Patterns of Variation in MHC Class II Beta Loci of the Little Greenbul (Andropadus virens) with Comments on MHC Evolution in Birds

Abstract

The hydraulic conductance of leaves (kleaf) was examined to determine whether this little understood component of the water transport pathway plays a role in governing leaf phenology of two deciduous dry forest trees (Calycophyllum candidissimum and Rhedera trinervis ). kleaf was monitored in parallel with stem hydraulic conductivity (Kstem) during the transition from wet to dry season. The relationships between declining photosynthetic quantum yield during senescence and declining kleaf and Kstem were compared. Divergent patterns were observed in the response of Kstem to seasonal drying; however, the behaviour of kleaf was essentially similar in both species. Large (five- to ten-fold) decreases in leaf hydraulic conductance occurred before, and during the later stages of leaf senescence. During senescence, declining kleaf, which continued until leaves were ultimately shed, was associated with a concomitant decline in quantum yield. We conclude that, in these species, the loss of hydraulic conductance of the leaf vascular system is linked to, and possibly responsible for, the loss of photosynthetic capacity during leaf senescence.