Physiological response of young trees of different apple cultivars to nitrogen and phosphorus deficiency

Objectives We studied the physiological response of different apple cultivars to P and N deficiencies, to guide the selection and breeding of nutrient efficient apple cultivars.

Methods A pot experiment was conducted, using three-year-old ‘Regal Gala’ /M9T337, ‘Fuji’ /M9T337 and ‘Honeycrisp’ /M9T337 as test materials (M9T337 is dwarf rootstock). In the 1/2 Hoagland nutrient solution, normal and low N levels (NO₃^– 15 and 1.5 mmol/L), and normal and low P levels (H₂PO₄^– 1.0 and 0.1 mmol/L) were prepared to make four treatments, as normal N and P (NNNP), normal N and low P (NNLP), low N and normal P (LNNP), and low N and P (LNLP). The apple seedlings were subjected to the treatments for total of 120 days. The growth, photosynthesis and chlorophyll fluorescence parameters, the activities of N and P metabolism enzymes, as well as the accumulation of N and P were investigated.

Results Compared to NNNP treatment, the total plant dry weight of ‘Regal Gala’ and ‘Honeycrisp’ reduced significantly, but ‘Fuji’ increased significantly in NNLP and LNNP treatments; NNLP reduced the chlorophyll b contents of ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’, increased F_v/F_m of ‘Regal Gala’ and ‘Fuji’ significantly; NNLP decreased the P_n of ‘Honeycrisp’ by 65.72%; LNNP did not increase the F₀ of ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’ significantly, and decreased the F_v/F₀ of ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’ by 7.45%, 2.47% and 8.59%, respectively. The carotenoid content of ‘Fuji’ was significantly lower under NNNP than under other treatments, while the carotenoid content of ‘Regal Gala’ or ‘Honeycrisp’ did not show significant change among the four treatments. Compared to NNNP, the NNLP, LNNP and LNLP treatments significantly reduced the leaf nitrite reductase (NiR) activity in ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’; decreased the leaf nitrate reductase (NR) activity and plant N accumulation of ‘Regal Gala’ , but did not affect those of ‘Fuji’. NNLP increased the leaf acid phosphatase (ACP) activity of ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’, with the largest increase in ‘Fuji’; LNNP and LNLP markedly decreased the leaf ACP activity of ‘Regal Gala’, ‘Fuji’ and ‘Honeycrisp’ compared with NNLP. Based on the quantitative evaluation function values Y of the radar plots, the adaptability to low P, low N, and low N and P were ranked in the order of ‘Fuji’>‘Honeycrisp’>‘Regal Gala’.

Conclusions Under P, N and N and P deficiency stress, ‘Fuji’ has significantly higher leaf carotene content for maintaining normal photosynthesis, has significantly increased leaf ACP and NR activities to improve the uptake and accumulation of P and N to ensure the normal plant growth. While the other two cultivars exhibite less adaptability for the N, P, and N and P deficiencies, of which the leaf photosynthesis of ‘Honeycrisp’ is significantly inhibited, and the leaf NO₃^– reduction process in ‘Gala’ is significantly weaker, and their growth are both significantly inhibited.