Accumulation of Calcium in the Centre of Leaves of Coriander (Coriandrum sativum L.) is Due to an Uncoupling of Water and Ion Transport.

The aim of this study is to understand the parameters
regulating calcium ion distribution in leaves. Accumulation
of ions in leaf tissue is in part dependent on
import from the xylem. This import via the transpiration
stream is more important for ions such as calcium
that are xylem but not phloem mobile and cannot
therefore be retranslocated. Accumulation of calcium
was measured on bulk coriander leaf tissue (Coriandrum
sativum L. cv. Lemon) using ion chromatography
and calcium uptake was visualized using phosphorimages
of 45Ca2+. Leaves of plants grown in hydroponics
had elevated calcium in the centre of the leaf
compared with the leaf margin, while K+ was distributed
homogeneously over the leaf. This calcium was
shown to be localised to the mesophyll vacuoles using
EDAX. Stomatal density and evapotranspiration (water
loss per unit area of leaf) were equal at inner and outer
sections of the leaf. Unequal ion distribution but
uniformity of water loss suggested that there was
a difference in the extent of uncoupling of calcium and
water transport between the inner and outer leaf. Since
isolated tissue from the inner and outer leaf were able
to accumulate similar amounts of calcium, it is proposed
that the spatial variation of leaf calcium concentration
is due to differential ion delivery to the two
regions rather than tissue/cell-specific differences in
ion uptake capacity. There was a positive correlation
between whole leaf calcium concentration and the
difference in calcium concentration between inner and
outer leaf tissue. Exposing the plants to increased
humidity reduced transpiration and calcium delivery to
the leaf and abolished this spatial variation of calcium
concentration. Mechanisms of calcium delivery to
leaves are discussed. An understanding of calcium
delivery and distribution within coriander will inform
strategies to reduce the incidence of calcium-related
syndromes such as tip-burn and provides a robust
model for the transport of ions and other substances
in the leaf xylem.