation rate of Mn from roots to leaves showed a trend of decreasing first and then increasing. The contents of K, P, Ca, Mg, Fe, Zn in leaves decreased, while the contents of K, P, Fe, Zn increased and those of Ca, Mg decreased in roots.

Conclusions The tolerance of rootstocks to excessive Mn stress is significantly different. A comprehensive evaluation suggests that the tolerance of the four rootstocks to Mn toxicity is in order of Ziyang Xiangcheng > Shatian pomelo > Red tangerine > trifoliate orange. It is suggested that Ziyang Xiangcheng should be chosen as rootstock in citrus orchard in high Mn soil.