Fluorescence F 0 of photosystems II and I in developing C3 and C4 leaves, and implications on regulation of excitation balance

This work addresses the question of occurrence and function of photosystem II (PSII) in bundle sheath (BS) cells of leaves possessing NADP-malic enzyme-type C4 photosynthesis (Zea mays). Although no requirement for PSII activity in the BS has been established, several component proteins of PSII have been detected in BS cells of developing maize leaves exhibiting O2-insensitive photosynthesis. We used the basal fluorescence emissions of PSI (F 0I) and PSII (F 0II) as quantitative indicators of the respective relative photosystem densities. Chl fluorescence induction was measured simultaneously at 680 and 750 nm. In mature leaves, the F m(680)/F 0(680) ratio was 10.5 but less in immature leaves. We propose that the lower ratio was caused by the presence of a distinct non-variable component, F c, emitting at 680 and 750 nm. After F c was subtracted, the fluorescence of PSI (F 0I) was detected as a non-variable component at 750 nm and was undetectably low at 680 nm. Contents of Chls a and b were measured in addition to Chl fluorescence. The Chl b/(a + b) was relatively stable in developing sunflower leaves (0.25\u20130.26), but in maize it increased from 0.09 to 0.21 with leaf tissue age. In sunflower, the F 0I/(F 0I + F 0II) was 0.39 \ub1 0.01 independent of leaf age, but in maize, this parameter was 0.65 in young tissue of very low Chl content (20\u201350 mg m 122) falling to a stable level of 0.53 \ub1 0.01 at Chl contents >100 mg m 122. The values of F 0I/(F 0I + F 0II) showed that in sunflower, excitation was partitioned between PSII and PSI in a ratio of 2:1, but the same ratio was 1:1 in the C4 plant. The latter is consistent with a PSII:PSI ratio of 2:1 in maize mesophyll cells and PSI only in BS cells (2:1:1 distribution). We suggest, moreover, that redox mediation of Chl synthesis, rather than protein accumulation, regulates photosystem assembly to ensure optimum excitation balance between functional PSII and PSI. Indeed, the apparent necessity for two Chls (a and b) may reside in their targeted functions in influencing accumulation of PSI and PSII, respectively, as opposed to their spectral differences

Responses of Aspen Leaves to Heatflecks: Both Damaging and Non-Damaging Rapid Temperature Excursions Reduce Photosynthesis

During exposure to direct sunlight, leaf temperature increases rapidly and can reach values well above air temperature in temperate forest understories, especially when transpiration is limited due to drought stress, but the physiological effects of such high-temperature events are imperfectly understood. To gain insight into leaf temperature changes in the field and the effects of temperature variation on plant photosynthetic processes, we studied leaf temperature dynamics under field conditions in European aspen (Populus tremula L.) and under nursery conditions in hybrid aspen (P. tremula × P. tremuloides Michaux), and further investigated the heat response of photosynthetic activity in hybrid aspen leaves under laboratory conditions. To simulate the complex fluctuating temperature environment in the field, intact, attached leaves were subjected to short temperature increases (“heat pulses”) of varying duration over the temperature range of 30 °C−53 °C either under constant light intensity or by simultaneously raising the light intensity from 600 μmol m−2 s−1 to 1000 μmol m−2 s−1 during the heat pulse. On a warm summer day, leaf temperatures of up to 44 °C were measured in aspen leaves growing in the hemiboreal climate of Estonia. Laboratory experiments demonstrated that a moderate heat pulse of 2 min and up to 44 °C resulted in a reversible decrease of photosynthesis. The decrease in photosynthesis resulted from a combination of suppression of photosynthesis directly caused by the heat pulse and a further decrease, for a time period of 10−40 min after the heat pulse, caused by subsequent transient stomatal closure and delayed recovery of photosystem II (PSII) quantum yield. Longer and hotter heat pulses resulted in sustained inhibition of photosynthesis, primarily due to reduced PSII activity. However, cellular damage as indicated by increased membrane conductivity was not found below 50 °C. These data demonstrate that aspen is remarkably resistant to short-term heat pulses that are frequent under strongly fluctuating light regimes. Although the heat pulses did not result in cellular damage, heatflecks can significantly reduce the whole plant carbon gain in the field due to the delayed photosynthetic recovery after the heat pulse