Spray applications of 3.49 millimolar 2-(4-chlorophenoxy)-2-methylpropanoic acid (CPMP) significantly decreased leaf chlorophyll loss, compared with the control, after 72 hours of subcompensation-point stress. Treatment with CPMP also consistently increased leafchlorophyll per unit area under nonstress greenhouse conditions. These effects may be due to increases in specific leaf weight produced by CPMP although the compound did not consistently act as a height retardant. The compound, 3-butyl-2-hydroxy4H-pyridoll,2-alpyrimidin4-one (BHPP), inhibited senescence under low CO2 conditions but did not decrease leaf light transmission at ambient CO2 levels. The cytokinin N'-benzyladenine (BA) retarded low CO2 stress senescence although greening effects were not observed. Neither 2-hydroxy-3-butynoic acid (HBA) nor its butyl ester, inhibitors of glycolate oxidase, influenced low CO2 survival. Cyclohexanecarboxylic acid (CHCA) and sodium naphthenate had no effect upon subcompensation-point senescence.Antisenescence effects of CPMP, BHPP, and BA do not appear to be directly attributable to effects upon the competing carbon paths of photosynthesis and photorespiration. Protection against low CO2 stress and increased chlorophyll synthesis under nonstress conditions may represent separate effects upon plastids by some of the compounds. This screen will identify compounds which inhibit photorespiratory senescence without decreasing the CO2 compensation point. (21) provided HBA. The nuclear magnetic resonance and IR absorption spectra of all of the above compounds were in agreement with the published data and confirmed the assigned structures.Cyclohexanecarboxylic acid (CHCA) and N6-benzyladenine (BA) were purchased from the Aldrich Chemical Co., Inc. and 3 Abbreviations: CPMP, 2-(4-chlorophenoxy)-2-methylpropanoic acid; BHPP, 3-butyl-2-hydroxy4H-pyrido[1,2-a]pyrimidin-4-one; BHB, butyl 2-hydroxy-3-butynoate; HBA, 2-hydroxy-3-butynoic acid; CHCA, cyclohexanecarboxylic acid; NaNAP, sodium naphthenate; T, transmission value; SLW, specific leaf weight; r, C02 compensation concentration.4 CPMP(2-(4-chlorophenoxy)-2-methylpropanoic acid) has been generally referred to as p-chlorophenoxyisobutyrate or CPIB in publications dealing with its effect upon animal peroxisomes and with its hypolipidemic activity.

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