Effects of low phosphorus stress on the growth and development in pecans (Carya illinoinensis)

Abstract:【Objective】In this experiment, the changes in growth, physiology and biochemistry of pecan were explored under the condition of phosphorus deficiency. It provided a scientific basis for applying phosphorus fertilizer in the pecan industry.【Methods】Six-month-old pecan seedlings were used in this study. The hydroponics method was used to explore the effect of normal phosphorus supply (1 mmol·L^-1 KH2PO4,CK), low phosphorus treatment (0.5 mmol·L^-1 KH2PO4, LP) and phosphorus-free treatment (0 mmol·L^-1 KH2PO4, NP) on the growth and development of pecan seedlings. The length of the aboveground part of the seedling (cm) was measured using a meter ruler while the stem diameter near the ground (mm) was measured using a vernier caliper. Seedling root length, diameter, surface area and volume were obtained by the Wanshen LA-S series plant image analyzer. The photosynthetic parameters of pecan leaves were measured with a Li-6800 portable photosynthetic measurement system (Li-Cor, Inc, USA), and a Pm2500 portable modulated chlorophyll slow-speed fluorimeter was used to measure pecan seedling leaves in vivo. Chlorophyll content was determined by direct extraction with 95% ethanol. Phosphorus content in roots was determined with Mo-Sb colorimetric method. Coomassie brilliant blue staining was used to determine protein staining. The total protein quantitative assay kit was used to measure the total protein content in leaves. The activity of superoxide dismutase (SOD) and peroxidase (POD) in leaves was analyzed using the total SOD assay and the POD assay kit, respectively. And the malondialdehyde (MDA) and acid phosphatase (ACP) contents of leaves were determined with an MDA assay and ACP assay kit, respectively. The above indicators were determined after 90 days of phosphorus deficiency treatment. The kits aforementioned were produced by Nanjing Jiancheng Bioengineering Institute. The cytological structure of mesophyll cells was observed under a transmission electron microscopy.【Results】Compared with the CK, the phosphorus content, protein content, ground diameter and seedling height of pecan significantly (p＜0.05) decreased by 10.57%, 30.79%, 5.03% and 8.40% , respectively after 90 days of low phosphorus treatment. The indicators mentioned above decreased by 59.21%, 51.48%, 10.57%, and 9.75%, respectively in NP. SOD, POD and MDA significantly (p＜0.05) increased in both treatments compared with CK. SOD, POD and MDA increased by 63.05%, 59.42% and 4.36%, respectively in LP, while 178.76%, 196.4% and 38.76% were found, respectively in NP. The ACP contents of LP and NP significantly increased by 33.84% and 78.59% compared with CK (p＜0.05), respectively. With NP treatment, the chloroplasts mesophyll cells of pecan seedlings were enlarged, and some of the chloroplasts had swollen into hemispheres and squeezed vacuoles towards the middle part. Phosphorus deficiency treatment had no significant effect on photosynthetic parameters, especially for net photosynthetic rate (Pn) and intercellular CO2 concentration (Ci). The correlation coefficients between chlorophyll A, chlorophyll B and total chlorophyll, and carotenoid were all greater than 0.98. On the other hand, the correlation coefficients between transpiration rate (Tr) and stomatal conductance (GSW), Ci and Pn were 1, 0.8 and 0.52, respectively. By comparing the stress degree of LP and NP, the stress degree of LP was below 40%, and the stress degree of NP was above 60%.【Conclusion】The results indicated that the pecan had better stress resistance and could tolerate low-phosphorus stress for a long time. However, the time under low phosphorus stress should not be too long; even if the phosphorus concentration was as low as 0.5 mmol·L^-1 , pecan was still under stress. Hence, from the point of view of the pecan industry, we shall not frequently apply phosphate fertilizers and shall not apply too much phosphate fertilizer for each time. Considering the ecological environment and the growth status of pecan, we recommend that the optimal phosphorus concentration is between 0.5 mmol·L^-1 and 1.0 mmol·L^-1. Excessive fertilization will not only cause the plants not to grow better but also lead to serious environmental problems. In conclusion, less application should be followed when applying phosphate fertilizer to pecans.