COORDINATION OF BRANCH ORIENTATION AND PHOTOSYNTHETIC PHYSIOLOGY IN THE JOSHUA TREE (YUCCA-BREVIFOLIA)

Despite the profusion of light in deserts, morphological adaptations t o increase light interception are common among desert plants. We studi ed branch orientation and related physiological parameters in the Moja ve Desert Joshua tree, Yucca brevifolia (Agavaceae). Azimuth and incli nation were measured on all leaf rosettes of 44 Y. brevifolia trees. I nterception of solar radiation was modeled for leaves in hypothetical rosettes facing due south and due north in December, March, and June. Carbon isotope discrimination, nitrogen content, and conductance to wa ter vapor were measured in leaves from north- and south-facing rosette s. Rosette azimuths were nonrandom; rosettes predominantly faced south east. North-facing rosettes were more steeply inclined than those faci ng south. The preponderance of south-facing rosettes reduces self-shad ing and increases interception of solar radiation during the winter-sp ring growth period. Stomatal conductance was higher for leaves in sout h-facing than in north-facing rosettes. Nevertheless, discrimination a gainst C-13 was less in leaves of south-facing rosettes, indicating th at average intercellular CO2 concentration was also lower. South-facin g whorls had higher leaf nitrogen content. Greater allocation of nitro gen to leaves in south-facing whorls probably results in those leaves having a greater photosynthetic capacity than their north-facing count erparts. Orientation of rosettes to increase interception of sunlight during the period most favorable for photosynthesis, coupled with allo cation of nutrients to maintain a higher photosynthetic capacity in th ose rosettes, should significantly increase whole-plant carbon gain in Y. brevifolia.