Physiological characterisation of magnesium deficiency in sugar beet: acclimation to low magnesium differentially affects photosystems I and II

Abstract

Magnesium deficiency in plants is a widespread problem, affecting productivity and quality in agriculture, yet at a physiological level it has been poorly studied in crop plants. Here, a physiological characterization of Mg deficiency in Beta vulgaris L., an important crop model, is presented. The impact of Mg deficiency on plant growth, mineral profile and photosynthetic activity was studied. The aerial biomass of plants decreased after 24 days of hydroponic culture in Mg-free nutrient solution, whereas the root biomass was unaffected. Analysis of mineral profiles revealed that Mg decreased more rapidly in roots than in shoots and that shoot Mg content could fall to 3 mg g⁻¹ DW without chlorosis development and with no effect on photosynthetic parameters. Sucrose accumulated in most recently expanded leaves before any loss in photosynthetic activity. During the development of Mg deficiency, the two photosystems showed sharply contrasting responses. Data were consistent with a down-regulation of PSII through a loss of antenna, and of PSI primarily through a loss of reaction centres. In each case, the net result was a decrease in the overall rate of linear electron transport, preventing an excess of reductant being produced during conditions under which sucrose export away from mature leaf was restricted.

Appendix: definition of photosynthetic parameters

PSII fluorescence parameters

Maximum quantum yield for primary photochemistry at time zero

(Butler and Kitajima 1975; Kitajima and Butler 1975)

$$ \varphi _{{{\text{Po}}}} = 1 - {\left( {F_{{\text{O}}} /F_{{\text{M}}} } \right)} $$

Actual quantum yield for primary phytochemistry at time t

(Paillotin 1976)

$$ \varphi _{{{\text{Pt}}}} = 1 - {\left( {F_{{\text{t}}} /F_{{\text{M}}} } \right)} = {\left( {F_{{\text{V}}} /F_{{\text{M}}} } \right)}{\left( {1 - V_{{\text{t}}} } \right)} = {\left( {F_{{\text{M}}} - F_{{\text{t}}} } \right)}/F_{{\text{M}}} $$

where relative variable fluorescence at the state t is given by V_t=(F_t−F_O)/(F_M−F_O)

Actual quantum yield for primary photochemistry at steady state under a given light condition

(Genty et al. 1989)

$$ \varphi '_{{{\text{PS}}}} = 1 - {\left( {F_{{\text{S}}} '/F_{{\text{M}}} '} \right)} = F_{{\text{V}}} '/F_{{\text{M}}} '{\left( {1 - V_{{\text{S}}} '} \right)} = {\left( {F_{{\text{M}}} ' - F_{{\text{S}}} '} \right)}/F_{{\text{M}}} ' = \Phi _{{{\text{PSII}}}} $$

where ′ refers to the light-adapted state, and relative variable fluorescence at the various states is given by V_S′=(F_S′−F_O′)/(F_M′−F_O′).

Rate of electron transport

$$ {\text{PSII ETR}} = \Phi _{{{\text{PSII}}}} {\text{*PFD}} $$

Performance index (PI) and driving force (DF) of PSII

(Hermans et al. 2003)

$$ \begin{array}{*{20}l} {{{\text{PI}}_{{{\text{ABS}}}} } \hfill} & { = \hfill} & {{{\left[ {\gamma /{\left( {1 - \gamma } \right)}} \right]}*{\left[ {\varphi _{{{\text{Po}}}} /{\left( {{\text{1}} - \varphi _{{{\text{Po}}}} } \right)}} \right]}*{\left[ {\Psi _{{\text{o}}} /{\left( {{\text{1}} - \Psi _{{\text{o}}} } \right)}} \right]}} \hfill} \\ {{} \hfill} & { = \hfill} & {{{\left\{ {{\left[ {{\left( {F_{{300\;\mu {\text{s}}}} - F_{{50\;\mu {\text{s}}}} } \right)}/{\left( {F_{{2\;{\text{ms}}}} - F_{{50\;\mu {\text{s}}}} } \right)}} \right]}*{\left( {F_{{\text{M}}} /F_{{\text{V}}} } \right)}} \right\}}} \hfill} \\ {{} \hfill} & {{} \hfill} & {{\;*{\left[ {F_{{\text{V}}} /F_{{\text{O}}} } \right]}*{\left[ {{\left( {F_{{\text{M}}} - F_{{2\;{\text{ms}}}} } \right)}/{\text{(}}F_{{2\;{\text{ms}}}} - F_{{\text{O}}} {\text{)}}} \right]}} \hfill} \\ \end{array} $$

$$ {\text{DF}} = {\text{Log}}{\left( {{\text{PI}}_{{{\text{ABS}}}} } \right)} = {\text{DF}}_{\gamma } + {\text{DF}}\varphi + {\text{DF}}_{\Psi } $$

where:

$$ \gamma = {\text{Chl}}_{{{\text{RC}}}} /{\text{Chl}}_{{{\text{tot}}}} = {\text{RC}}/({\text{ABS}} + {\text{RC}}) $$

$$ \varphi _{{{\text{Po}}}} = {\text{TR}}/{\text{ABS}} $$

$$ \Psi _{{\text{o}}} = {\text{ET/TR}} $$

PSI absorbance parameters

Maximum signal amplitude induced by far-red light on dark-adapted leaves

(Weis and Lechtenberg 1989)

$$ \Delta S_{{{\text{MAX}}}} \sim {\left( {{\text{P700}}} \right)}{\text{total}} $$

Signal amplitude following a light-to-dark transition on light-adapted leaves

(Weis and Lechtenberg 1989)

$$ \Delta S \sim {\text{P}}700^{ + } $$

Rate of PSI electron transport calculated on a total P700 basis

(Clark and Johnson 2001)

$$ {\text{PSI ETR}}_{{total}} = {\left( {\Delta S} \right)}{\text{*}}k $$

Rate of PSI electron transport per P700 oxidised fraction

(Ott et al. 1999)

$$ {\text{PSI ETR}}_{{per\;RC}} = {\left( {\Delta S{\text{/}}\Delta S_{{{\text{MAX}}}} } \right)}{\text{*}}k $$

where k is the rate constant for P700 re-reduction.