Stem photosynthesis in a desert ephemeral, Eriogonum inflatum : Morphology, stomatal conductance and water-use efficiency in field populations.

Seasonal patterns in plant morphology, phenology, and physiology were monitored in several populations of Eriogonum inflatum, a desert ephemeral which produces a large photosynthetic inflorescence above a basal leaf rosette. Green stems accounted for 66-77% of whole plant photosynthetic surface area when integrated over a developmental cycle, whereas only 40-67% of the yearly transpirational water loss could be attributed to stems. Stems were found to have lower nitrogen and chlorophyll contents than leaves, and lower stomatal conductance under all physiological conditions encountered. However, because stems occur later in the year than leaves, comparison of physiological patterns was complicated by the two structures being exposed to different climatic regimes during their developmental cycles. Stems exhibited higher δ13C values than leaves, indicating that stems operated at higher water-use efficiencies than leaves, at least during periods when both leaves and stems were present. Higher water-use efficiency in stems of E. inflatum is attributed to both more conservative water use patterns and to their vertical orientation, allowing stems to remain photosynthetically active longer into the dry season after senescence of the horizontal leaf rosette.