Effects of root exudation of apple rootstocks on phosphorus acquisition efficiency under low phosphorus

Abstract：【Objective】The aim of this study was to explore the effects of root exudates on the phosphorus acquisition efficiency of apple rootstocks under different phosphorus supply levels, so as to provide the theoretical basis for solving the common problem of phosphorus deficiency in apple orchards. An additional calcium phosphate treatment was also designed to investigate the effect of root exudation on alleviating phosphorus stress.【Methods】Seven kinds of apple rootstocks including Pingyi Tiancha (Malus hupehensis Rehd.), Baleng (M. robusta Rehd.), M26 (M. pumila Mill.), M9 (M. pumila Mill.), Qiuzi (M. prunifolia), Chistock-1 (M. xiaojinensis) and Shandingzi (M. baccata Borkh.) were used as experimental materials. The differences in tree growth, root exudates, root architecture, photosynthesisand fluorescence parameters, and phosphorus efficiency of each rootstock under normal phosphorus (CK) (0.5 mmol· L- 1 KH2PO4), low phosphorus (LP) (0.01 mmol· L- 1 KH2PO4), and additional calcium phosphate treatments (LP-Ca3(PO4)2) (0.01 mmol·L-1 KH2PO4+Ca3(PO4)2) for 50 days were studied. After 50 d, the plants were oven-dried for 72 h. The dried plants were digested using a microwave oven (Mars, CEM, USA). An inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 5300DV, PerkinElmer, USA) was used to measure the phosphorus content. Root images were collected with an EPSON scanner and root total length, root surface area, root total volume and root tips were analyzed with the software WinRHIZO version 5.0 (Regent Instruments Inc., Canada). The photosynthetic and fluorescence parameters of leaves were measured with a Li-6400 XT portable photosynthetic measurement system (Li-Cor Biosciences, Lincoln, USA). The effects of root exudates on phosphorus acquisition efficiency of apple rootstocks were investigated by correlation and principal component analysis. All experiments had eight biological replicates. Statistical analyses were performed using IBM SPSS Statistics (Version 25, SPSS, Chicago, IL, USA) and Origin software (2022, Origin Lab, Northampton, MA, USA).【Results】Compared with the normal phosphorus treatment, the root-shoot ratios of Pingyi Tiancha, M26, M9, and Chistock-1 were significantly increased under low phosphorus condition, and the increase rate of Chistock-1 was 49.5%. The root-shoot ratio of Shandingzi was the largest after additional application of calcium phosphate. Except for root total surface area, the other three root indexes were significantly affected by low phosphorus treatment. The root total length of Pingyi Tiancha, Baleng, M26 and M9 increased after low phosphorus treatment, of which Baleng increased by 37.5% compared with CK. Compared with normal phosphorus, the root total volume of Baleng and M26 increased significantly, and the number of root tips of Pingyi Tiancha, M9 and Chistock-1 increased significantly under low phosphorus treatment, and the increase rate of Chistock-1 reached 89.2%. Under low phosphorus treatment, the PAE of Chistock-1 was the highest. After additional application of calcium phosphate, the PAE of Chistock- 1 and Shandingzi increased significantly compared with low phosphorus treatment, with an increase rate of 40% in Chistock-1. Low phosphorus treatment resulted in a significant decrease in the photosynthetic rate in Shandingzi leaves compared to CK. Low phosphorus treatment significantly increased citric acid secretion rate in the rootstocks except M26, and the citric acid secretion rate in Chistock-1 was the highest. After low phosphorus treatment, the change pattern of malic acid secretion rate was similar to that of the citric acid secretion rate. The malic acid secretion rate of Chistock-1 was significantly higher than that of the remaining rootstocks except Baleng. M26 was the lowest in malic acid secretion among the seven rootstocks.【Conclusion】The seven apple rootstocks tested differed significantly in their tolerance to low phosphorus stress. Under low phosphorus stress, the secretion of organic acids was positively correlated with the phosphorus acquisition efficiency of apple rootstocks. The phosphorus acquisition efficiency of Chistock-1 was the highest, and its citric acid and proton secretion rates were all higher than those of the other six rootstocks. At the same time, its malic acid secretion rate was significantly higher than that of the other rootstocks except Baleng.