Geoscience Reference
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high-light grown algal cells. Alternatively, photosynthetic elec-
tron transport can also be inhibited specifically at PSII in the
presence of DCMU (3-(3,4-diclorophenyl)-1,1-dimethylurea.
Under these conditions, PSII is unable to reduce PQ to PQH 2
pool oxidised by PSI. This produces a green phenotype which
mimics low-light grown cells.
The maintenance of cellular energy balance is called photo-
stasis, which is dependent chloroplast-mitochondrial interac-
tions. For example, the mitochondrial Moel protein is thought
to regulate the transcription of mitochondrial genes involved
in the maintenance of the mitochondrial respiratory electron
transport, however, under high light, the moc1 mutant, which
lacks this mitochondrial protein, is unable to up-regulate rates
of respiration to match the production of fixed carbon by pho-
tosynthesis. The block in mitochondrial electron transport
slows the rate of respiratory carbon metabolism which, in
turn, causes a feedback inhibition in the rate of photosynthetic
electron transport. This also results in the reduction of the PQ
pool in the chloroplast. This is an excellent example of the link
between chloroplast and mitochondrial metabolism and its
importance in the regulation of the gene expression.
Acclimation to
drought effects
at different
plant parts
One of the long-term effects of water deficit is a reduction in
vegetative growth. Shoot growth, and especially the growth of
leaves, is generally more sensitive than root growth. In a study
in which water was withheld from maize plants, for example,
there was a significant reduction of leaf expansion when tis-
sue water potentials reached −0.45 MPa and growth was com-
pletely inhibited at −1.00 MPa. At the same time, normal root
growth was maintained until the water potential of the root
tissues reached −0.85 MPa and was not completely inhibited
until the water potential dropped to −1.4 MPa. Reduced leaf
expansion is beneficial to a plant under conditions of water
stress because it leads to a smaller leaf area and reduced
transpiration.
Roots are generally less sensitive than shoots to water stress.
Apparently, osmotic adjustment in roots is sufficient to maintain
water uptake and growth down to much lower water potentials
than is possible in leaves. Relative root growth may actually be
enhanced by low water potential, such that the shoot-root ratio
will change in favour of the proportion of roots. An increase
in the root-shoot ratio as the water supply becomes depleted
is clearly advantageous, as it improves the capacity of the root
system to extract more water by exploring larger volumes of
soil. A changing root-shoot ratio is accompanied by a change
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