The heat tolerance of 8 temperate- and 1 subtropical-origin C3 species as well as 17 tropical-origin ones, including C3, C4, and CAM species, was estimated using both F0-T curve and the ratio of chlorophyll fluorescence parameters, prior to and after high temperature treatment. When leaves were heated at the rate of ca. 1 °C min-1 in darkness, the critical temperature (Tc) varied extensively among species. The Tc's of all 8 temperate-origin species ranged between 40-46 °C in winter (mean temperature 16-19 °C), and between 32-48 °C in summer (mean temperature ca. 30 °C). Those for 1 subtropical- and 12 tropical-origin C3 species ranged between 25-44 °C and 35-48 °C, and for 1 CAM and 4 C4 species were 41-47 and 45-46 °C, respectively. Acclimating three C3 herbaceous plants at high temperature (33/28 °C, day/night) for 10 d in winter caused their Tc's rising to nearly the values measured in summer. When leaves were exposed to 45 °C for 20 min and then kept at room temperature in darkness for 1 h, a significant correlation between RFv/m (the ratio of Fv/Fm before and after 45 °C treatment) and Tc was observed for all tested temperate-origin C3 species as well as tropical-origin CAM and C4 species. However, F0 and Fv/Fm of the tropical-origin C3 species were less sensitive to 45 °C treatment, regardless of a large variation of Tc; thus no significant correlation was found between their RFv/m and Tc. Thus Tc might not be a suitable index of heat tolerance for plants with wide range of environmental adaptation. Nevertheless, Tc's of tropical origin C3 species, varying and showing high plasticity to seasonal changes and temperature treatment, appeared suitable for the estimation of the degree of temperature acclimation in the same species. and J.-H. Weng, M.-F. Lai.
So far the photorespiration rate (RP) in a leaf has been determined as the difference between the net photosynthetic rates (PN) measured in 21 % O2 air (PN21%) and 3 % O2 air (PN3%). In the C3 plant Vigna radiata and the C4 plant Amaranthus mongostanus L., PN and chlorophyll fluorescence quenching in leaves were monitored simultaneously. RP of leaves in situ was estimated as termed RPE from the electron transport rates through photosystem 2 (PS2), and compared with RPO (PN3% - PN21%). In V. radiata RPO was 11.9 µmol(CO2) m-2 s-1 and the ratio of RPO to PN21% was 42.2 %, whereas the ratio of RPE to PN21% was 25.7 %. This suggests that RPO may be over-estimated for the real RP in normal air. In A. mongostanus, PN was almost not changed with a decrease in O2 concentration from 21 to 3 %, whereas the quantum yield of PS2 was evidently affected by the change in O2 concentration. This fact shows the presence of photorespiration in this C4 species, where RPE was equivalent to 3.8 % of PN21%. and Y. Yoshimura, F. Kubota, K. Hirao.
In order to evaluate the photosynthetic activity of a C3 leaf from the electron transport rate (ETR) of photosystem 2 (PS2), a new method was devised and examined using leaves of sweet potato. In this method, both surfaces of a leaf were sealed with transparent films to stop the gas exchange between the leaf and the atmosphere; hence the functions of both photosynthetic assimilation (CO2 uptake) and photorespiration (CO2 release) are restricted to the inside of the leaf. After both functional rates became equally balanced, ETR of the sealed leaf (ETRseal) was determined from the chlorophyll fluorescence. The measurements were conducted at different irradiances and leaf temperatures and by using leaves of different age. Under each measurement condition, ETRseal showed a close positive relationship with the photosynthetic potential, or the gross photosynthetic rate measured in the air of 2 % O2 (PG2%) before sealing. ETRseal may become an indicator to estimate or evaluate the photosynthetic activity of C3 leaves. and Haimeirong, F. Kubota, Y. Yoshimura.