Effects of N deficiency and resupply of N nutrient on banana growth and root morphological parameters

Abstract: 【Objective】The purpose of the paper was to understand the effects of nitrogen deficiency and restoration of nitrogen nutrients on banana seedling growth, root morphological parameters and development of typical deficiency symptoms in order to provide theoretical basis for banana N nutrition diagnosis and fertilization.【Methods】The experimental design was a randomized complete block with 3 treatments. The treatments were supplied in quartz sand culture in a pot experiment. The three treatments were banana supplied complete Hoagland nutrient solution (control, T1), minus nitrogen (-N) nutrient solution (T2) and restoration of nitrogen involving resupply whole nutrients solution after N deficiency symptoms were observed (T3). Each banana seedling with 6 green leaves was transplanted to a pot of 13 cm (bottom) to 16 cm (top) in diameter and 15 cm in height. The banana seedling was rinsed with deionized water for one week before treatments were implemented. T1, T2 and T3 had 90, 33 and 15 pots as 90, 33 and 15 repetitions respectively. The banana seedlings of the control were irrigated with modified complete Hoagland nutrient solution, in which the molar ratio of N and K2O changed into 1:2. T2 supplied Hoagland nutrient solution of N deficiency. T3 was applied with Hoagland nutrient solution of N deficiency until typical mild N deficiency symptoms developed in 50 percent of banana seedlings, when they were supplied complete Hoagland nutrient solution. Morphological characteristics of banana seedlings were observed every day after the treatments. The nitrogen deficiency symptoms and time of its appearance were recorded in detail. Chlorosis of the leaves was obvious, which was regarded as mild nitrogen deficiency symptoms. When the plants showed slow and weak growth, they were considered to have moderate nitrogen deficiency symptom. Stagnated plant growth with some dead leaves was classified as severe nitrogen deficiency symptom. Sampling of seedlings showing mild and moderate nitrogen deficiency was conducted at 31 days and 67 days after nitrogen deficiency treatment, respectively, when mild and moderate nitrogen deficiency symptoms in 50 percent of banana seedlings in T2 were observed. Banana shoot and root samples were collected from T1and T2. Sampling time of T3 was 32 days after the recovery of nitrogen supply, when nitrogen deficiency symptoms in 50 percent of banana seedling disappeared. Banana shoot and root samples were collected from all the treatments. The fresh weight, dry weight and water content of shoot and root were measured. The root morphological parameters were analyzed with WinRHIZO software. The length, surface area, volume, mean root diameter of total root and root with diameters of ＜2.0 mm, 2.0 mm to 4.5 mm and ＞4.5 mm were tested.【Results】 Nitrogen mild deficiency symptoms could be observed 7 days after N deficiency treatment. The leaves developed a chlorosis in mature leaves, which then became evident in middle leaves. Moderate nitrogen deficiency symptoms in banana seedlings were observed with the time of nitrogen deficiency. Banana seedlings began to grow slowly, and then new sprouting leaves became smaller. The height and circumference of the stems increased slowly. Thus the banana seedlings became small and thin. The growth of the seedlings became extremely slow and then stagnated with the development of severe nitrogen deficiency symptom. The lower leaves died faster, and the number of dead leaves increased while the number of green leaves decreased. Phenotypic characters of banana seedlings returned to normal and symptoms of nitrogen deficiency disappeared 32 days after recovery of nitrogen supply. However, the effects of N deficiency severity and N deficiency-resupply on shoots and roots of banana seedlings were different. Mild nitrogen deficiency inhibited the shoot growth of banana seedlings. There was no difference in the dry matter of shoots between N deficiency treatment and the control. On the contrary, mild nitrogen deficiency stimulated root growth. Compared with the control, the dry matter of roots significantly increased. Moderate nitrogen deficiency significantly inhibited the growth of banana seedlings. Compared with the control, the dry matter of shoots decreased by 19.25%. However the effect of nitrogen deficiency on root growth was weak and the dry matter of roots had no obvious difference from the control. 32 days after N deficiency-resupply, the morphological symptoms of N deficiency disappeared completely, and the dry matter of shoots and whole plants increased by 6.08% and 5.24%, respectively, but still was significantly lower than those of the control. Compared with the control, total root length of N deficiency decreased significantly. Total root length of N deficiency was 58.3% of the control at mild N deficiency period and 49.3% at moderate N deficiency period. The length of fine roots with a diameter ＜2 mm decreased. Average root diameter and total root volume of N deficiency increased obviously. The average root diameter and total root volume of N deficiency were 1.85 times and 2.01 times of the control, respectively. The volume of the roots ＞2 mm in diameter increased. However, the difference in average root diameter and total root volume between the N deficiency and the control became smaller with the increase of nitrogen stress time. The roots tended to be normal when improved Hoagland complete nutrient solution was resupplied to the banana seedlings experiencing N deficiency. Compared to N deficiency, the total root length, especially the length of fine roots with a diameter ＞2 mm increased significantly and the average root diameter, total root volume in N deficiency- resupply treatment decreased significantly. However there was a significant difference between the N deficiency- resupply and the control. 【Conclusion】The phenotypic symptoms of nitrogen deficiency in banana can reflect the nitrogen nutrition status of banana in time. The symptoms of nitrogen deficiency can be restored in a short time by timely supplementation of nitrogen fertilizer, but the recovery period of dry matter and root growth lags behind the recovery phenotypic symptoms. In banana cultivation, banana seedlings should be properly fertilized based on soil fertility and nutritional characteristics, so as to avoid apply N fertilizer after significant N deficiency symptoms develop.