Photosynthesis in Chlorophyllus Stem Tissue and Leaves of Cercidium floridum: Accumulation and Distribution of 14C from 14CO2

The contribution of phlotosynithesis by green stem tissue to the total carbohydrate economy of p1lants has been of interest to physiologists and ecoologists (6,9,10,11,12). Priestley (11) suggested that it seemed unlikely that stem photosynthesis contributed significantly to the requirements of most deciduous trees during leafless seasons. Pearson and Lawrence (9, 10) measured photosynthetic activity in the stem cortex of the quaking aspen (Populus tremuloides) with an iiodinestarch test, and suggested that co,rticular photosynthesis was a significant factor in its distribution withtin a mainly coniferous vegetative formation. Strain and Johnson (12) mea;suired CO, absoirption by green stems of P. tremutloides and agreed with the latfter workers. With the desert shrub ocotillo (Fouquieria splendens), however, Mooney and Stra,in (6) were not able tio detect net CO2 absorption during a drought-induced leafless condition. Nevertheless, the adaptive significance of stem photo,synthesis by woody desert perennials, which m,ay be leafless duiring dry seasoins, remains an attractive hypothesiis. In this report, we present evidence th,at stem photosynthesis by the Palo Verlde (Cercidiun floridum Benth.) is quiallitatively similar to leaf photo,synthesis when the plants are in leaf, and is of stuch a magnitude that it could be biologicallly significanit. Cercidium floridum is a smiall leguminouts tree occurring in washes and sandy areas below 400 m in the Colorado Desert of Cali,forni,a, in Arizona, and in the states of Baja California and Sonora, Mexico (7). The smalil leaves (1-2 cm) bear 1 to 3 pairs of pinnae. Typical;ly, the trees are leafless during much of the year (14); however, in some years the leaves may persist. Cannon (3) descrihed the location of the chlorophyll in the stem.s. Plants were selected from several uinder investigation at tihe University of California's Boyd Desert Res-earch Laboratory, near Palm Desert, California. Experiments were conducted on foliated and leafless stems, approximately 2 mm in diameter. The stems were exposed to closed atmospheres containing '-CO2 and 'air for 10 or 20 minute periods. The intact tissne was pliaced in 50 ml glass test tulbes which contained 5 ,ur NaH'4CO3 (,specific activity _ 52 pjuc/pm'ole) in 0.7 ml 10 mm tris buffer (pH 7.4). A pipet was inserted and the system sealed with Apiezon Q. Excess 85 % lactic acid was injected through the pipet to release 11CO2 from the NaH4iCO3. The sealed tube was immersed in a 1-Tliter glass jar containing cdis'tilled water and ice to control tempera,tture. Temperature was measured with a shielded mercury thermometer sealed in a similarly immersed test tube. At the end of the 10 or 20 mintute periods, the leaves and/or stems were wasXhed in d'istiIlled water and frozen uinider dry ice. The tissue was returned to the laboratory whil,e frozen and extracted with!in 2 hours. Surface area wias determine'd by measulrements taken at the beginning o'f the experiment. Chlorophyll was extracted wiith cold 80 % acetone and ana!lyzed by the method of Arnon (1). Tissuees varied from 0.300 to 0.716 g fresh weight. The products of photosynthesis were ani alyzed essentially by the method's of Bli'ght 'and Dyer (2) anid Canvin and Beevers (4), as previously r eported (13). After grinding in liquid N9, neutral (carbohydrates), basic (amino acids), ancd acidic ('org,anic phosphates anid a.cids) fractions were separated by ion exchange chromatography. Aliqtuiots (0.2 mll) of the fra'ctions were counted for radioactivity, and resullts expressed as disintegration's per minute (dpm) cm-2 hr-1. Slimillar aliiqiiots of the Mlipid-containing chiloro,form component were counted. In so'me experiments, the methanol-chloroform-insoluble fraction was counted uising CabO-Sil Gel as a suspens'ijon medium. In general, in leaves and stems the h'i'ghest levels .1 Supported by a grant, GB-4146, from the National Science Foundation. 2 National Aeronautics and Space Administration predoctoral fellow.