PARENT SESSION
Oral Session # 24: Photosynthesis and Respiration.
Presiding: A Leakey
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 203.

Topography/canopy effects on the photochemical performance of jojoba, a broad-leaved Sonoran Desert evergreen shrub.

Hamerlynck, Erik¹, Huxman, Travis², ¹ Department of Biological Sciences, Newark, NJ, USA² Department of Ecology and Evolutionary Biology, Tucson, AZ, USA

ABSTRACT- We used PSII chlorophyll fluorescence to examine the varying effects of leaf canopy position and plant topographic exposure on photochemical dynamics at the height of early-summer pre-monsoonal drought and high temperature stress in jojoba (Simmondsia chinensis), an unusual Sonoran Desert shrub that possesses an extensive broad-leaved evergreen canopy. Predawn water potentials were lowest in plants at ridgetop and eastern exposures (-9.5 and -9.1 MPa, respectively), and highest in western exposure plants (-8.0 MPa). Mean predawn levels of optimal PSII photochemical yield (Fv/Fm) were 0.63, 0.54 and 0.74 for ridge, eastern- and western-exposure plants, respectively, with minor differences between inner- and outer-canopy leaves. During the day, canopy-position effects on Fv/Fm and NPQ, an index of engagement of thermal-dissipative photoprotection, were topographic-specific. In general, Fv/Fm and NPQ were higher in shaded inner-canopy leaves, while light-adapted PSII yield (_PSII)was lower compared to outer-canopy leaves. However, in eastern-exposure plants Fv/Fm increased and NPQ remained constant in inner-canopy leaves, while Fv/Fm and NPQ in outer-canopy leaves declined through the day. This latter pattern was apparent at higher parameter levels in outer- and inner-canopy leaves at ridgetop and western-exposure locations. These findings show that the dense canopy of jojoba might serve to ameliorate exposure to high early-morning photon flux, allowing for sustained photosynthetic activity deeper in the canopy during limiting soil-water conditions, thereby facilitating jojoba's persistence across topographically complex desert landforms.

Key words: Jojoba, Chlorophyll fluorescence, Sonoran Desert, Water stress