317 Photosynthetic Characteristics of Two Cycas micronesica Leaf Cohorts

in HortScience
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  • 1 College of Agriculture & Life Sciences, Univ. of Guam, Mangilao, GU 96923

Cycas micronesica is an arborescent cycad with sclerophyllous, long-lived compound leaves that are produced in synchronized pulses. The photosynthetic characteristics of leaves in two sequential cohorts of ≈2 and ≈11 months after leaf expansion were determined in this study. Fluorescence yield following 30-min of light exclusion or from leaves engaged in photosynthesis under ambient light was measured throughout several 24-h periods to determine maximum quantum efficiency of PSII photochemistry and quantum efficiency under ambient light. Maximum quantum efficiency was similar for the two cohorts throughout the nocturnal period. Maximum quantum efficiency and quantum efficiency under ambient light declined following exposure to daily direct sun but recovered quickly each afternoon. This daily decline was greater for the older cohort than the younger cohort. Net carbon dioxide assimilation (Pn) was also determined using gas exchange, and light saturated Pn of the older cohort was 75% to 85% of that for the younger cohort during the daily maximum at late morning. Pn of the older cohort increased more slowly in the morning and declined more rapidly in the afternoon than did Pn of the younger cohort. Apparent quantum yield determined by gas exchange was similar for the two cohorts in the absence of extended sun exposure. However, this characteristic declined during midday, and the decline was greater for the older cohort. These results indicate that photosynthetic capacity of older C. micronesica leaf cohorts remains high, and these older leaves may substantially contribute to the plant's overall carbon economy. However, the number of hours during the day in which these older leaves reach their photosynthetic capacity is less than for the younger leaves.

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