Maturation of Atriplex halimus L. leaves involves changesin the molecular regulation of stomatal conductance under highevaporative demand and high but not low soil water content
Under well-watered condition, stomatal conductance (gs) and photosynthetic rate (A) of expanding and expanded leaves of
Atriplex halimus peaked at noon despite the midday decline in the leaf relative water content, suggesting deviation from
typical isohydric behaviour. However, the expanding leaves had higher gs and A than the expanded ones. When light intensity
was temporarily increased, A and gs were enhanced in both types of leaves though to a higher level in the expanding leaves.
In well-watered expanded leaves: (1) A was mainly dependent on gs rather than photosynthetic capacity; gs was controlled
by internal factors, thereby limiting water loss via transpiration (E); (2) the accumulation of total soluble sugars (TSS) along
with increased Rubisco protein could be a subsidiary factor limiting A; (3) TSS and ABA seem to act in co-ordination to
up-regulate ABA-dependent genes controlling gs and (4) the significant induction of DREBs suggests a role in maintaining
high relative water content in these leaves compared to the expanding ones. In expanding leaves of well-watered plants, high
A along with Rubisco down-regulation and elevated TSS suggests that A was regulated by signals coordinating carbon and
nitrogen balance and the elevated ABA could be involved in regulating the hydraulic activity to enhance cell expansion and
facilitate leaf growth. Both expanded and expanding leaves behaved in typical isohydric manner under water stress, which
did not involve the accumulation of ABA suggesting that stomatal closure was primarily stimulated by hydraulic rather than
chemical signals