The effects of nitrogen (N) supply restriction on the CO2 assimilation and photosystem 2 (PS2) function of flag leaves were compared between two contrastive Japanese rice cultivars, a low-yield cultivar released one century ago, cv. Shirobeniya (SRB), and a recently improved high-yield cultivar, cv. Akenohoshi (AKN). Both cultivars were solution-cultured at four N supply levels from N4 (control) to N1 (the lowest). With a reduction in N-supply, contents of N (LNC), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and chlorophyll (Chl) in flag leaves decreased in both cultivars. In parallel with this, the net photosynthetic rate (PN), mesophyll conductance (g m), and stomatal conductance (gs) decreased. PN was more dominantly restricted by gm than gs. The values of PN, gm, and RuBPCO content were larger in AKN than SRB at the four N supply levels. The content of Chl greatly decreased with N deficiency, but the reduction in the maximum quantum yield of PS2 was relatively small. Quantum yield of PS2 (ΦPS2) decreased with N deficiency, and its significant cultivar difference was observed between the two cultivars at N1: a high value was found in AKN. The content ratio of Chl/RuBPCO was also significantly low in AKN. The low Chl/RuBPCO is one of the reasons why AKN maintained a comparatively high PN and ΦPS2 at N deficiency. The adequate ratio of N distribution between Chl and RuBPCO is the important prerequisite for the efficient and sustainable photosynthesis in a flag leaf of rice plant under low N-input. and E. Kumagai, T. Araki, F. Kubota.
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.
Of the four tested sweet potato cultivars having different features in growth and yield, cv. Koganesengan (KOG) was sustainable in photosynthetic activity through young to aged leaves under drought. One of the causes for this phenomenon may be stomatal conductance (g s) of this cultivar that was relatively high in both aged and drought-imposed leaves. In these leaves the non-photochemical quenching (NPQ) was low and the quantum yield of photosystem 2 (Φe) was high, compared to those of the other cultivars. This helps to prevent excessive accumulation of chemical energy in leaves and a decrease in photoinhibition damage to the photosynthetic function, by which KOG sustains a relatively high photosynthetic activity under the drought and alleviates functional deterioration caused by leaf age. and Haimeirong, F. Kubota.