Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2008
Publication Date: 3/5/2009
Citation: Hamerlynck, E.P., Huxman, T.E. 2009. Ecophysiology of two Sonoran Desert evergreen shrubs during extreme drought. Journal of Arid Environments. 73: 582-585.
Interpretive Summary: This study compares how two evergreen Sonoran Desert shrubs –– the broadleaved jojoba (Simmondisa chinensis) and creosotebush (Larrea tridentata), which has very small leaves –– cope with excessive light levels under conditions of unprecedented summer drought. Even after near-normal July precipitation levels, the two plants had extremely low predawn water potentials, indicating that they were still experiencing severe water stress after experiencing over 11 months without measurable rainfall. Chlorophyll fluorescence analysis showed that active creosote bush was better able to cope with high light levels than jojoba, and that the outer leaves of jojoba may act as a “sunscreen”, allowing inner-canopy leaves to maintain physiological activity under prolonged drought conditions. These findings show that leaf-level physiological differences accompanied differences in canopy structure in these long-lived desert shrubs.
Technical Abstract: Recent drought across the arid Southwest US may be especially problematic for evergreen desert species that maintain leaves through dry periods. In July, 2002 we compared the ecophysiogical performance of the microphyllous creosotebush (Larrea tridentata) to broadleaved jojoba (Simmondisa chinensis) growing on a ridgetop, east- and west-facing slopes to assess how differences in leaf habit affect these species’ ability to withstand severe drought. Standardized precipitation index (SPI) from 100 years precipitation data showed July 2002 rainfall was normal, but the 12-month period ending July 2002 was of unprecedented aridity (SPI = -2.71). Predawn water potentials were extremely low, but water stress was more severe in jojoba (-8.1 to -9.5 MPa) than Larrea (-6.6 to -9.4 MPa), possibly due to its more extensive leaf area display. Chlorophyll fluorescence showed Larrea had less evidence of photo-oxidative damage, having higher optimal photochemical efficiency (Fv/Fm), intrinsic photochemical efficiency (Fv’/Fm’) photosystem II yields ('PSII) and photoprotective non-photochemcial quenching (NPQ). Outer-canopy leaves of jojoba were less photochemically competent that inner-canopy leaves, and these canopy differences varied with topographic position. Our findings show these desert evergreens have distinct photoprotective strategies, and that the persistent outer-canopy in jojoba may facilitate inner-canopy activity under severe drought.