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Physiological assessment of magnesium deficiency in broad beans (Vicia faba L.)

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Hariadi, Yuda Cahyoargo (2005) Physiological assessment of magnesium deficiency in broad beans (Vicia faba L.). PhD thesis, University of Tasmania.

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Abstract

Despite the wide recognition of Mg2+ deficiency in several soil types, there
has been little documentation of the relationship between Mg2+ concentration in the
soil solution and plant growth, yield responses or the kinetics of development of
deficiency symptoms in crops. Most of the literature deals with forest species, with
only a handful of crops being characterized. Even in the latter case, these papers
address very severe cases of Mg2+ deficiency. Meanwhile, in many crops, visual
symptoms of Mg2+ deficiency are either virtually absent, or become obvious only at
later stages of plant development. One of these species is broad bean (Vicia faba L).
In this study, broad beans were used as a case study to characterize the development
of Mg2+ deficiency symptoms in plants and make a comparative evaluation of the
suitability of various physiological characteristics as prospective tools for early
diagnosis of Mg2+ deficiency. The ultimate goal of this study was to suggest an
efficient screening tool for early diagnosis of Mg2+ deficiency in crop species and to
explain cellular mechanisms underlying observed changes at the whole-plant level. In a series of glasshouse experiments, growth characteristics were measured at
regular intervals (on a monthly basis) from plants grown in a wide range of Mg 2+
concentrations (from 1 to 200 ppm) in soil solution. Those data were then correlated
with plant yield responses, pigment composition and nutrient content in leaves as well
as with visual deficiency symptoms. At the age of 4 weeks, no visual symptoms of
deficiency were evident even for plants grown at 1 ppm (severe Mg2+ deficiency).
Shoot growth characteristics were very similar for a wide range of treatments,
although a pronounced difference in plant yield was observed at the end of the
experiment. It appears that neither plant biomass nor leaf area is a good indicator for
diagnosis of Mg2+ deficiency in broad beans. Although pigment analysis revealed
some difference between treatments, at no age was it possible to distinguish between
moderately Mg2+ deficient (10 or 20 ppm) and sufficient (50 to 80 ppm) treatments.
Leaf elemental analysis for Mg2+ content remained the most sensitive and accurate
indicator of Mg2+ deficiency in broad beans. However, its suitability for screening
purposes is jeopardised by hight cost of analysis and a significant amount of time
involved.
In a search for rapid screening tools for Mg2+ deficiency in crops at early
stages of plant ontogeny, kinetics of leaf photosynthetic responses and changes in
electrophysiological characteristics of broad bean leaves were studied. No apparent
correlation between plant age, Mg2+ supply level and leaf stomatal conductance (gs)
and transpiration rate (E) were found. A significant difference in CO2 assimilation
became obvious only at week 8. Chlorophyll fluorescence analysis was more suitable
for screening purposes, with significant (P=0.05) difference in the maximum quantum
efficiency of PSII (Fv/Fm ratio) between Mg2+-deficient and sufficient plants as early
as 2 weeks after seedling emergence. However, even in this case, only "extreme"
Mg2+ treatments were differentiated. The most sensitive potential screening tool was measurement of light-induced
changes in the leaf surface electric potential (an integrated change in membrane
potential of hundred of cells on the leaf surface). A significant difference in the
magnitude of leaf electrical responses to light, measured by means of the surface
potential technique, was found between optimal (Mg50) and deficient leaves as early
as 2 weeks after the beginning of the experiment. Not only severe (Mg 1), but also
moderately deficient (Mg10) leaves showed a significantly lower magnitude of
response. At the age of 4 weeks, it became possible to "rank" all treatments according
to their predicted bean yield. Therefore, it appears that leaf electric measurements warrants testing as a potential screening tool for Mg 2+ deficiency, at least in broad
bean plants. To establish a causal link between light-induced electrical events in bean
leaves and membrane-transport processes at the plasma membrane of bean mesophyll
cells, a series of experiments were conducted using a non-invasive ion selective
microelectrode (MIFE) technique. Netfluxes of Fr, K+, Ca2+ and Mg2+ were
measured from leaves, grown at various Mg 2+ levels, and light-induced kinetics of
flux changes were characterised with high (5 sec) temporal resolution. To my
knowledge, this is the first report on net Mg2+ flux measurements from higher plants
in physiological literature.
Based on the stoichiometry of ion flux changes and results of pharmacological
experiments, it was suggested that at least two mechanisms are involved in Mg 2+
uptake across the plasma membrane of bean mesophyll cells. One of them is a nonselective
cation channel (NSCC), also permeable to K + and Ca2+. The other
mechanism, operating at concentrations below 30 iiM, was suggested to be H+7Mg2+
exchanger. Experiments performed on leaves grown at different levels of Mg2+
availability (from deficient to excessive) showed that Mg 2+ availability has a
significant impact on the activity of plasma membrane transporters for Ca 2+, K+ and
H+. Among other findings, our data suggests that Mg 2+ availability during plant
growth may significantly affect light-induced Ca 2+ flux "signatures" and thus regulate
(directly or indirectly) signal transduction between light photoreceptors and plasma
membrane effectors (ion channels). A possible causal link between light-induced
changes in fluxes of other ions (Fr, K+, and Mg2+) and whole-plant responses to Mg2+
deficiency is discussed.

Item Type: Thesis (PhD)
Keywords: Magnesium deficiency diseases in plants, Fava bean
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2005. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:10
Last Modified: 11 Mar 2016 05:54
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