and more markedly in plants grown at elevated CO2 concentra-
tion. The reason for this increase in soluble sugars may be that
under a surplus of CO2 high amounts of starch are synthesized in
mature, photosynthetically active leaves. Also, this increase could
be the result of senescence promoting a decline in the functional
and structural integrity of cell membranes,thereby accelerating the
membrane lipid catabolism which produces sugars by gluconeoge-
nesis (Buchanan-Wollaston et al., 2003b; Lim et al., 2007). Sugars
regulate many metabolic and development processes, and in some
of them, hexokinase is involved as a sugar sensor. Hexokinase may
be responsible for sugar-dependent senescence regulation, since its
over-expression inhibits plant growth, decreases photosynthetic
activity and induces senescence rapidly (Wingler et al., 2004).
Exposure of cucumber plants to elevated CO2 concentrations was
previously found to result in a concomitant increase in starch and
soluble sugars inleaves, and a decrease inthenitrate content(Lario