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Notes: Photosynthetic Life Span of Attached Poplar Leaves Under Favorable Controlled Environmental Conditions

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Several CO2 exchange variables--net photosynthetic rate (Ps), stomatal and mesophyll resistances to CO2, and internal leaf CO2 concentration--were monitored in aging attached leaves of Populus clone 'Tristis #1' grown in a greenhouse under long days and moderate temperatures with minimum self and mutual shading of monitored leaves. One-half of the leaves monitored were treated with 10-3M 6-benzylaminopurine to try to delay leaf senescence. 6-benzylaminopurine had only small and temporary effects on CO2 exchange variables and no effect on leaf yellowing. Net photosynthetic rate declined to negligible levels by 65 days after full leaf expansion, whether leaves were green, green with yellow spots, or undergoing general yellowing. Stomatal resistance to CO2 increased after full expansion to only moderate levels over the same period. Mesophyll resistance to CO2 changed oppositely and proportionately to Ps to levels extremely limiting to Ps. Internal leaf CO2 concentration increased near the end of the monitoring period. Photosynthetic life spans were no longer than for leaves of the same clone grown in the field in the same locale. Stomatal control, chlorophyll degradation, and loss of photosynthetic capacity may be independent processes in aging poplar leaves. Green poplar leaves are not necessarily capable of CO2 fixation late in their life span. Caution is advisable when using yellowing as a criterion of leaf senescence. Forest Sci. 31:700-705.

Keywords: Leaf senescence; benzyladenine; leaf aging; mesophyll resistance; stomatal resistance

Document Type: Miscellaneous

Affiliations: Research Plant Physiologist, USDA Forest Service, North Central Forest Experiment Station, Forestry Sciences Laboratory, Rhinelander, WI 54501

Publication date: September 1, 1985

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