Characteristics Of Photosynthesis In Cycad Species

Cycads (Cycadaceae, Stangeraceae, Zamiaceae) the most primitive living seed plants have been protected by the Convention on International Trade listed in Endangered Species of Wild Fauna and Flora. Few studies have characterized photosynthesis of cycads in relation to photosynthetic product accumulation and competition in plant communities. Thus, an investigation of some cycad species was aimed to probe the photosynthetic characteristics in relation to environment factors. Licor-6400 portable photosynthesis system and PAM-2100 modulatory fluorometers were used in the study.Two years old plants of seven native Cycas: Cycas panzhihuaenasis, C. taitungensis, C. elonga, C.debaoensis, C.miquelii, C.shiwandashanica, C.micholitzii and one foreign species of Zamia furfuracea were used in the study.Net photosynthetic rates (P_n) of the cycad species were generally very low. Diurnal variations showed typical double-peak patterns. The major reason for midday depression was non-stomatal regulation limitation factors. It is speculated that less efficient transport system in cycad could be another main reason for the low photosynthetic efficiency.Parameters of light and CO₂ response curves were all in a lower level, such as apparent quantum yield(0.0132～0.0322μmol CO₂·mol^-1photons), light compensation point (85.99～177.45μmol·mol^-1 carboxylation efficiency(0.0116～0.0373). These features don't favor the accumulation of photosynthetic product. The response of C.shiwandashanica, C.debaoensis and C.miquelii to CO₂ showed non-saturation up to 2000μmol·mol^-1, which probably reflected some characteristics of their ancestors living environment.Optimum leaf temperature for photosynthesis is in range of 24.28～33.03℃, so that high temperature at noon was also negative to cycad photosynthesis accumulation. It could be concluded that their ancestors probably derived from Warm Temperate Zone, but not tropical zone.It can be concluded that there are two main adaptation mechanisms to cope with increased solar radiation through chlorophyll fluorescence analysis: one is increasing thermal energy dissipation and photorespiration, such as Cycas panzhihuaenasis, C.taitungensis and Zamia furfuracea; the other is Enhancement of photosynthetic capacity, such as C.elonga, C.debaoensis, C.miquelii, C.shiwandashanica, C.micholitzii.Preliminary study on the photoinhibition of cycads has been done. In photoinhibition disposal, the leaf colors and the content of chlorophyll and carotenoid were different among these species. The content of chlorophyll and carotenoid of C.panzhihuaensis decreased less than 10% compared to the original, but C.micholitzii decreased to more than 50%. This result implies that photosynthetic apparatus of C.panzhihuaensis presented higher tolerant potential to photoinhibition.In brief, all of the features mentioned above related to their endangered. It is suggested that cycads presented various adaptability to the environment and abundant genetic diversity.This paper is for the first time trying to understand photosynthesis of cycads in detail.