Thick sun leaves have a larger construction cost per unit leaf area than thin shade leaves. To re-evaluate the adaptive roles of sun and shade leaves, we compared the photosynthetic benefits relative to the construction cost of the leaves. We drew photosynthetically active radiation (PAR)-response curves using the leaf-mass-based photosynthetic rate to reflect the cost. The dark respiration rates of the sun and shade leaves of mulberry (Morus bombycis Koidzumi) seedlings did not differ significantly. At irradiances below 250 µmol m-2 s-1, the shade leaves tended to have a significantly larger net photosynthetic rate (PN) than the sun leaves. At irradiances above 250 µmol m-2 s-1, the PN did not differ significantly. The curves indicate that plants with thin shade leaves have a larger daily CO2 assimilation rate per construction cost than those with thick sun leaves, even in an open habitat. These results are consistently explained by a simple model of PAR extinction in a leaf. We must target factors other than the effective assimilation when we consider the adaptive roles of thick sun leaves. and M. Tateno, H. Taneda.
Citrus volkameriana (L.) plants were grown for 43 d in nutrient solutions containing 0, 2, 14, 98, or 686 µM Mn (Mn0, Mn2, Mn14, Mn98, and Mn686, respectively). To adequately investigate the combined effects of Mn nutrition and irradiance on photosystem 2 (PS2) activity, irradiance response curves for electron transport rate (ETR), nonphotochemical quenching (qN), photochemical quenching (qP), and real photochemical efficiency of PS2 (ΦPS2) were recorded under 10 different irradiances (66, 96, 136, 226, 336, 536, 811, 1 211, 1 911, and 3 111 µmol m-2 s-1, I66 to I3111, respectively) generated with the PAM-2000 fluorometer. Leaf chlorophyll content was significantly lower under Mn excess (Mn686) compared to Mn0; its highest values were recorded in the treatments Mn2-Mn98. However, ETR and ΦPS2 values were significantly lower under Mn0 compared to the other Mn treatments, when plants were exposed to irradiances ≥96 µmol m-2 s-1. Furthermore, Mn0 plants had significantly higher values of qN and lower values of qP at irradiances ≤226 and ≥336 µmol m-2 s-1, respectively, than those grown under Mn2-Mn686. Irrespective of Mn treatment, the values of ΦPS2 and qN decreased, while those of qP increased progressively by increasing irradiance from I136 to I3111. Finally, Mn2-Mn98 plants were less sensitive to photoinhibition of photosynthesis (≥811 µmol m-2 s-1) than the Mn686 (≥536 µmol m-2 s-1) and Mn0 (≥336 µmol m-2 s-1) ones. and I. E. Papadakis ... [et al.].
Changes in various components of photosynthetic apparatus during the 6-d dark incubation at 25 °C of detached control and DCMU-treated Triticum aestivum L. leaves were examined. The rate of photosystem 2 (PS2) activity was decreased with increase of the time of dark incubation in control leaves. In contrast to this, DCMU-treated leaves demonstrated high stability by slowing down the inactivation processes. Diphenyl carbazide and NH2OH restored the PS2 activity more in control leaves than in DCMU-treated leaves. Mn2+ failed to restore the PS2 activity in both control and DCMU-treated samples. Similar results were obtained when Fv/Fm was evaluated by chlorophyll fluorescence measurements. The marked loss of PS2 activity in dark incubated control leaves was primarily due to the loss of D1, 33, and 23 kDa extrinsic polypeptides and 28-25 kDa LHCP2 polypeptides. and N. Nedunchezhian, K. Muthuchelian, M. Bertamini.
Tolerance of photosystem 2 (PS2) to high temperature in apple (Malus domestica Borkh. cv. Cortland) leaves and peel was investigated by chlorophyll a fluorescence (OJIP) transient after exposure to 25 (control), 40, 42, 44, and 46 °C in the dark for 30 min. The positive L-step was more pronounced in a peel than in leaves when exposed to 44 °C. Heat-induced K-step became less pronounced in leaves than in peel when exposed to 42 °C or higher temperature. Leaves had negative L-and K-steps relative to the peel. The decrease of oxygen-evolving complex (OEC) by heat stress was higher in the peel than in the leaves. OJIP transient from the 46 °C treated peel could not reach the maximum fluorescence (Fm). The striking thermoeffect was the big decrease in the relative variable fluorescence at 30 ms (VI), especially in the leaves. Compared with the peel, the leaves had less decreased maximum PS2 quantum efficiency (Fv/Fm), photochemical rate constant (KP), Fm and performance index (PI) on absorption basis (PIabs) and less increased minimum fluorescence (F0) and non-photochemical rate constant (KN), but more increased reduction of end acceptors at PS1 electron acceptor side per cross section (RE0/CS0) and per reaction center (RE0/RC0), quantum yield of electron transport from QA - to the end acceptors (ϕ R0) and total PI (PIabs,total) when exposed to 44 °C. In conclusion, PS2 is more thermally labile than PS1. The reduction of PS2 activity by heat stress primarily results from an inactivation of OEC. PS2 was more tolerant to high temperature in the leaves than in the peel. and L.-S. Cheng, L. Chen.