Marie-Odile Jordan

A simulation model of the three-dimensional architecture of the maize root system

In order to study the nutrient and water uptake of rootsin situ, we need a quantitative three-dimensional dynamic model of the root system architecture.The present model takes into account current observations on the morphogenesis of the maize root system. It describes the root system as a set of root axes, characterised by their orders and their inter-node of origin. The evolution of the simulated pattern is achieved by three processes, occuring at each time step: emission of new primary root axes from the shoot, growth and branching of existing root axes. The elongation of an axis depends on its order, inter-node and local growing conditions. Branches appear acropetally at a specified distance from the apex and from former branches, along ranks facing xylem poles, with a branching angle specific to their order and inter-node.From the three-dimensional branched patterns simulated by the model, various outputs, such as root profiles or cross-section maps can be computed, compared to observed data and used as inputs in uptake models. A number of examples of such possible outputs are presented.

Influence of nitrogen availability on shoot development in young peach trees [Prunus persica (L.) Batsch]

Abstract. It is commonly stated that nitrogen (N) influences biomass accumulation in plants. For trees, however, a precise characterisation of shoot response to N and its impact on architecture is lacking. We attempted to study on the phytomer scale the effects of N limitation on shoot growth components, i.e. leaf emergence rate, final internode length and branching on the main and secondary axes of 1-year-old peach trees [Prunus persica (L.) Batsch]. Trees were grown on recycled nutrient solutions in which N concentration was restored once a week. We used two hydroponic set-ups in which weekly N availability, i.e. amount of N per tree, differed being either low (N1) or high (N2). Net N availability was defined weekly as the relative amount of N remaining in each set-up before solution replenishment. It declined with time and three periods of contrasting net N availability were identified. During these periods, leaf emergence rate and final internode length were similar on the main axis of N1 and N2 trees, so too was the distribution of secondary axes along the main axis. Secondary axes responded to N limitation by decreasing their growth components according to their position along the main axis. Differences were most important for basal secondary axes. Leaf emergence rate and final internode length responded similarly to N availability depending on axis order and position in the tree. It was concluded that N availability, particularly during the period of maximum growth of axes, influenced the shoot growth components and thus tree architecture.

Maize nodal root ramification: Absence of dormant primordia, root classification using histological parameters and consequences on sap conduction

Maize plants grown in field conditions were used to describe the histological organisation of the nodal roots, those of their laterals, and also to test the presence of critical stages where the subsequent capability for growth and development of young laterals was determined irreversibly. The absence of undeveloped primordia, which stop their development before boring through the nodal mother-root epidermis, proved that the number of laterals could not be regulated between the differentiation and the emission stage. Cross sections performed on nodal roots beared by the internodes 2, 4 and 6 and their long (>3 cm) and short (<3 cm) laterals showed that:uThe short laterals have almost the same root and stele diameter, cortex width and xylem size regardless of the insertion position of their mother root. Consequently they stopped their development at nearly the same stage, which seems to be concomitant with late metaxylem vessel differentiation. Thus, 92% of these short roots had only one of these vessels. On the contrary, the dimensions of the tested histological parameters, as well as the numbers of late and early metaxylem vessels and of xylem poles increased with internode rank for both long laterals and nodal roots. Moreover, for the nodal roots issued from internode 6, important decreases (up to 50%) in the sizes of most of the measured parameters were observed along the roots with the distance from the stem insertion point. Complementary experiments performed in mist boxes, under strict homogeneous conditions, showed that the observed variability is not generated by the culture medium.Up to 750 μm, the root diameter is correlated to most of the histological characters tested.Nodal roots have a higher proportion of xylem than lateral roots. Consequently their conduction abilities, evaluated using the C/E ratio of Hagen-Poiseuilles law, were particularly important.No relationship was found between the diameter of a nodal root and those of its laterals.

Autumnal N storage determines the spring growth, N uptake and N internal cycling of young peach trees

Although N storage determines early spring growth in trees, the usefulness of autumn N supply remains unclear as N uptake decreases in autumn, but could be restored earlier in spring to compensate for low N cycling. We intended here to evaluate the effects of autumn N supply on N uptake, storage and cycling, and spring growth. Four levels of N fertilisation were applied to 1-year-old peach trees, between the end of shoot growth and leaf fall. In spring, N supply was 15N labelled. Organ dry weights and concentrations of 14N, 15N, starch and soluble sugars were evaluated after the first growth flush. Bud development had previously been described in the same trees by Jordan et al. (Trees-Struct Func 23:235–245, 2009). Fertilisation promoted autumn N uptake, spring N uptake and growth up to a threshold level, since no differences were evidenced between the three highest N treatments. The variability in tree 14N contents was related to the number of phytomers per tree in autumn, i.e. to tree size. In spring, the depletion of the perennial structures was independent of treatment, indicating a complete mobilisation of the N stores. Spring growth was related to the amounts of cycling N, and spring N uptake was in turn proportioned to shoot and fruit growth. The lower N uptake of the N limited trees was not due to a C shortage since these trees displayed the highest starch concentrations. We conclude that a moderate autumn fertilisation improved spring growth and fruit production (Jordan et al. in Trees-Struct Func 23:235–245, 2009) and that a deficit of N storage could not be compensated for by an increase in spring N uptake.

The effect of autumn N supply on the architecture of young peach ( Prunus persica L.) trees

Irrigation and fertilisation were recently considered as useful tools to control tree shape, and reduce pruning costs. The role of the N reserves, which determined spring growth, was considered to be essential. We intended therefore to evaluate its effects on peach tree architecture. Four levels of N fertilisation were applied on 1-year-old trees, from the end of shoot growth to leaf fall. In subsequent spring, each bud fell into one of the ten classes of positions previously defined within the crown. Its development was followed weekly from burst to June. Fertilisation promoted growth until a threshold level, since no differences were evidenced between the three highest N treatments. Fall N did not affect burst but the further transformation of the buds into rosettes, proleptic or ramificated axes. Crown base was little affected. Fall N increased the number of proleptic axes on most median and upper positions. Axes lengthening and thickening were limited on the median positions, promoted at crown top. The variations concerned the mean internodes lengths, not the number of phytomers per axis. Sylleptic ramification was limited to the crown outer parts, and decreased with fall N. Treatment did neither affect the fruit dry weights, nor the ratio between the number of leaves and the number of fruits. Fruit number was proportioned to vegetative growth by blossoming and fruit set. We conclude that a moderate autumn fertilisation improved orchard productivity, but favoured vegetative growth in the crown outer parts. Additional pruning may therefore be required to control tree shape.

Nitrate availability influence on the growth components of young peach trees

Plant growth may be defined as the combination of two processes: phytomer production (or apparent plastochrone) and phytomer growth (internode elongation). We used this approach in the aim to analyse peach tree growth quantitatively both in space and time and to determine which growth components are influenced by nitrogen availability.