Nichols, Brandt by lev17755


									                                Nichols, Brandt
Phosphorus, Zinc, and Manganese Interactions in Hydroponically
                        Grown Maize
                      Faculty Mentor: Dr. Von D. Jolley, Plant and Wildlife

Maize (Zea maize L.) roots prolifically explore soil and this requires less intensive phosphorus
(P) fertilization than less efficient species, such as potato (Solanum tuberosum L.). Crops are not
grown in isolation and species requiring high P are often grown in rotation with low P requiring
species which may be problematic. High soil P is known to potentially induce deficiencies of
micronutrients such as zinc (Zn) and manganese (Mn) in species like maize. The Zn-induced P
deficiency is the most commonly observed and researched P micronutrient interaction. Our
research focused on this important micronutrient interaction in hydroponic treatments. Growing
the plants hydroponically allowed a direct analysis of the micronutrient interactions without the
interference of soil.

Zinc is absorbed by plants as a cation (Zn2+) and P is taken up by plants as the phosphate
(H2PO4-1 or HPO4-2) anion. These cations and anions have an electrical attraction to each other,
facilitating the formation of a chemical bond that can form in either the soil or plant tissue. If
excess P binds a large amount of the Zn normally available to the plant, the result can be a P-
induced Zn deficiency. This generally results in reduced Zn uptake and reduced growth
(Marschner, 1986).

Although the P-Zn interaction is more commonly studied, other micronutrient interactions have
also been observed. Safaya (1976) established a linkage with the P-Zn interaction with Mn, iron,
and copper uptake by maize. Recent work has revealed a complex relation among P, Zn, and Mn
in potato (Nichols et al., 2007). Because of this complex relationship between P, Zn, and Mn in
potato, this research also involved observation of Mn levels as P and Zn increased. Overall, our
studies with maize were designed to test the impact of variable levels of P or Zn as other nutrient
levels remained constant in order to reveal information about P, Zn, and Mn relationships.

Bukvic et al. (2003) found that differences existed between inbred maize lines in element uptake
ability and biomass production with various P and Zn fertilization treatments. Since maize
hybrids are available that vary in susceptibility to Zn deficiency in the field, we compared two
hybrids (one susceptible and the other resistant to Zn deficiency) at variable levels of P and Zn.
Seeds of two maize hybrids (Zn-inefficient and Zn-efficient) were germinated separately in
darkness on steel screens covered with moist cheesecloth with dilute complete nutrient solution
reaching the bottom of the stainless steel screens. Three days after the radicles emerged,
seedlings were transferred into containers with dilute, complete nutrient solution and placed in an
environmental growth chamber to elongate. Healthy maize plants of uniform size were then
transferred into nutrient solutions. The plants were then randomly suspended through holes in
opaque plastic lids into the buckets of complete nutrient solutions (Bernards et al., 2002). Plants
were grown in this solution for 14 days prior to transfer into treatment solutions.

The nutrient solutions for experiment 1 had five levels of P (8, 32, 128, 512 and 2048 µM P) and
40 µM Zn, while solutions for experiment 2 consisted of five levels of Zn (0.05, 20, 40, 80 or
160 µM Zn) and 128 µM P. After 18 days in their variable treatment solutions the plants were
then harvested to allow quantifying of dry weight and nutrient concentrations.

Variable Zn and P solutions generally resulted in identifiable ranges of deficiency, sufficiency,
and toxicity as reflected in maize yields. Both experiments showed that optimum levels of P and
Zn produced more yield than at deficient or toxic levels. At deficient levels of P and Zn, the
stems and leaves had substantial purpling along with stunted growth. Plants grown at high P or
Zn levels had less growth and branching in the roots.

The impact of increasing P levels on Zn content in maize partially explains the P-Zn interaction
in that Zn accumulated in maize roots as solution P increased. At the same time, Mn
concentration increased in shoots as solution P increased. These two observations reveal more
about the three way P-Zn-Mn interaction as Zn is trapped in the roots while Mn is able to travel
into the shoots, possibly revealing competition for absorption at high P levels. The combination
of these interactions could result in decreased transport of Zn to shoots or reduced physiological
activity of Zn in shoots under marginal soil Zn. However, despite these results in the roots, only
the first increment of P (32 µM P) reduced maize Zn shoot content below that of the control.

Treating maize with increasing levels of Zn produced reductions of P in both shoots and roots
above 20 µM Zn. Root Mn increased dramatically above 0.05 µM Zn, peaked at 20 and 40 µM
Zn, and then declined at higher solution Zn levels. Fageria (2001) suggested that besides
nutrient interactions between oppositely charged ions, interactions may occur between ions
whose chemical properties are similar enough to compete for site of adsorption and transport on
plant root surfaces or within plant tissues. Zn and Mn have similar stable oxidation states (Zn2+,
Mn2+) and therefore may be susceptible to these interactions, causing the observed patterns of
Mn in plant roots.

Contrary to our previous hypothesis, these experiments show a greater impact of solution Zn on
P translocation rather than high solution P levels reducing Zn translocation. Total removal of P,
Zn and Mn were generally similar for both hybrids, except for total removal of P in the variable
Zn experiment when the Zn-inefficient (susceptible to Zn deficiency) hybrid removed more P
than the Zn-efficient hybrid. This higher P removal in Zn inefficient hybrid could help explain
the field observed differences in susceptibility to Zn deficiency. Because different results appear
depending on whether marginal or sufficient nutrient levels exist, further study in maize should
focus on varying P levels but at solution Zn levels between 0.05 and 40 µM Zn.

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