These data support the idea that leaf growth drives the dynamics of …

• Abstract
• Introduction
• Materials and methods
• Results
• Discussion and conclusion
• Acknowledgements
• References
• Figures

Biology Articles » Botany » Developmental changes in shoot N dynamics of lucerne (Medicago sativa L.) in relation to leaf growth dynamics as a function of plant density and hierarchical position within the canopy » Introduction

Introduction

- Developmental changes in shoot N dynamics of lucerne (Medicago sativa L.) in relation to leaf growth dynamics as a function of plant density and hierarchical position within the canopy

 

Understanding of the N distribution within a canopy is relevant for the analysis of behaviour of an individual plant in a dense stand where competition for light, minerals, and water may be occurring between plants. For that it is necessary to know the developmental effect of growth on N acquisition and distribution within the plant in relation with the level of competition with neighbouring plants.

It is generally accepted that even when there is an ample supply of N, the shoot N concentration in plants within a dense canopy decreases as the plants grow. Lemaire and Salette (1984) and Lemaire et al. (1985) showed that for perennial grasses and lucerne, the N accumulation (N_sh) in plant shoots at any given time was related to the shoot mass (W_sh) during growth process according to an allometric function:

Nsh = a(Wsh)b

(1)

or

[Nsh] = a(Wsh)b-1

(1')

where [N_sh] is the N concentration in the shoot, a represents the quantity of N required to produce the first unit of shoot mass (W_sh=1), and b represents the ratio between the relative N accumulation rate (dN_sh/N_shdt) and the relative growth rate (dW_sh/W_shdt).

Two different processes may be involved in the decrease of shoot N concentration with shoot mass in a dense canopy. (i) According to Caloin and Yu (1984) and Greenwood et al. (1990), plants can be viewed as being composed of two compartments. The first, the metabolic compartment, is involved in growth processes with a high N concentration and, the second, the structural compartment, with a low N concentration. The proportion of these two compartments within the total plant mass would decrease as plants get bigger, leading to a decrease in shoot N concentration. Following Hardwick (1987) the metabolic compartment can be considered to scale with leaf area, while the shoot mass would scale with shoot volume, so the decrease in shoot N concentration should parallel the decrease in leaf area:shoot mass ratio (Lemaire and Gastal, 1997). (ii) According to Charles Edwards et al. (1987), Hirose et al. (1988), and Lemaire et al. (1991), the allocation of N in leaves within a dense canopy is not uniform and more or less parallels the light distribution profile. As a crop canopy develops, an increasing proportion of leaves are shaded and the average N concentration of plants within the canopy therefore declines.

The first objective of this paper is to analyse to what extent these two hypothesises can be combined in order to explain the plant N concentration decrease during the growth process. The second one is to analyse how the presence of neighbouring plants can affect the shoot N dynamics of any individual plant in a dense stand.

Previously published data on plant growth, shoot N accumulation dynamics, and LA expansion obtained on lucerne plants growing either under controlled conditions at low plant density or under field conditions in a dense canopy (Kim et al., 1993; Avice et al., 1997a) were used in order to analyse the effect of the intensity of competition. Lucerne is a non-clonal dicotelydonous herb showing the following features: (i) the separation between leaf and stem fraction is easy to achieve and each of these two fractions can be related to each of the two conceptual compartments, metabolic for the leaf and structural for the stem. This is not as evident for other species, such as grasses, where the morphological differences between photosynthetic and structural tissues are not as clear; (ii) individual plants can easily be identified within a canopy, which is not possible with plants with clonal propagation such as grasses; and (iii) it can be supposed that a steady-state non-limiting condition for plant N nutrition can be more easily achieved during the regrowth period under field conditions for a legume species with both N₂ fixation and soil N mineral sources. Thus, lucerne appears to be a good plant model for generating data as well as for explaining the relationships between individual plant development, canopy structure, and N accumulation dynamics in shoots.