Alternate Application of Osmotic and Nitrogen Stresses to Partial Root System: Effects on Root Growth and Nitrogen Use Efficiency

ABSTRACT

Effects of alternately applying osmotic and nitrogen (N) stresses to partial root zone were studied on maize seedlings. The maize seedlings were grown in Hoagland's nutrient solution with the roots divided equally into two containers. Half of the root system was subjected to an osmotic stress (W) using PEG 6000 added in the solution at −0.2 MPa and/or nitrogen stress (N) with the N-free Hoagland's solution. After 6 days, four of such treatments, C (control), W, N, and WN were shifted to the other halves of the root systems. Results showed that when compared to the control, total root dry weight and the root/shoot ratio increased markedly for all the stress treatments. There were no statistically significant differences in shoot biomass and N accumulation of the whole plant among all the treatments. Nitrogen use efficiency for the N and WN was significantly enhanced when compared to the C and W treatments, and the shoot N concentrations were less for N and WN than for C and W. The root vitality of the non-treated halves under the W, N, or WN treatments increased by 21.1–65.6% when compared to the other halves of the same root system or by 19.7–68.9% when compared to the control roots. Such a compensation of root function was evident during our days 1, 3, and 5 observations. After each alternation, rates of biomass accumulation of all previously stressed half root systems were significantly higher than the other halves or the controls. Our results suggested that significant compensation mechanisms of growth and functions in the root system occurred both when N and osmotic stresses were localized in part of the root system and during the subsequent recovery from either of such stresses.

Keywords:

• root growth
• nitrogen stress
• nitrogen use efficiency
• osmotic stress
• partial root zone
• maize (Zea mays)

ACKNOWLEDGMENTS

We are grateful for grants from the Chinese National Natural Science Foundation (50339030, 50279043), the 2001AA242091 Project, and the earmarked grant support from RGC of Hong Kong University Grants Council (HKBU 2041/01M).

Notes

^a The root vitality was measured as TTC reduction intensity (μ g TTC h^{− 1} g^{− 1}RFW). Codes C, W, N, WN indicate the four treatments that half of roots was exposed to Hoagland, Hoagland added with PEG at −0.2 MPa, N-free Hoagland, and both PEG and N-free Hoagland solutions, respectively. Codes C₁, C₂ represent two halves of roots of control and foot index of the treatment codes represent the non-stressed (c) and stressed halves (s) of root system of W, N, and WN treatments, respectively. Values are means of five replicates and index by different letters suggest significant difference at P_0.05 level within the same row

^b The proportional increase compared with the average of C₁ and C₂.

^c The proportional increase compared with the other half root system.

^a Codes refer to the first step of the experiment and are explained in the legend of Table 1. All the stress treatments were shifted to the other halves of the root system and then shifted back after 6 days interval each. Values are means of five replicates and index by different letters suggest significant difference at P_0.05 level within the same row.

^a Codes refer to the first step of the experiment and are explained in the legend of Table 1. All the stress treatments were shifted to the other halves of the root system and then shifted back after 6 days interval each. Values are means of five replicates and index by different letters suggest significant difference at P_0.05 level within the same row.

^a Codes refer to the first step of the experiment and are explained in the legend of Table 1. All the stress treatments were shifted to the other halves of the root system and then shifted back after 6 days interval each. Values are means of five replicates and index by different letters suggest significant difference at P_0.05 level within the same row.