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Maize root system morphogenesis as affected by phosphorus deprivation A. Mollier, S. Pellerin 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.
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 mol 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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