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Maize root system
morphogenesis as affected by phosphorus deprivation
A. Mollier, S. Pellerin
INRA, laboratoire d'Agronomie
71, avenue Edouard-Bourleaux
B.P. 81, 33883 Villenave d'Ornon Cedex, France.
mollier@bordeaux.inra.fr
Introduction
The morphological properties of roots are important factors determining
the uptake of less mobile nutrients such as phosphorus in soils. Contradictory results are
found in the literature about the effect of phosphorus deficiency on root growth. Several
authors have observed an enhanced root growth on P deficient plants (Anuradha et al.,
1991), whereas other authors have reported a reduction of root growth under P deficiency
(Hajabbasi et al., 1994). Such apparent contradictions probably reflect differences
between experimental works about the studied system (individual root or whole root
system), the variable under consideration (root biomass or root elongation), the time
scale of the experiment (short term or long term) and all other experimental conditions
which may interfere with the measured process. A clarification was required before any
attempt to model root growth under P deficiency. The objective of this work was to
investigate on maize the immediate and subsequent effects of P deprivation on root biomass
and root system morphogenesis.
Methods
Maize plants (Zea mays L. cv Volga) were grown hydroponically on
individual containers (36 cm high, 31 cm diameter). Between emergence and the 5 visible
leaf stage, the P concentration in solution was kept close to 10 µmol L-1. When plants
were at the 5 visible leaf stage, half of them (52 plants) were deprived from P. Leaf
elongation, axile root emergence, elongation of axile root and progression of the lateral
front were then followed daily on 8 plants randomly chosen per experimental treatment.
Eight plants per treatments were also sampled every three days for biomass measurement and
detailed observations on roots (number and length of laterals). All relevant environmental
variables were recorded throughout the experiment (air and nutrient solution temperature,
photosynthetically active radiation (PAR), and vapor pressure deficit of air)
Results
Leaf elongation dropped rapidly after P starvation . Biomass accumulation
in shoots was also reduced in P deprived plants. Root biomass was slightly higher in P
deprived plants three days after P deprivation, but became lower thereafter. The
root/shoot ratio was higher for P deprived plants at all sampling dates. An almost similar
relationship was found between the production of total biomass and the cumulated PAR
intercepted by plants for both treatments. The PAR intercepted by P deprived plants was
very low because of the large reduction of their leaf area.
The appearance of new axile roots was drastically delayed on P deprived
plants, but the elongation rate of emerged axile roots was maintained throughout the
experiment. The number of first-order laterals per unit length of axile root was not
affected by P deprivation. On the opposite, the elongation rate of first-order laterals
and their final length were drastically reduced.
Conclusions
The effect of P deprivation on total dry matter production per plant was
almost fully accounted for by the effect of P starvation on leaf growth and its subsequent
effect on PAR interception. Root growth was slightly enhanced a few days after P
starvation, but became lower thereafter. The elongation rate of axile roots was maintained
whereas elongation of laterals was drastically reduced. A very similar response is
observed when root growth is limited by availability in carbohydrates (Aguirrezabal et
al., 1993). It is concluded that the root system response to P deficiency is mainly
explained by the consequence of P deficiency on the carbon budget of the plant.
References
Aguirrezabal, L. A. N., et al., 1993. Carbon nutrition, root branching and
elongation: Can the present state of knowledge allow a predictive approach at a
whole-plant level? Environmental and Experimental Botany, 33, 121-130.
Anuradha, M., et al., 1991. Promotion of root elongation by phosphorus
deficiency. Plant and Soil, 136, 273-275.
Hajabbasi, M. A., et al., 1994. Phosphorus effects on root growth and development in
two maize genotypes. Plant and Soil, 158, 39-46.
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