Tissue Water Potential, Photosynthesis, C‐Labeled Photosynthate Utilization, and Growth in the Desert Shrub Larrea divaricata Cav.

Abstract

Tissue water potential is the most important factor throughout the seasons controlling phenological events, photosynthesis, and productivity of Larrea divaricata growing in Deep Canyon near Palm Desert, California. Growth of reproductive structures was initiated at the time of highest tissue water potential and ceased as water potential decreased. Percentage foliation correlated strongly with dawn water potential (r = 0.89). The elongation rate of stems and the rate of node production were both dependent on tissue water potential. Leaf growth and node growth proceeded at varying rates throughout the year, providing a continuous sink for photosynthates. Photosynthesis rates ranged from 9.02 mg CO2 incorporated per day per gram dry weight of leaf tissue in September to an estimated 74.7 mg CO2 in early February. Net photosynthesis and relative productivity correlated very strongly with dawn water potential (r = 0.93 and r = 0.97, respectively). Larrea plants were labeled at 1— to 2—month intervals with photosynthetically incorporated 14CO2to determine the utilization in growth and storage of photosynthate fractions produced at various times throughout the year. Tissue was subsampled at similar intervals, and the activity in various metabolic compounds (sugar, starch, lipid, organic acid, amino acid, protein, cellulose, and cell—wall materials) was analyzed. The utilization of photosynthates in the various fractions was similar in all seasons. No appreciable mobilization into and out of storage materials was apparent. Never dormant, Larrea remains metabolically active and forms new tissue throughout the year. This growth pattern may be an important adaptation allowing Larrea to exist in a wide range of geographical and climatic areas, and, perhaps owing to the species' tropical affinities, it might have been a preadaptation to the desert environment.