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Changes in activities of photosynthetic enzymes and photochemical processes were followed with aging of vegetative and flag leaves of wheat (Triticum aestivum L. cv Roy). Activities of stromal enzymes began to decline prior to photochemical activities. In general, total soluble protein and the activities of ribulose-1,5-bisphosphate carboxylase and NADPtriose-phosphate dehydrogenase decUned in parallel and at an earlier age than leaf chlorophyll (Chl), leaf photosynthesis, and photosynthetic electron transport activity. Leaves (8) in Phaseolus vulgaris L. observed a continual decline in electron transport per mg Chl, which suggested a gradual degradation of -all chloroplasts. Studies of electron transport as well as other physiological parameters are needed before definitive conclusions can be made as to the loss of whole chloroplasts from wheat leaves.In the present study, the time course of leaf development from full expansion through senescence was investigated in vegetative and flag leaves of wheat to determine the basis for reduction of photosynthesis. Specific objectives of this study were to determine (a) if electron transport activities or stromal enzyme activities declined first and (b) if there was a general decline in the photosynthetic functioning of all chloroplasts or if there was a preferential degradation of whole chloroplasts.Senescence is a coordinated, deteriorative growth process that is initiated at full maturity and ultimately leads to the death of a cell, organ, or organism. To characterize a tissue that is undergoing senescence, it is necessary to identify the sequential order in which various parameters change. Butler and Simon (5) reported from ultrastructural studies that aging is first expressed in the chloroplast. Thus, changes in the physiology of this organelle are important to our understanding of senescence.In leaves, senescence is typically characterized by a decline in MATERIALS AND METHODS Plant Material. Triticum aestivum L. cv Roy was grown in a growth chamber with a photon fluence rate of 3 x 1 -O mol/m2. s (400-700 nm) and a 15-h photoperiod (22°C day/17°C night). For vegetative leaves, the second leaf to emerge was studied from the time of full expansion (about 14 d from planting) through senescence. Flag leaves were sampled from the point of emergence of the head from its sheath (arbitrarily designated day 0) through senescence. Flag leaves were fully expanded at this stage. Reproductive development was induced by vernalization of 12-d-old seedlings for 6 weeks at 2 to 4°C. All plants were fertilized twice weekly with Hoagland solution.Two separate plantings of both vegetative and flag leaf material were used. Two to four replications of each parameter for each age were performed and the results from both plantings were averaged.Leaf Area, Chi, and Protein Assays. Leaf area was measured with a Li-Cor3 area meter (model LI-3000, Li-Cor, Inc.). Chl and Chl a/b ratios were determined by the procedure of Arnon (1).

MATERIALS AND METHODS Plant Growth and Chloroplast Isolation. Spinach (Spinacia oleracea L.) plants were grown in the greenhouse or growth chamber-grown (12-h photoperiod, 22°C day/170C night). Leaves were ground briefly in a Waring Blendor2 using an iced slurry containing 0.33 M sorbitol, 10 mm Na4P207 (pH 6.5), 5 mM MgC12, and 2 mm isoascorbate (12). The chloroplasts were pelleted by centrifugation at 2,000g for 5 s. Chloroplasts from greenhousegrown spinach were washed in a buffer containing 0.4 M sorbitol, 50 mM Hepes-NaOH (pH 7.6), 10 mm NaHCO3, and 5 mm EDTA. Chloroplasts were resuspended carefully in a medium containing 0.33 M sorbitol, 50 mm Hepes-NaOH (pH 7.6), 1 mm MnCl2, 1 mM MgCl2, and 2 mm EDTA. The final suspension of chloroplasts (70-80% intact) contained about 0.5 mg/ml of Chl.02 Evolution. Rates of 02 evolution were determined polarographically using a Clark-type platinum O2 electrode placed in a 2-ml. H20-jacketed cuvette at 250C. The reaction mixture contained 0.33 M sorbitol, 50 mM Hepes-NaOH (pH 7.6), 1 mm MnCl2, 1 mm MgCl2, 2 mm EDTA, 250 units catalase, 5 mM NaHCO3, 30 to 60 ,ig Chl, and 0.25 to 0.5 mm Pi. A 75 w flood lamp that emitted 60 nE/cm2.s provided illumination for photosynthesis.Stromal pH and KV. Stromal pH was determined by the equilibration of radioactive bicarbonate ions across the envelope following the method developed by Heldt and Sauer (7). Polyethylene tubes were set up containing 30 ll 2.5 N NaOH in the bottom of the tube with a 70-,ul layer of silicone oil (Wacker AR 200) 2Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

Effects of 22 herbicides on the synthesis of RNA and protein were investigated in excised tissues. The assays measured ATP and orotate incorporation into RNA, leucine incorporation into protein, and the gibberellin-controlled induction of a-amylase Statistical analysis of the average responses measured in the four assays suggested that 14 of the herbicides inhibited RNA and protein biosynthesesin vivo.The most inhibitory chemicals were 4-hydroxy-3,5-diiodobenzonitrile (ioxynil), 2-sec-butyl-4,6-dinitrophenol (dinoseb), 3',4'-dichloropropionanilide (propanil), 2,3,5-trichloro-4-pyridinol (pyriclor), and isopropylm-chlorocarbanilate (chlorpropham). The activity of an RNA polymerase isolated from corn(Zea maysL.) tissue was inhibited maximally at approximately 20% by ioxynil, dinoseb, and pyriclor. Hence, interference with RNA polymerase, which performs a pivotal role in RNA and protein biosyntheses, does not fully account for all inhibitions.