Former studies using the chlorophyll fluorescence technique on evergreen
Photosynthesis of broad-leaved evergreens is sensitive to frost events in winter and this might play a role in limiting their distributional range. Frost can reduce the activity of RuBisCO, inhibit photosynthetic electron transport or even cause irreversible damage to the photosynthetic apparatus (
The central populations of
In winter, however, leaves of evergreen plants may become photoinhibited, as induced by low temperatures in combination with high light intensities, thereby reducing photosynthetic efficiency, a phenomenon known in conifers of the temperate and boreal zones (
In this study we additionally investigate the potential of
Five saplings of
Measurements were carried out from November 2013 to July 2014, always following the same procedure (principle shown in
Low photosynthetic induction strongly limits carbon gain (
Photosynthetic induction Aind(t) was then calculated according to
where A(t) is net photosynthesis at time t. From this, times to reach 50% of full induction (t50%A) were evaluated.
Subsequent to the induction measurements, photosynthetic light response curves were measured in these fully induced leaves. When ambient temperatures during induction measurements rose by a few degrees, leaf temperature was re-adjusted (to 1 °C above ambient). After leaves were acclimatized, measurements started at PAR of 500 µmol m-2 s-1, followed by a stepwise decrease. Data were recorded at PAR of 500, 400, 300, 100, 50, 20, 10, 5 and 0 µmol m-2 s-1, allowing for a new steady-state of CO2 exchange (after 2.5 min) between light steps. From light response curves, several parameters were evaluated: light compensation point (Icomp) as the light at which net photosynthesis is zero; light saturation of photosynthesis (Isat) as the light at which 90 % of Amax is reached; apparent quantum yield of CO2 assimilation (Φi) estimated as the slope of the initial linear part of the light response curve (usually between PAR of 10 and 50 µmol m-2 s-1 but above the so-called “Kok effect”); respiration in light (RI) estimated by extending this linear part to the
Maximum quantum yield of PSII (Fv/Fm) in darkened leaves was determined using a MINI-PAM® (Heinz Walz GmbH, Effeltrich, Germany -
where Fv is the maximal variable fluorescence and Fm and F0 the maximal and minimal fluorescence yields of the darkened samples respectively. Dark acclimation was initiated for at least 15 min prior to the measurements using leaf clips (DLC-8).
To evaluate statistical differences between groups of leaves measured in the field and the same leaves measured in the laboratory, paired
Daily radiation, air temperature and duration of frosts during the period of measurements are shown in
Respiration during winter was relatively low, since RD and RI clearly depended on temperature in leaves of all plants and over the whole period (
Slower activities at low temperatures could also be observed for the velocity of photosynthetic induction, which depends on the activity of enzymes in the Calvin-cycle and on stomatal opening (see review by
There were no such clear responses to temperature in other photosynthetic parameters (Amax, Icomp, Isat - data not shown). Amax(gross) as measured in the field changed with time and from this, three distinct phases could be visually distinguished (
During phase I, Amax(gross) was not only affected by Tleaf, but very likely also by the acclimation to long-lasting low air temperatures (and frosts) in combination with low radiation (
During the following weeks until April (phase II), Amax(gross) in the field was lower for the same or higher Tleaf compared to phase I, and remained at a relatively consistent low rate despite the increasing temperature (
From April onwards (phase III), when temperature and radiation in the field markedly increased (
The apparent quantum yield of CO2 assimilation (Φi,
In order to quantify the short-term recovery of photosynthesis from ambient winter conditions, plants were always taken into the laboratory for one night after having been measured in the field. After about 13 to 15 h at laboratory temperature, measurements were repeated here and compared to both their photosynthetic performance in the field and that of the reference plants. Although measurements in the laboratory were always performed under the same conditions (Tleaf = 21 °C), all three phases of photosynthetic performance described above could be observed here (
Phase I: during the first weeks Amax(gross), Φi and Fv/Fm of the same leaves reached significantly higher values in the laboratory (open triangles in
To evaluate the potential of photosynthetic parameters to recover in the long-term (
Phase II: the frost-hardening in phase I (
At this point one might question the artificial determination of phases. Based on Amax(gross) one could argue for the beginning of phase II after the last observed recovery in February, whereas our decision was based on the absolute amount of Amax(gross) remaining low from the end of January onwards (
Phase III: with the beginning of spring at the end of March, photosynthesis increased again both in the field and in the laboratory (
There are two completely different major traits in plants concerning mechanisms for adjusting photosynthesis to winter conditions, probably as a consequence of different leaf longevities. While herbaceous winter annuals (
We observed decreasing light-saturated photosynthetic rates in the field from November to February which could not be explained by decreasing temperatures alone (
In Central Europe,
These findings seem contradictory to chlorophyll fluorescence measurements, which indicate a much better potential for photosynthesis on warm days in winter. From experiments by
Under high light conditions, especially in winter when temperatures are low, photosynthesis of
For boreal coniferous forests,
Although Amax(gross) clearly rose again with the beginning of spring, leaves exhibited lower photosynthetic rates as compared to the previous autumn. A reason for these lower rates might be that photosynthesis has not yet fully recovered from winter, or leaves measured here during early and late spring could have come close to the end of their life spans. On average, leaves of
Similar to evergreen conifers, evergreen
We thank Margaret Eppli (Hohenheim, Stuttgart, Germany) for proofreading the manuscript and correcting the English.
Illustration of the temporal schedule of the experiment. Further explanations in the text.
Climatic conditions at the field site. (a): radiation; (b): air temperature; (c): duration of frost (including 0 °C); and (d): mean temperature and sum of radiation over the five days prior to each set of field measurements.
Respiration in darkness (RD) and in light (RI) at different leaf temperatures (Tleaf) as obtained over the whole experimental period (November to July). Values are means ± SE (n = 1-8). RD is also described as the regression: RD = -0.1806 - 0.0291 · Tleaf + 0.0018 · T2leaf - 0.0001 · T3leaf; r2=0.73.
Time to reach 50 % of full photosynthetic carbon gain induction (t50%A) in relation to leaf temperature (Tleaf). Values are means ± SE (n = 1-12). t50%A is also described as the regression: t50%A = 0.0233 · exp(352.6577 / (Tleaf + 53.5026)); r² = 0.61.
Leaf temperature (a) and light-saturated gross photosynthesis (b) derived from light response curves as measured in the field (means ± SE, n = 3). I, II, III show different phases of photosynthetic performance based on the magnitude of Amax(gross) (for further explanation see the text).
Courses of light-saturated gross photosynthesis (a), apparent quantum yield of CO2-assimilation (b) and maximal quantum yield of PSII (c) over time in leaves of reference plants (closed circles, n = 1-2), in leaves of plants measured in the field (closed triangles, n = 3) and the same leaves measured in the laboratory after one night of acclimatization (open triangles, n = 3); all means ± SE. Graph (d) shows the level of significance (p-value) derived by paired
Changes in recovery of photosynthetic parameters in the long-term: photosynthetic parameters of field plants which were measured after one night of acclimatization in the laboratory (see open triangles in
Simplified scheme of potential carbon gain (Amax(gross), Φi) of