Rates and Roles of Cyclic and Alternative Electron Flow in Potato Leaves

Abstract

Measurements of 810 nm transmittance changes in leaves, simultaneously with Chl fluorescence, CO(2) uptake and O(2) evolution, were carried out on potato (Solanum tuberosum L.) leaves with altered expression of plastidic NADP-dependent malate dehydrogenase. Electron transport rates were calculated: J(C) from the CO(2) uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O(2) evolution rate, J(F) from Chl fluorescence parameters and J(I) from the post-illumination re-reduction speed of PSI donors. In the absence of external O(2), J(O) equaled (1.005 +/- 0.003) J(C), independent of the transgenic treatment, light intensity and CO(2) concentration. This showed that nitrite and oxaloacetate reduction rates were very slow. The Mehler-type O(2) reduction was evaluated from the rate of electron accumulation at PSI after the O(2) concentration was decreased from 210 to 20 mmol mol(-1), and resulted in <1% of the linear flow. J(F) and J(I) did not differ from J(C) while photosynthesis was light-limited, but considerably exceeded J(C) at saturating light. Then, typically, J(F) = 1.2 J(C) and J(I) = 1.3 J(C), and J(F) -J(C) and J(I) -J(C) depended little on CO(2) and O(2) concentrations. The results showed that the alternative and cyclic electron flow necessary to compensate variations in the ATP/NADPH ratio were only a few percent of the linear flow. The data do not support the requirement of 14H(+)/3ATP by the chloroplast ATP synthase. We suggest that the fast PSI cyclic electron flow J(I) - J(C), as well as the fast J(F) - J(C) are energy-dissipating cycles around PSI and PSII at light saturation.

Fast cyclic electron transport around photosystem I in leaves under far-red light: a proton-uncoupled pathway?

“…Considering the oxidatively titrated PS I density of 1.6 and 2.2 lmol m -2 in these two leaves, the CET rate was 32 e -s -1 per PS I in each case. Such a fast CET is less subject to experimental error because it dominates total electron flow, and is not based on a marginal difference between total PS I turnover and fast LET as measured previously under WL (nevertheless, a comparable maximum CET of about 30 e -s -1 per PS I was observed under WL in potato by Laisk et al 2007). Though relatively fast, the maximum CET reported here is still much slower than the potential maximum LET in sunflower (about 200 e -s -1 per PS I, Laisk et al 2005).…”

The Importance of Energy Balance in Improving Photosynthetic Productivity

“…Cyclic electron flow appears to be minimally engaged (less than 14% of linear electron flow) under nonstressed conditions in C 3 plants (Cruz et al, 2005;Fan et al, 2007;Laisk et al, 2007;Livingston et al, 2010aLivingston et al, , 2010b, either because linear electron flow nearly meets the ATP required for chloroplast metabolism or because other processes (WWC, malate valve) are sufficient to balance the ATP/NADPH budget. However, cyclic electron flow appears to be important for C 4 photosynthesis, carbon-concentrating mechanisms in green algae, and coping with environmental stress Jia et al, 2008;Kohzuma et al, 2009), where additional ATP may be required.…”

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J–I–P rise

“…(7) Cyclic electron transport around PSI, CET-PSI (see Joliot et al 2006;Shikanai 2007;Laisk et al 2007Laisk et al , 2010Joliot and Johnson 2011;Johnson 2011), may also affect the fast FI. Lazár (2009), based on mathematical simulation of the O-J-I-P transient, has considered a direct influence of CET-PSI on the fluorescence transient, which is much more significant at lower, than higher, light intensities.…”