The Differences Of Adaptive Morpho-physiological Responses To Phosphorus Deficiency Between Maize, White Lupin And Faba Bean

Phosphorus (P) is one of the macro-elements influencing maize growth and development. Chemical P fertilizers are easily fixed in soils, which causes low availability of P fertilizer. Plants have developed different adaptive morpho-physiological responses in roots to P deficiency, which increase P availability in the rhizosphere and thus P uptake by roots. Plant species differ in their root adaptive mechanisms under P deficiency. Illustration of the adaptive mechanisms of maize under P-limitation, comparison of the differences in the rhizosphere processes of maize and high P-efficient plants like white lupin and faba bean, and revelation of the influence of root activity on P fractions of the rhizosphere soil, are important for exploitation of plant potential to use soil P and management of fertilizer application. The present studies focused on the adaptive mechanism of maize roots to P deficiency using white lupin and maize as reference, by combining hydroponic culture, rhizoboxes in situ research and sand culture together. The main results were summarized as follows:(1) The results of the hydroponic culture with low P (LP,1μmol L-1) and high P (HP,250μmol L-1) supply (7-16 days after treatments) demonstrated that, regardless of nitrogen (N) forms [Ca(NO3)2 or NH4NO3], P deficiency significantly decreased shoot growth while increased root growth of faba bean and maize; increased the total root length, axial root length, lateral root density and specific root length of maize, but not of white lupin. Phosphorus deficiency enhanced the release of protons and organic acid anions, and acid phosphatase activity of the roots of both legumes but not of maize. Compared with Ca(NO3)2 supply, NH4NO3 dramatically increased proton release by roots but did not alter root morphology or physiology of the three species in response to LP.(2) The results of the rhizoboxes experiments with low P (LP,10 mg P kg-1 soil) or high P (HP,150 mg P kg-1 soil) supply (50 days after treatments) further confirmed that P deficiency markedly enhanced the release of protons, organic acid anions and acid phosphatase activity from faba bean roots but not from maize roots. The P concentration in rhizosphere soil solution collected by suction using micro-rhizons from faba bean was greater than that in the bulk soil, but the P concentration in the rhizosphere solution of maize was less than that in the bulk soil. The concentrations of different inorganic P fractions of the rhizosphere soil of faba bean were significantly increased compared with those of the bulk soil, while the concentrations of organic P fractions of the rhizosphere soil of faba bean were decreased compared with those of the bulk soil. No significant differences were found on the concentrations of soil P fractions between rhizosphere and bulk soil in maize.(3) The ability of maize, white lupin and faba bean to utilize sparingly soluble P sources including AIPO4 (Al-P), FePO4 (Fe-P) and hydroxyapatite (Ca10-P) was examined in sand culture for 25 and 33 days. The concentration of the available inorganic P (Pi) in the growth medium supplied with sparingly soluble Al-P increased with the prolongation of cultural period. Maize could utilize this Pi during the culture, while white lupin and faba bean could not.In conclusion, this dissertation demonstrated that, different from white lupin and faba bean which had markedly changes in root morphology and physiology, morphological variation is the main adaptive strategy in maize roots in response to P deficiency. A large root system is therefore important for maize to access soil available nutrients, especially immobile P. The P concentration of the rhizosphere soil solution of maize was depleted significantly, and no changes were found in concentrations of different soil P fractions between rhizosphere and bulk soil. The rhizosphere processes of maize roots could not activate soil inorganic or organic phosphorus. Nitrogen form [NH4NO3 or Ca(NO3)2] could not fundamentally alter the responses of root morphology and physiology of the three species to P deficiency, although NH4NO3 enhanced proton release compared with Ca(NO3)2 supply.