Response and tolerance of apple rootstocks to low nitrogen stress

Objectives The tolerance of seven apple rootstocks to low nitrogen stress was studied for the selection and breeding of apple rootstocks.

Methods Using the sand culture method,1/2 Hoagland nutrient solution as basic nutrient solution, normal N supply (NN, 5 mmol/L NO₃^–) and low N stress (LN, 0.5 mmol/L NO₃^–) were imposed. Seven annual apple rootstocks were tested―dwarf root stocks T337, Nic29, Pajam2, B9, 71-3-150; semi-dwarf rootstock Qingzhen 2, and vigorous stock (Malus baccata L. Borkh.). After the rootstocks were treated for 60 days, the growth of new shoot, biomass accumulation, root morphology, nitrate reductase activity of leaf and root, as well as nitrogen accumulation were measured. The tolerance to low nitrogen stress in apple rootstocks was evaluated by membership function fuzzy evaluation method.

Results Under normal N supply, the dry weight of total plant, root/shoot ratio, and nitrogen use efficiency of the vigorous stock M. baccata L. Borkh. were significantly higher than those of the five dwarf rootstocks, Pajam 2 had the highest dry matter accumulation in total plant and B9 had the highest root/shoot ratio among the dwarf rootstocks. The growth rate of new shoot and leaf NR activity of Nic29, a dwarf rootstock, were significantly higher than that of the remaining six rootstocks. The root nitrate reductase activity of the semi-dwarf Qingzhen 2 was significantly higher than that of the other rootstocks, which facilitated the nitrogen accumulation in root and plant.Compared with the normal N supply, the growth of new shoot in T337, Nic29 and M. baccata L. Borkh. was significantly inhibited under sub optimum nitrogen condition; the growth of new shoot in B9, Pajam2 and Qingzhen 2 was not significantly affected; while the growth of new shoot in 71-3-150 was significantly higher, the leaf area increased, the root biomass accumulation increased and the ratio of root/shoot increased significantly, reaching 2.59 times that of the control. Under N deficiency condition, all root parameters of T337, B9, Pajam2 and M. baccata L. Borkh. were significantly reduced, with T337 exhibiting drastic decrease, while root parameters such as total root surface area, total root length, total root volume and number of root tips increased significantly in 71-3-150, and total root surface area, total root length, root volume and root branches increased in Nic29, but the number of root tips decreased in Nic29. The leaf nitrate reductase activity of apple rootstocks decreased under nitrogen deficiency condition, and increased by 3.70, 5.16, 2.85 and 5.14-times in root of B9, Nic29, Pajam2 and M. baccata L. Borkh., respectively, compared with normal nitrogen supply. Under nitrogen deficiency condition, nitrogen accumulation in leaves, stems, and roots of T337, B9, Pajam2 and Qingzhen 2 decreased, so as nitrogen accumulation in total plant decreased, with the largest decrease in Qingzhen 2. However, nitrogen use efficiency of B9, Nic29, Pajam2 and Qingzhen 2 all increased significantly, with the largest increase in Qingzhen 2, and nitrogen accumulation in both root and plant of 71-3-150 all increased significantly. The seven apple rootstocks were classified into three tolerance types based on tolerance index to nitrogen deficiency of above nineteen indicators, including the growth rate of new shoot, root parameters, nitrate reductase activity, nitrogen accumulation and nitrogen use efficiency, combined with the fuzzy evaluation method of the affiliation function and cluster analysis: Class I was the most tolerant rootstock (71-3-150); Class II was the less tolerant rootstock (Nic29, M. baccata L. Borkh., B9 and Qingzhen 2); Class III was the least tolerant rootstocks (Pajam2 and T337).

Conclusions Under normal nitrogen supply, the vigorous stock M. baccata L. Borkh. and the semi-dwarf Qingzhen 2 outperformed the dwarf rootstocks in terms of biomass accumulation, root development and nutrient uptake and utilization, but they were less adaptable to nitrogen deficiency condition. Apple rootstocks adapt to nitrogen deficiency by increasing nitrogen use efficiency, of which the tolerant rootstocks were less inhibited in growth and adapt to nitrogen deficiency conditions by regulating their own physiological characteristics, increasing biomass and nutrient accumulation in root, and improving the ratio of root/shoot.