Plant breeding for organic and sustainable, low-input agriculture

The pattern of N uptake in time, N translocation and root growth as possible factors
determining genotype x environment interactions in wheat (Triticum aestivum L.)
Jörg Peter Baresel, Hans-Jürgen Reents
Technical University of Munich, Chair of organic Farming, Freising, Germany
Baking quality of wheat grown under organic management in Germany often does not match the
requirements of the milling and baking industry. Like in other species and for more general
criteria, it has therefore often been proposed, to breed special varieties adapted to the conditions
of organic farming. There are two ways to improve baking quality: one is to improve protein
quality, another possibility is an improvement of nitrogen (N) efficiency and thus, grain protein
content. Subject of this contribution is N efficiency, which is here intended as the ratio of grain N
uptake to N availability. Objectives of our trials were, to explain a part of the genotype x
environment interactions on N efficiency, analyzing N uptake, N partitioning and N translocation
processes of selected varieties in conditions of organic farming. Our focus is specifically on the
temporal patterns of N dynamics.
Six varieties (Altos, Batis, Bussard, Capo, Dream and Naturastar) were grown in replicated plots
over three years in four locations, differing in yield potential. All trials were conducted on
organic farms. Above-ground N uptake was determined at the development stages EC 32 (begin
of stem elongation), 62 (begin of flowering) and 95 (ripeness). At EC 95, grain and straw N
content were determined separately. Soil samples for determination of mineral soluble N were
taken in early spring and at the EC stages 32, 62 and 95 from surface to a depth of 90 cm. Total
mineralization and mineralization in single sub-periods, N uptake before and after anthesis and N
translocation from vegetative tissues to the grain were estimated basing on these data. Root
growth of the same six varieties was observed nondestructively in a separate trial in cylindrical
vessels of transparent polyethylene. The height of the vessels was 1.4 m, their diameter 25 cm.
The estimated global amount of mineralized N ranged from 160 to 220 kg/ha in the whole cycle;
approx. 70-120 kg (60%) were absorbed by the plant canopy. Mineralization during grain filling
varied from 0 to 15 \% of totally mineralized N. The variability of mineralization among
environments was highest after anthesis. There were significant differences in total N uptake of
the varieties. Differences between varieties were low in the earlier development stages, while N
uptake during the grain filling stage, despite contributing little to total uptake, were most
variable, but only in environments with relatively high N availability. Thus, genetic differences
of N efficiency depended mainly on uptake ability in the later stages. The pattern of root
development in time varied among the varieties and was in part correlated with the temporal
pattern of N uptake: consequently, the differences in N efficiency and in N uptake in time may
be partly due to differences in root growth. Significant differences could also be found in the
proportion of direct N uptake after anthesis and N translocation from vegetative plant tissues into
the grain.
Three main conclusions may be drawn from the results: (1) Organic farming may include very
different environments, with different requirements on varieties, also with respect to the
physiology of N uptake and utilization. (2) The pattern of N uptake in time, and N translocation
processes are of importance: uptake efficiency in late development stages may be useful in
environments with relatively high N availability, genotypes with a uptake efficiency in earlier
development stages and a high translocation efficiency may be better adapted to environments
with low N availability. (3) Root growth and the temporal pattern of root development may play
a role in N uptake efficiency.
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