1Graduate School of Agricultural and Life Sciences, The University of Tokyo, Nishi-tokyo, Tokyo 188-0002, Japan. www.fm.a.u-tokyo.ac.jp/index-e.html Email firstname.lastname@example.org
2Graduate School of Agricultural and Life Sciences, The University of Tokyo, www.fm.a.u-tokyo.ac.jp/index-e.html Email email@example.com
3Graduate School of Agricultural and Life Sciences, The University of Tokyo, www.ab.a.u-tokyo.ac.jp/aeb/ Email firstname.lastname@example.org
The dynamics of the elongation growth of roots in peanut plants (Arachis hypogaea L.) was investigated from viewpoints of developmental morphology. Peanut plants were grown in root boxes and the roots were traced every two days until 55 days after sowing. The frame of the whole root system was defined by the taproot and some of the 1st-order lateral roots emerged from basal part of taproot that elongated long and formed 2nd-order lateral roots. In the time course of elongation rate of whole root system, two peaks appeared during the observed period. In the interval period between the two growth peaks, emergence and elongation of 1st-order lateral roots were synchronically suppressed with declined elongation of taproot. In cases where the axis of taproot stopped elongation completely, existing 1st-order lateral roots grew vigorously and compensated the lost growth of the taproot. Therefore, there is close relation and interaction between the tap root elongation and the development of lateral roots in peanut root system.
1st-order lateral roots play important roles in architecture of peanut root system. Close relation was found between taproot elongation and development of lateral roots.
Peanut, taproot, 1st-order lateral root, root box
Most leguminous plants form root nodules on their roots that can fix N2 available from the atmosphere. Efficient use of symbiotic nitrogen fixation in legumes is an important strategy to establish sustainable agriculture. For this purpose, it is necessary to understand the architecture and development of root systems, but there are few reports about root system formation in legumes except soybean (Isoi and Yoshida 1991; Yoshida et al. 1988). Peanut (Arachis hypogaea L.) is one of major leguminous crops in the world and an important constituent crop in some crop rotation systems. In this study, development of taproot system in peanut was observed using the root box method.
The study was conducted in a glass house in Field Production Science Center of the University of Tokyo (Tokyo, Japan) in 2003. Peanut cultivar Chibahandachi, a major cultivar in Japan, was cultured in five root boxes (100cm, in depth, 50cm in width, about 2cm in thickness). The root boxes were filled with fertilized soil (N:P:K= 0.4 :1.5 : 0.4 g/kg, Kureha Chemical Industry Co., Ltd.). The root boxes were immersed in a water pool to uniformly water the soil and gravitationally drained for one day after taking out from the pool. Peanut was sown in the center of the soil surface in each root box on July 29, 2003. 50ml of water was supplied to the soil surface every other day after sowing. The observation side of root box made of the clear acrylic board was covered with tracing paper to trace the root. The roots were traced using a marker pen with different colors to identify observation date (Figure 1) every other day after sowing. This transparent side of root boxes was covered with aluminum foil throughout the growth period except the times of observation to avoid lighting to roots. The length of roots traced to the tracing paper was measured and the number of 1st-order lateral roots was counted for every 2cm segment of taproot. The elongation rate of taproot, 1st-order and higher-order lateral roots for every two days was calculated respectively. The root boxes were disassembled to carefully wash out the root system at 55 days after sowing (DAS), because taproot or a long lateral root reached the bottom of root box. The total length of the sampled roots was measured with the root scanner (Comair Root Length Scanner).
Figure 1. Traced root system using marker pens (55 DAS)
The length of traced roots that appeared in the observed side of the root box was approximately 9m at 55DAS, which is about 25% of the real total root length measured by root scanner. The frame of root distribution of whole root system was defined by taproot and several 1st-order lateral roots emerged from basal part of taproot (Figure 1).
The time course of elongation rate of the whole traced root system showed two peaks during the observed period. A steep and high peak appeared 10 DAS, and a gradual and low peak on 36 DAS. Mainly the elongation of 1st-order lateral roots contributed to the 1st peak, and those of 2nd- and 3rd-order lateral roots as well as 1st lateral roots contributed to the 2nd peak (Figure 2).
During 14-18 DAS, the valley between these two peaks of elongation rate in whole root system, the elongation rate of taproot once declined but increased again in three of the five observed plants. In the other two plants, the taproot fully stopped the elongation during 14-18 DAS and did not recover after that. The timing of this suppression of root growth may correspond to exhaustion of nutrient store in seed. Because the 1st and 2nd branches started development in the shoot at the same time, shoot-root relationship should also be taken account to consider the cause of this pausing of root growth. In case that the taproot completely stopped its elongation, 1st-order lateral roots that had already formed increased elongation rate, and compensated the lost length of the taproot (Figure 1, Figure 3).
Figure 2. Elongation rate of individual components of the root system
The growth of 1st-order lateral roots varied depending on the position in the axis of taproot (Figure 4). The 1st-order lateral roots formed in basal part of taproot elongated long and formed higher-order lateral roots. The number and length of 1st-order lateral roots decreased at around 20-50cm from the base of taproot even in the three plants that continued taproot elongation until the end of the observed period. This part with suppressed development of lateral roots was the apical part of the main axis when the taproot growth declined during 14-18DAS. Thus, the pausing of taproot elongation and the suppression of lateral-root development appear to have been synchronized to each other.
Figure 3. The elongation rate of taproot and 1st-order lateral roots that elongated 15cm or more in four individual plants (A-D).
Figure 4. The number and length of 1st-order lateral roots along the taproot. Each letter indicates a plant in a different root box.
The dynamics of individual components of the root system and their interactive relationship in peanut was clearly illustrated in this study. The 1st-order lateral roots contributed most to the total root length. The growth of the whole root system declined, and then re-activated with vigorous elongation of 1st- and higher-order lateral roots. During the period of declined growth, both the taproot elongation and the emergence and growth of new lateral roots were synchronously suppressed. There should be close relation between the taproot elongation and the development of 1st-order lateral roots. For the spatial arrangement of the root system in soil, the taproot and the 1st-order lateral roots that emerge from the basal part of the taproot are important. Indeed, some of the 1st-order lateral roots formed in basal part appeared to compensate the role of the taproot with their elongation and branching, when the taproot stopped elongation in some of the plants.
Isoi T and Yoshida S (1991). Low nitrogen fixation of common bean (Phaseolus vulgaris L.) Soil Science and Plant Nutrition 37, 559-563.
Yoshida S, Isoi T and Hasegawa H (1988). Influence of farmyard manure application on the distribution of nodule formation and nitrogen-fixing activity of soybean grown in a plastic faced-root box. Japan Journal of Soil Science and Plant Nutrition 59, 182-189. (in Japanese with English summary)