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Leaf photosynthetic and solar-tracking responses of mallow, Malva parviflora, to photon flux density
Plant Physiology and Biochemistry 47 (2009) 946–953
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Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy
Research article
Leaf photosynthetic and solar-tracking responses of mallow, Malva parviflora, to photon flux density Dennis H. Greer a, *, Michael R. Thorpe b a b
School of Agriculture and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia ¨ lich, D-52425 Ju ¨ lich, Germany Phytosphere Insitute, Forschungszentrum Ju
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2009 Accepted 4 June 2009 Available online 13 June 2009
Malva parviflora L. (mallow) is a species that occupies high-light habitats as a weedy invader in orchards and vineyards. Species of the Malvaceae are known to solar track and anecdotal evidence suggests this species may also. How M. parviflora responds physiologically to light in comparison with other species within the Malvaceae remains unknown. Tracking and photosynthetic responses to photon flux density (PFD) were evaluated on plants grown in greenhouse conditions. Tracking ability was assessed in the growth conditions and by exposing leaves to specific light intensities and measuring changes in the angle of the leaf plane. Light responses were also determined by photosynthesis and chlorophyll fluorescence. Leaves followed a heliotropic response which was highly PFD-dependent, with tracking rates increasing in a curvilinear pattern. Maximum tracking rates were up to 20 h1 and saturated for light above 1300 mmol (photons) m2 s1. This high-light saturation, both for tracking (much higher than the other species), and for photosynthesis, confirmed mallow as a high-light demanding species. Further, because there was no photoinhibition, the leaves could capture the potential of an increased carbon gain in higher irradiance by resorting to solar tracking. Modelling suggested the tracking response could increase the annual carbon gain by as much as 25% compared with leaves that do not track the sun. The various leaf attributes associated with solar tracking, therefore, help to account for the success of this species as a weed in many locations worldwide. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Malva parviflora Carbon gain Chlorophyll fluorescence Light response Leaf angle Photosynthesis Sun tracking
1. Introduction Light is the predominant driver for plant growth and it has been of long standing interest to assess how plants respond to this important environmental variable [30]. Much research has been devoted to the physiological comparisons between those plants grown in sun compared with those in shade [4,14,30]. Many of these studies have shown sun leaves invest proportionately more capacity in fixing CO2 while shade leaves invest proportionately more in capturing light. Photosynthetic light response curves have been an integral method of assessing such responses between sun and shade species. Another group of plants that have received considerable attention are those that track the sun, that is, those that maintain the leaf blade facing the sun throughout the day [13,24]. This capacity for leaf movement in response to light has also been well documented in several species of the Malvaceae [16,27,35], Fabaceae [5,23,25,26], desert annual species [13] and many others [32].
* Corresponding author. Tel.: þ61 2 6933 2725; fax: þ61 2 6933 2107. E-mail address: [email protected] (D.H. Greer). 0981-9428/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.plaphy.2009.06.002
Generally, leaf movement is seen as a means of enhancing photosynthetic carbon gain through higher light interception [20,23]. Mallow (Malva parviflora L.) is an annual native of Eurasia but has become well established as a weedy species in many orchards and vineyards throughout Australia, New Zealand and the USA [10], especially where high-light exposures occur. While the tracking capacity of this species appears not to have been assessed, field observation of mallow (Greer, unpublished) indicates the leaves can rotate generally from an easterly orientation in the morning to a westerly orientation in the afternoon. The related species Malva neglecta, however, has been well documented to track the sun [16]. This species can move in response to photon flux densities (PFD) as low as 70 mmol (photons) m2 s1. In another species of the Fabaceae, Melilotus indica, tracking rates were also light-saturated at a low PFD of 100 mmol (photons) m2 s1 [34]. It is not clear if the low-light saturation of the sun tracking process of both these species was an intrinsic property of the tracking process or perhaps a consequence of the growth condition. For example, M. neglecta was photosynthetically light-saturated at 500–600 mmol (photons) m2 s1 [16], indicative of a more shade-adapted plant. However, solar tracking is known to be influenced by blue light [11,16,41], possibly through phototropin receptors [9]. Vogelmann
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2.1. Leaf angle-tracking response to the growth conditions The angle of leaves at the start of the day was generally below about 20 but it increased significantly (Fig. 1) throughout the day by bending of the petiole to reach a maximum angle of about 65– 70 5 towards the end of the day. Additional measurements of other plants that were remeasured overnight indicated that the leaf angle at the start of the day averaged 17 3 , reached 71 4 by the end of the day, and then overnight the average leaf angle returned to 22 3 , thus not significantly different from the angle at the start of the previous day. These results show the angle of the mallow leaves shifted significantly towards the vertical throughout the day and then overnight returned to the start of day position.
Ps (μmol CO2 m-2 s-1)
10 0
Across three times of the day, there were significant differences (P < 0.001) in mallow leaf photosynthesis, stomatal conductance and transpiration in relation to PFD (Fig. 2). Leaves with an average angle of 40 and a temperature of 23 C had average light-saturated photosynthetic rates of 27.6 0.3 mmol (CO2) m2 s1 and reached 95% light-saturation at an average PFD of 1420 15 mmol (photons) m2 s1 (Fig. 2A). These data were statistically (P < 0.01) higher than leaves measured later in the day at 70 but not for those leaves at 17, with maximum rates at 24.0 1.3 and 26.4 0.9 mmol (CO2)
B
0.15 0.10 0.05 0.00 3
C
2 17o
1
44o 70o
0 2.2. Leaf gas exchange responses to PFD
A
20
0.20 gs (mol H2O m-2 s-1)
2. Results
30
E (mmol H2O m-2 s-1)
and Bjo¨rn [41], for instance, have shown blue light alone could account for 85% of a white light tracking response. Relatively low intensities are also required to saturate blue-light responses [16,36], in keeping with the low-light saturation of the tracking response. Malva species are generally thought to perceive bluelight signals by the leaf lamina [24]. Given that growth conditions and differentiation into sun and shade attributes involve leaf physiology, it may be that these attributes could also affect the sun tracking response. The objective of this study was, therefore, to characterise the photosynthetic light response of M. parviflora leaves grown in highlight conditions and then to quantify their tracking responses to different photon flux densities. These light responses were assessed by measuring leaf angle changes, gas exchange and chlorophyll fluorescence.
947
0
400
800
1200
PFD (μmol (photons)
1600 m-2
2000
s-1)
Fig. 2. Light responses of net photosynthesis (Ps, A), stomatal conductance (gs, B) and transpiration (E, C) (Mean SE, N ¼ 6) for intact fully expanded leaves of Malva parviflora that tracked under natural light throughout the day. Each leaf was constrained at the angle it had attained while its light response was recorded (less than 20 min). Each line is the hyperbolic tangent which best fits the data (see text). Responses to irradiance were measured using the Licor 6400-02B LED lamp.
m2 s1 and light-saturation at 1370 20 and 1240 18 mmol (photons) m2 s1 respectively. There were no statistically significant differences in the initial slopes of photosynthetic response to PFD between leaves at the different leaf angles, the mean apparent (not CO2-saturated) photon yield (slope of PFD response) was 0.050 0.004 mol CO2 mol (photon)1. Stomatal conductance (Fig. 2B) and transpiration (Fig. 2C) varied much less than photosynthesis in response to PFD for leaves in different angle classes, however, there were consistent differences. Leaves at 40 had significantly (P < 0.01) higher stomatal conductance and transpiration for all PFDs than leaves at 17 or 70 to the horizontal. At the higher PFDs (>800 mmol (photons) m2 s1), stomatal conductance was significantly (P < 0.05) higher in leaves at 70 than at 17 but the differences in transpiration were not significant (P > 0.05). 2.3. Photon yields in relation to leaf angle
Fig. 1. The time-course of the leaf angle between fully expanded leaves and the horizontal (Mean SE, N ¼ 8) for glasshouse-grown Malva parviflora plants. The line is the best fit to a sigmoid regression.
There were small but significant differences in the rates of oxygen evolution in relation to PFD when mallow leaves shifted from near horizontal to near vertical (Fig. 3). Photon yields declined significantly (P < 0.05) from 0.104 0.003 mmol (O2) mmol (photons)1 for leaves inclined at 16 to 0.094 0.005 mmol (O2) mmol
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20
Cos(θ) [0 = Vert., 1 = Horiz.]
O2 evolution (μmol O2 m-2 s-1)
1.0
15
10
Leaf angles o 16 o 42 o 73
5
0
0
20
40
60
80
100
120
140
160
0.8
0.6
0.4
265 μmol m-2 s-1 430 μmol m-2 s-1 1320 μmol m-2 s-1
0.2
0.0
0
100
180
PFD (μmol (photons) m-2 s-1) Fig. 3. Photosynthetic oxygen evolution in relation to PFD (Mean SE, N ¼ 3–4) for Malva parviflora leaves at different leaf angles as indicated. Leaf disks were collected at different times of the day when the leaves had attained the desired angle. Each line is a best fit linear regression, the slope giving the photon yield. The line for the mid angle has been excluded for clarity only.
(photons)for leaves at 42 . Further, though not significant (P > 0.05), yields decreased to 0.089 0.003 mmol (O2) mmol (photons)1 for leaves at 73 , late in the day. Thus, across the whole day, photon yield declined significantly (P < 0.01) while leaf angle was also changing. 2.4. Changes in leaf angle in response to PFD When exposed to a constant horizontal light beam in the laboratory treatment system, the angle of leaves initially shifted over time from the horizontal at a relatively fast rate but this progressively declined in a curvilinear pattern as the leaves approached the vertical (Fig. 4). Leaf angle was transformed to its cosine since this was found to linearize the plot and enabled rates of tracking to be related to PFD. The rate and extent of change was highly dependent on irradiance (Fig. 5), with leaves exposed to a PFD of 265 mmol (photons) m2 s1 tracking by an equivalent 40 in 7 h whereas leaves exposed to about 1320 mmol (photons) m2 s1 tracked by about 80 in the same time.
200
300
400
500
Time of exposure (min) Fig. 5. The cosine-transformed angle attained by Malva parviflora leaves (Mean SE, N ¼ 6) at regular intervals during continuous exposure to horizontal light of various intensities. Each value is the mean of six measurements – three leaves on two plants. The lines are linear regressions.
Thus, the rate at which the leaves tracked towards the light was highly PFD-dependent (Fig. 6), in a curvilinear pattern. Tracking rates were low at low PFDs and increased in a generally linear pattern up to a PFD of about 600 mmol (photons) m2 s1. With further increases in PFD, the rate increased asymptotically to a maximum of about 0.15 0.01 Cos(q) h1 when PFD exceeded about 1300 mmol (photons) m2 s1. There was, therefore, a general similarity of the PFD-responses of photosynthesis (Fig. 2) and the tracking-rate of cosine-transformed leaf angle (Fig. 6). 2.5. Changes in chlorophyll fluorescence light responses in relation to leaf tracking Prior to any exposure to the tracking light beam, mallow leaves had a typical PFD-dependent fluorescence response (Fig. 7) in that photochemical yield (Fs/Fm’) and qP declined while ETR and NPQ both increased with increasing PFD. After a 7 h exposure to the tracking beam that caused, in the case shown, a shift of 50 towards light, there was a steeper PFD-dependent and a greater decrease in
0.20
Rate of change (Cos(θ) h-1)
80
Leaf angle (o)
60
40
20
0.15
0.10
0.05
0 0
100
200
300
400
Time of exposure (min) Fig. 4. Time-course of leaf angle (Mean SE, N ¼ 6) for Malva parviflora leaves at an air temperature of 23 C during an exposure to horizontal light (700 mmol (photons) m2 s1) provided by a water-screened tungsten halogen lamp. The leaf was initially close to horizontal and shifted towards the vertical and facing the lamp. The line is the best fit to a sigmoid curve.
0.00
0
400
800
1200
1600
PFD (μmol (photons) m-2 s-1) Fig. 6. The effect of PFD on the rate of change in the cosine of leaf angle (Mean SE, N ¼ 6) of Malva parviflora leaves. Each point is the mean of three - four leaves measured over two separate days and the fitted line is the best fit to a sigmoid function.
D.H. Greer, M.R. Thorpe / Plant Physiology and Biochemistry 47 (2009) 946–953
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Fig. 7. PFD-responses (Mean SE, N ¼ 3) for the quantum yield of PSII (A, Yield), electron transport rates (B, ETR), photochemical quenching (C, qP), and non-photochemical quenching (D, NPQ) for Malva parviflora leaves prior to (closed circles) and after tracking (open) towards a light source by 50 . The PFD-responses were carried out according to Greer and Halligan [18]. The leaves were exposed throughout the day to a horizontal light beam of 500 mmol (photons) m2 s1 at a temperature of 23 C.
photochemical yield, compared with the initial state. However, there were no differences in the PFD-dependent decline in qP. In accordance with the differences in photochemical yield, ETR was reduced by about 35% after the leaf shifted towards the light. NPQ also increased at a faster rate and was about 35% higher at 1800 mmol (photons) m2 s1 than the pre-exposure control. Similar results were observed at other leaf angles (not shown). 2.6. Benefits of leaf tracking to carbon gain Using the fitted photosynthetic light response, and a model for clear-sky radiation at given time and latitude [6], we calculated the diel patterns of net carbon uptake for a leaf tracking the sun in comparison with a permanently horizontal leaf, with both leaves receiving equal diffuse radiation. Predicted photosynthetic rates at two latitudes in midsummer (Fig. 8A,B) show that the benefit of tracking is much higher if the sun is further away from the zenith for much of the day, as occurs with long twilights in high latitudes. The tracking benefit in midsummer is 13% at 15 and 21% at 65 latitude. The latitudinal variation of the annual carbon exchange for both tracking and horizontal leaves was calculated, firstly for an ‘evergreen’ leaf (active for the full year), and then for an ‘annual’ leaf (active in summer only: May 1 to Sept 30 in northern latitudes). For the ‘evergreen’ leaf, there was a decline from a maximum carbon uptake at the equator to lower values at high latitudes (Fig. 8), but an increase in uptake at higher latitudes for the ‘annual’ leaf. The benefit of tracking is more pronounced at higher latitudes in both annual and evergreen plants (Fig. 8), although the benefit was also not as great as at lower latitudes. 3. Discussion M. parviflora leaves responded heliotropically by bending the plane of the leaf towards a white light source, and reorientated back
to be horizontal in the dark. This is in keeping with the behaviour of the leaves throughout the day in the greenhouse growth conditions, where the leaves shifted from near horizontal to near vertical. While the changes in leaf angle in these growth conditions were affected by late afternoon shading in the greenhouse, nevertheless, these shifts in leaf angle were reminiscent of the behaviour of this species in field conditions, where the leaves rotate from an easterly-facing orientation to a westerly-facing orientation throughout the day in both apple orchards and vineyards (Greer, unpublished). The capacity of the M. parviflora leaves to track towards a light source is well in keeping with other Malva species [1] and genera within the Malvaceae [27]. However, the tracking response of M. parviflora leaves was highly dependent on the PFD, with tracking rates increasing markedly as the PFD increased, at least up to about 1300 mmol (photons) m2 s1 (Fig. 6). Maximum recorded rates of tracking by the mallow leaves in this study were 0.15 Cos(q) h1, and about 15 h1. In comparison, the sun tracking Melilotus indicus had maximum rates of 65 h1 [34] and Lupinus succulentus had maximum rates of 13 h1 [41]; mallow leaves were thus intermediate in the maximum tracking rates. By contrast, Phaseolus vulgaris plants grown a PFD of 200 mmol (photons) m2 s1 also responded to the incident PFD, with tracking rates ranging from 50 to 150 h1 with increasing PFDs from 400 to 2000 mmol (photons) m2 s1 [25]. However, the PFDs were directed specifically at the pulvinule of the bean leaves and hence were not directly comparable with the mallow response since the PFD was directed at the leaf blade where the photosensitive region is probably located [24]. Comparable changes in leaf inclination over the course of a day were reported by Schwartz et al. [34] for M. indicus and also for L. succulentus (recalculated from [41]), with that shown for mallow leaves. Furthermore, the Melilotus responded asymptotically to PFD in a similar pattern to that shown here. However, there was a distinct difference in that the light response for Melilotus was saturated in PFDs of 50–100 mmol (photons) m2 s1, markedly below (ca. 8%) that for mallow leaves. By contrast, for the related
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Fig. 8. The modelled consequences of tracking for net carbon exchange. The diel rhythm of net photosynthesis (Ps), responding (as in Fig. 2) only to light, with intensity and angle given by a clear-sky radiation model [6], are shown for a horizontal leaf (dotted) and a solar-tracking leaf (solid line), along with solar zenith angle (dashed line), for latitudes A: 15 and B: 60 . The modelled annual net photosynthesis for tracking and horizontal leaves (C: evergreen and D: annual leaf-durations), and the benefit of tracking, are shown as a function of latitude. For modelling purposes, the annual leaf season was defined as May 1 to September 30.
M. neglecta, the rate of change, (recalculated from [16]), was 0.13 0.01 Cos(q) h1 and well in keeping with maximum rates measured for M. parviflora. However, the rate for M. neglecta was determined at a PFD of 100 mmol (photons) m2 s1; the comparable rate for M. parviflora at the same PFD was considerably slower at about 0.025 Cos(q) h1 (Fig. 7). Unfortunately, the tracking response to PFD for M. neglecta appears not to have been determined. For L. succulentus, maximum rates of tracking at 95 mmol (photons) m2 s1 were 0.006 Cos(q) h1 (equivalent to 13 h1, (recalculated from [41]). Although the maximum tracking rate of mallow leaves was similar to those of many other species, their tracking response is apparently unique in saturating at a much higher light intensity (close to 75% full sunlight). One beneficial consequence of this high saturation irradiance is that the mallow leaves will track very slowly if shaded (for example during sunflecks or scattered clouds), and will not be far from the appropriate angle when exposed again to the solar beam. Despite reorientation of the mallow leaves towards a light source, which would have increased the intercepted PFD per unit leaf surface area [15,31], there were few differences in the photosynthetic capacity and only slight (15%) decreases in photon yield. Thus, the photosynthetic response was not affected by changes in light history caused by leaf movement. Conforming to this result, photosynthetic response was not affected by large shifts in leaf angle in several other solar tracking species [32]. However, for mallow, both the saturated photosynthetic rate of about 25 mmol (CO2) m2 s1 and the saturation PFD of 1200–1400 mmol (photons) m2 s1, were much higher than those parameters for M. neglecta (7.5 mmol (CO2) m2 s1 and 500–600 mmol (photons) m2 s1 [16]). The relatively low-light saturation of the tracking response of M. neglecta, compared to M. parviflora, was, therefore, probably
a consequence of growth conditions; M. neglecta a shade-grown plant and M. parviflora, a sun-grown plant. M. neglecta photosynthesis does not, however, usually exhibit shade characteristics in natural growth conditions [3,40]. The high rates of photosynthesis and relatively high-light saturation for photosynthesis in the mallow leaves, nevertheless, occur in other high-light demanding sun tracking species such as alfalfa [38], cotton [12], soybean [23] and the weedy species Abutilon theophrasti [22]. Prior to tracking when the leaves were still in low light, the responses of mallow leaves to PFD for photochemical yield, electron transport rate, proportions of open PSII reaction centres and nonphotochemical quenching were similar to those reported for mallow by Bjo¨rkman and Demmig-Adams [8] and also for many other species [2,18,21]. However, after mallow leaves tracked towards the light, the photochemical yield, ETR and NPQ were all affected whereas the oxidation state of PSII was unchanged. Proportions of functional PSII reaction centres hence appeared to be unaffected by the tracking and consequent exposure to high PFDs. By contrast, the reductions in photochemical yield when the leaves tracked towards the light source could be attributed almost entirely to increased non-radiative energy dissipation, presumably manifested by an increase in xanthophyll-cycle activity [8,29]. Consistent with a high NPQ capacity, Bilger and Bjo¨rkman [7] showed that M. parviflora leaves had higher rates of de-epoxidation of violaxanthin than cotton. This suggests that mallow leaves have a high capacity for xanthophyll-mediated thermal dissipation, as well as a high rate of carbon assimilation which saturates at an unusually high irradiance. Having established that in mallow leaves there is little photoinhibition even if the intercepted PFD was enhanced by solar tracking, we calculated the benefit of tracking for this plant’s
D.H. Greer, M.R. Thorpe / Plant Physiology and Biochemistry 47 (2009) 946–953
carbon exchange on both daily and annual bases. We considered a wide range of latitudes, since mallow has been reported to grow in virtually all latitudes up to 65 (http://data.gbif.org) and, as our objective was merely to explore the consequences of tracking for plants with the measured photosynthetic light response (Fig. 2), we chose not to consider other important environmental factors such as leaf temperature, which would be essential for understanding the role of tracking in the species’ growth. Because photosynthetic rate approaches saturation as PFD increases, a horizontal leaf is saturated for most of the day at low latitudes where the sun remains nearer than about 20 to the zenith, and the carbon benefit of tracking is only about 13%. In fact, plants in the field are more likely to be shaded at such times when the sun is nearer the horizon and so the benefit of tracking will tend to be lower than the model suggested. At higher latitudes where the sun is away from the zenith for a larger fraction of the day, the benefit rises to about 25% at 60 latitude. The annual carbon exchange is obviously greater for an evergreen plant, as leaves are active for the most of the year (Fig. 8), but the benefit from leaf tracking is much greater at high latitudes, due to photosynthesis when the sun is nearer the horizon and horizontal leaves are poorly illuminated. Interestingly, at low latitudes, the benefit of tracking is virtually identical, at 15%, for both the evergreen and annual leaf habit. But at 60 latitude, the benefit rises to 42% for evergreen and 25% for annual plants. Of course, these benefits may well be counterbalanced by other factors such as the lower temperatures of high latitudes. These benefits in carbon gain are, nevertheless, comparable with those measured in solar tracking compared with horizontally fixed leaves of the desert annuals Lupinus arizonicas and Malvastrum rotundifolium [17]. In conclusion, M. parviflora leaves have a high capacity to track towards a light source and the rate of tracking is extremely PFDdependent. Significantly higher PFDs are required to saturate the tracking rate in this species compared with other sun tracking species. Furthermore, the pattern of tracking response conforms closely with the photosynthetic response to light, suggesting mallow leaves are high-light demanding. Consistent with this, few changes in intrinsic photochemistry, photon yield or light-saturated photosynthesis occurred as the leaf angle shifted to maximize its absorption of photons. In other words, mallow leaves were relatively resistant to high-light induced photoinhibition. The modelling also suggested the annual carbon gain was increased about 25% as a consequence of the solar tracking habit. These various attributes may well account for the success of this species as a weed in many locations. 4. Materials and methods 4.1. Plant material and growth conditions Mallow (M. parviflora L) seeds were collected from plants growing in the wild. The seeds were germinated in a gravel/ vermiculite/pumice (70/15/15% v/v) growing medium in 1.2 L pots and raised in greenhouse conditions of natural summer photon flux densities (PFD, see [19]) and average day/night temperatures of 25 and 15 C, respectively. The plants were supplied with a modified Hoaglands nutrient solution twice daily. 4.2. Tracking capacity in greenhouse conditions The angle of mallow leaves was measured at regular intervals across several days for plants growing in the greenhouse conditions. Leaf angles were measured relative to the horizontal along the main plane of the leaf with a purpose-built protractor. This was comprised of two stainless steel arms, 200 20 mm each
951
with a slot towards one end. The slot measured 80 6 mm and the two arms were attached with a wing-nut through their respective slots. One arm was fixed in a vertical position to a wooden platform. This enabled the angle of any leaf plane from below the horizontal to near vertical to be measured. Leaves at the start of the day were typically near zero, i.e. close to horizontal. On each measurement occasion, four leaves each of two separate plants were measured. 4.3. Leaf gas exchange responses during tracking Light responses of individual mallow leaves that were tracking in natural light were measured with an open gas exchange system with a 3 2 cm leaf chamber and a LED lighting system (LI-6400, Licor, Lincoln, NE, USA). Leaves were measured at selected times of the day between 0900 and 1600 h, with the leaf constrained in the chamber for less than 20 min at the natural leaf angle while determining light responses to photon flux densities from 0 to 2000 mmol (photons) m2 (leaf area) s1 using the Licor light source. All measurements were conducted at 23 C, a leaf vapour pressure deficit of 1.2 kPa and at 350 ppm CO2. Three separate leaves on each of two plants at each time were measured and conducted over three separate days. Leaf angle was measured prior to each measurement. 4.4. Photon yield determinations The photon yield of individual mallow leaves was measured using detached 10 cm2 leaf disks using a leaf disk oxygen electrode (LDE, Hansatech, Kings Lynn, UK). PFDs were provided with a halogen lamp (LS2, Hansatech, Kings Lynn, UK) and neutral density filters and the photon yield determined on an absorbed light basis following the procedure of Greer and Halligan [18]. Each leaf disk was collected from a single leaf at selected times through the day and the leaf angle determined beforehand. Measurements were conducted on different leaves over three separate days. 4.5. Treatment system for leaf tracking response to different PFDs On each occasion, a plant was removed from the greenhouse at the start of the day and placed in a laboratory-based treatment system. This was comprised of a 75 W, 10 Halogen lamp (Hi Spot 120, Sylvania, Drummondville, Quebec, Canada) which illuminated the plant with a horizontal beam, and a vertical water-screen placed between the lamp and the plant. An additional fan was directed at the lamp for further heat dissipation. Horizontal photon flux densities at each target leaf were measured with a lightsensitive diode calibrated against a quantum sensor (LI-190SA, Licor Inc, Lincoln, NB, USA). To alter PFD, plants were positioned at different distances from the light source. Each plant was maintained at the same position over a whole day (typically 6–7 h). Before and during the measurement day for each PFD, the angles of three leaves on each of at least two plants were measured with the protractor at intervals of about 40 min. 4.6. Measurements by chlorophyll fluorescence Immediately prior to and after about 450 min exposure in the treatment system to specific PFDs, fluorescence light responses for selected leaves were determined using a fluorometer (PAM 2000, Walz, Effeltrich, Germany) and an external controlled lamp (2030B, Walz) according to the methods described by [21]. Chlorophyll fluorescence parameters, i.e. photochemical yield of electron flow to PSII (Fs/Fm’), oxidation state of PSII (qP), electron transport rate (ETR) and non-photochemical quenching (NPQ),
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were determined following van Kooten and Snel [39]. For the ETR determinations, leaf absorptance was measured in a laboratorydesigned integrating sphere as described by [18]. Each leaf was measured in situ within the treatment system. Each fluorescence response was recorded on at least three leaves on separate plants for each PFD. 4.7. Data analyses Leaf angle, q, was transformed to its cosine, Cos q, since this showed an approximately linear change with time. Denoting
dðcos qÞ dt
g ¼
(1)
Then
dq dt
g ¼ ðsin qÞ
(2)
To infer the rate of angle-tracking from the g, the cosine-transformed angle rate, the leaf angle is important:
g dq ¼ dt sin q
(3)
All data were analysed statistically using general linear models [33] for which least squares means and standard errors were determined. Curve fitting used the non-linear functions of Origin [28]. Photosynthetic light response data were fitted by a rectangular hyperbola from which we derived light saturation attributes according to Greer and Halligan [18]. 4.8. Modelling the effects of solar tracking on carbon gain A model of clear-sky irradiance [6] was combined with our fitted photosynthetic light response to calculate the photosynthetic rate of tracking and horizontal leaves, and the percentage benefit from tracking in comparison with a permanently horizontal habit. The clear-sky model accounted for atmospheric absorption and scattering of the extraterrestrial beam, giving direct and diffuse solar radiation (QB, QD: W m2) for a given latitude and time. The corresponding irradiances for the photosynthetic spectrum were calculated as 1.68QB and 2.88QD [37]. For simplicity, diffuse radiation was taken to be hemispherically isotropic, so that incident radiation driving the photosynthesis of tracking and horizontal leaves, respectively, was
PFDs ¼ 1:68B cosðZÞ þ 2:88QD PFDH ¼ 1:68QB þ 2:88QD
mmol m2 s1
mmol m2 s1
(4) (5)
The annual carbon exchange, AT, AH, for each scenario was calculated as the integral of the leaf photosynthetic rate, both where leaves remained active throughout the year, i.e. ‘‘evergreen’’, and also for a leaf duration from spring to autumn, i.e. ‘‘annual’’ (1 May to 31 September for northern latitudes). We used a 30 min timestep and thus
A ¼ 1:8 106
X
PFD
mol m2
(6)
The benefit of tracking, B, was calculated as a percentage:
As B ¼ 100 1 AH
(7)
Acknowledgments We thank Wayne Scott for technical assistance for this project. The study was also technically supported by Evert Van Thoor while on an Internship from Wageningen University and Research Centre for a Masters degree in Agricultural Engineering. We also thank SAS Australia for their generous support to the senior author for supplying software.
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Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy
Research article
Leaf photosynthetic and solar-tracking responses of mallow, Malva parviflora, to photon flux density Dennis H. Greer a, *, Michael R. Thorpe b a b
School of Agriculture and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia ¨ lich, D-52425 Ju ¨ lich, Germany Phytosphere Insitute, Forschungszentrum Ju
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2009 Accepted 4 June 2009 Available online 13 June 2009
Malva parviflora L. (mallow) is a species that occupies high-light habitats as a weedy invader in orchards and vineyards. Species of the Malvaceae are known to solar track and anecdotal evidence suggests this species may also. How M. parviflora responds physiologically to light in comparison with other species within the Malvaceae remains unknown. Tracking and photosynthetic responses to photon flux density (PFD) were evaluated on plants grown in greenhouse conditions. Tracking ability was assessed in the growth conditions and by exposing leaves to specific light intensities and measuring changes in the angle of the leaf plane. Light responses were also determined by photosynthesis and chlorophyll fluorescence. Leaves followed a heliotropic response which was highly PFD-dependent, with tracking rates increasing in a curvilinear pattern. Maximum tracking rates were up to 20 h1 and saturated for light above 1300 mmol (photons) m2 s1. This high-light saturation, both for tracking (much higher than the other species), and for photosynthesis, confirmed mallow as a high-light demanding species. Further, because there was no photoinhibition, the leaves could capture the potential of an increased carbon gain in higher irradiance by resorting to solar tracking. Modelling suggested the tracking response could increase the annual carbon gain by as much as 25% compared with leaves that do not track the sun. The various leaf attributes associated with solar tracking, therefore, help to account for the success of this species as a weed in many locations worldwide. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Malva parviflora Carbon gain Chlorophyll fluorescence Light response Leaf angle Photosynthesis Sun tracking
1. Introduction Light is the predominant driver for plant growth and it has been of long standing interest to assess how plants respond to this important environmental variable [30]. Much research has been devoted to the physiological comparisons between those plants grown in sun compared with those in shade [4,14,30]. Many of these studies have shown sun leaves invest proportionately more capacity in fixing CO2 while shade leaves invest proportionately more in capturing light. Photosynthetic light response curves have been an integral method of assessing such responses between sun and shade species. Another group of plants that have received considerable attention are those that track the sun, that is, those that maintain the leaf blade facing the sun throughout the day [13,24]. This capacity for leaf movement in response to light has also been well documented in several species of the Malvaceae [16,27,35], Fabaceae [5,23,25,26], desert annual species [13] and many others [32].
* Corresponding author. Tel.: þ61 2 6933 2725; fax: þ61 2 6933 2107. E-mail address: [email protected] (D.H. Greer). 0981-9428/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.plaphy.2009.06.002
Generally, leaf movement is seen as a means of enhancing photosynthetic carbon gain through higher light interception [20,23]. Mallow (Malva parviflora L.) is an annual native of Eurasia but has become well established as a weedy species in many orchards and vineyards throughout Australia, New Zealand and the USA [10], especially where high-light exposures occur. While the tracking capacity of this species appears not to have been assessed, field observation of mallow (Greer, unpublished) indicates the leaves can rotate generally from an easterly orientation in the morning to a westerly orientation in the afternoon. The related species Malva neglecta, however, has been well documented to track the sun [16]. This species can move in response to photon flux densities (PFD) as low as 70 mmol (photons) m2 s1. In another species of the Fabaceae, Melilotus indica, tracking rates were also light-saturated at a low PFD of 100 mmol (photons) m2 s1 [34]. It is not clear if the low-light saturation of the sun tracking process of both these species was an intrinsic property of the tracking process or perhaps a consequence of the growth condition. For example, M. neglecta was photosynthetically light-saturated at 500–600 mmol (photons) m2 s1 [16], indicative of a more shade-adapted plant. However, solar tracking is known to be influenced by blue light [11,16,41], possibly through phototropin receptors [9]. Vogelmann
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2.1. Leaf angle-tracking response to the growth conditions The angle of leaves at the start of the day was generally below about 20 but it increased significantly (Fig. 1) throughout the day by bending of the petiole to reach a maximum angle of about 65– 70 5 towards the end of the day. Additional measurements of other plants that were remeasured overnight indicated that the leaf angle at the start of the day averaged 17 3 , reached 71 4 by the end of the day, and then overnight the average leaf angle returned to 22 3 , thus not significantly different from the angle at the start of the previous day. These results show the angle of the mallow leaves shifted significantly towards the vertical throughout the day and then overnight returned to the start of day position.
Ps (μmol CO2 m-2 s-1)
10 0
Across three times of the day, there were significant differences (P < 0.001) in mallow leaf photosynthesis, stomatal conductance and transpiration in relation to PFD (Fig. 2). Leaves with an average angle of 40 and a temperature of 23 C had average light-saturated photosynthetic rates of 27.6 0.3 mmol (CO2) m2 s1 and reached 95% light-saturation at an average PFD of 1420 15 mmol (photons) m2 s1 (Fig. 2A). These data were statistically (P < 0.01) higher than leaves measured later in the day at 70 but not for those leaves at 17, with maximum rates at 24.0 1.3 and 26.4 0.9 mmol (CO2)
B
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E (mmol H2O m-2 s-1)
and Bjo¨rn [41], for instance, have shown blue light alone could account for 85% of a white light tracking response. Relatively low intensities are also required to saturate blue-light responses [16,36], in keeping with the low-light saturation of the tracking response. Malva species are generally thought to perceive bluelight signals by the leaf lamina [24]. Given that growth conditions and differentiation into sun and shade attributes involve leaf physiology, it may be that these attributes could also affect the sun tracking response. The objective of this study was, therefore, to characterise the photosynthetic light response of M. parviflora leaves grown in highlight conditions and then to quantify their tracking responses to different photon flux densities. These light responses were assessed by measuring leaf angle changes, gas exchange and chlorophyll fluorescence.
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1600 m-2
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Fig. 2. Light responses of net photosynthesis (Ps, A), stomatal conductance (gs, B) and transpiration (E, C) (Mean SE, N ¼ 6) for intact fully expanded leaves of Malva parviflora that tracked under natural light throughout the day. Each leaf was constrained at the angle it had attained while its light response was recorded (less than 20 min). Each line is the hyperbolic tangent which best fits the data (see text). Responses to irradiance were measured using the Licor 6400-02B LED lamp.
m2 s1 and light-saturation at 1370 20 and 1240 18 mmol (photons) m2 s1 respectively. There were no statistically significant differences in the initial slopes of photosynthetic response to PFD between leaves at the different leaf angles, the mean apparent (not CO2-saturated) photon yield (slope of PFD response) was 0.050 0.004 mol CO2 mol (photon)1. Stomatal conductance (Fig. 2B) and transpiration (Fig. 2C) varied much less than photosynthesis in response to PFD for leaves in different angle classes, however, there were consistent differences. Leaves at 40 had significantly (P < 0.01) higher stomatal conductance and transpiration for all PFDs than leaves at 17 or 70 to the horizontal. At the higher PFDs (>800 mmol (photons) m2 s1), stomatal conductance was significantly (P < 0.05) higher in leaves at 70 than at 17 but the differences in transpiration were not significant (P > 0.05). 2.3. Photon yields in relation to leaf angle
Fig. 1. The time-course of the leaf angle between fully expanded leaves and the horizontal (Mean SE, N ¼ 8) for glasshouse-grown Malva parviflora plants. The line is the best fit to a sigmoid regression.
There were small but significant differences in the rates of oxygen evolution in relation to PFD when mallow leaves shifted from near horizontal to near vertical (Fig. 3). Photon yields declined significantly (P < 0.05) from 0.104 0.003 mmol (O2) mmol (photons)1 for leaves inclined at 16 to 0.094 0.005 mmol (O2) mmol
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20
Cos(θ) [0 = Vert., 1 = Horiz.]
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PFD (μmol (photons) m-2 s-1) Fig. 3. Photosynthetic oxygen evolution in relation to PFD (Mean SE, N ¼ 3–4) for Malva parviflora leaves at different leaf angles as indicated. Leaf disks were collected at different times of the day when the leaves had attained the desired angle. Each line is a best fit linear regression, the slope giving the photon yield. The line for the mid angle has been excluded for clarity only.
(photons)for leaves at 42 . Further, though not significant (P > 0.05), yields decreased to 0.089 0.003 mmol (O2) mmol (photons)1 for leaves at 73 , late in the day. Thus, across the whole day, photon yield declined significantly (P < 0.01) while leaf angle was also changing. 2.4. Changes in leaf angle in response to PFD When exposed to a constant horizontal light beam in the laboratory treatment system, the angle of leaves initially shifted over time from the horizontal at a relatively fast rate but this progressively declined in a curvilinear pattern as the leaves approached the vertical (Fig. 4). Leaf angle was transformed to its cosine since this was found to linearize the plot and enabled rates of tracking to be related to PFD. The rate and extent of change was highly dependent on irradiance (Fig. 5), with leaves exposed to a PFD of 265 mmol (photons) m2 s1 tracking by an equivalent 40 in 7 h whereas leaves exposed to about 1320 mmol (photons) m2 s1 tracked by about 80 in the same time.
200
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Time of exposure (min) Fig. 5. The cosine-transformed angle attained by Malva parviflora leaves (Mean SE, N ¼ 6) at regular intervals during continuous exposure to horizontal light of various intensities. Each value is the mean of six measurements – three leaves on two plants. The lines are linear regressions.
Thus, the rate at which the leaves tracked towards the light was highly PFD-dependent (Fig. 6), in a curvilinear pattern. Tracking rates were low at low PFDs and increased in a generally linear pattern up to a PFD of about 600 mmol (photons) m2 s1. With further increases in PFD, the rate increased asymptotically to a maximum of about 0.15 0.01 Cos(q) h1 when PFD exceeded about 1300 mmol (photons) m2 s1. There was, therefore, a general similarity of the PFD-responses of photosynthesis (Fig. 2) and the tracking-rate of cosine-transformed leaf angle (Fig. 6). 2.5. Changes in chlorophyll fluorescence light responses in relation to leaf tracking Prior to any exposure to the tracking light beam, mallow leaves had a typical PFD-dependent fluorescence response (Fig. 7) in that photochemical yield (Fs/Fm’) and qP declined while ETR and NPQ both increased with increasing PFD. After a 7 h exposure to the tracking beam that caused, in the case shown, a shift of 50 towards light, there was a steeper PFD-dependent and a greater decrease in
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0 0
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Time of exposure (min) Fig. 4. Time-course of leaf angle (Mean SE, N ¼ 6) for Malva parviflora leaves at an air temperature of 23 C during an exposure to horizontal light (700 mmol (photons) m2 s1) provided by a water-screened tungsten halogen lamp. The leaf was initially close to horizontal and shifted towards the vertical and facing the lamp. The line is the best fit to a sigmoid curve.
0.00
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400
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PFD (μmol (photons) m-2 s-1) Fig. 6. The effect of PFD on the rate of change in the cosine of leaf angle (Mean SE, N ¼ 6) of Malva parviflora leaves. Each point is the mean of three - four leaves measured over two separate days and the fitted line is the best fit to a sigmoid function.
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Fig. 7. PFD-responses (Mean SE, N ¼ 3) for the quantum yield of PSII (A, Yield), electron transport rates (B, ETR), photochemical quenching (C, qP), and non-photochemical quenching (D, NPQ) for Malva parviflora leaves prior to (closed circles) and after tracking (open) towards a light source by 50 . The PFD-responses were carried out according to Greer and Halligan [18]. The leaves were exposed throughout the day to a horizontal light beam of 500 mmol (photons) m2 s1 at a temperature of 23 C.
photochemical yield, compared with the initial state. However, there were no differences in the PFD-dependent decline in qP. In accordance with the differences in photochemical yield, ETR was reduced by about 35% after the leaf shifted towards the light. NPQ also increased at a faster rate and was about 35% higher at 1800 mmol (photons) m2 s1 than the pre-exposure control. Similar results were observed at other leaf angles (not shown). 2.6. Benefits of leaf tracking to carbon gain Using the fitted photosynthetic light response, and a model for clear-sky radiation at given time and latitude [6], we calculated the diel patterns of net carbon uptake for a leaf tracking the sun in comparison with a permanently horizontal leaf, with both leaves receiving equal diffuse radiation. Predicted photosynthetic rates at two latitudes in midsummer (Fig. 8A,B) show that the benefit of tracking is much higher if the sun is further away from the zenith for much of the day, as occurs with long twilights in high latitudes. The tracking benefit in midsummer is 13% at 15 and 21% at 65 latitude. The latitudinal variation of the annual carbon exchange for both tracking and horizontal leaves was calculated, firstly for an ‘evergreen’ leaf (active for the full year), and then for an ‘annual’ leaf (active in summer only: May 1 to Sept 30 in northern latitudes). For the ‘evergreen’ leaf, there was a decline from a maximum carbon uptake at the equator to lower values at high latitudes (Fig. 8), but an increase in uptake at higher latitudes for the ‘annual’ leaf. The benefit of tracking is more pronounced at higher latitudes in both annual and evergreen plants (Fig. 8), although the benefit was also not as great as at lower latitudes. 3. Discussion M. parviflora leaves responded heliotropically by bending the plane of the leaf towards a white light source, and reorientated back
to be horizontal in the dark. This is in keeping with the behaviour of the leaves throughout the day in the greenhouse growth conditions, where the leaves shifted from near horizontal to near vertical. While the changes in leaf angle in these growth conditions were affected by late afternoon shading in the greenhouse, nevertheless, these shifts in leaf angle were reminiscent of the behaviour of this species in field conditions, where the leaves rotate from an easterly-facing orientation to a westerly-facing orientation throughout the day in both apple orchards and vineyards (Greer, unpublished). The capacity of the M. parviflora leaves to track towards a light source is well in keeping with other Malva species [1] and genera within the Malvaceae [27]. However, the tracking response of M. parviflora leaves was highly dependent on the PFD, with tracking rates increasing markedly as the PFD increased, at least up to about 1300 mmol (photons) m2 s1 (Fig. 6). Maximum recorded rates of tracking by the mallow leaves in this study were 0.15 Cos(q) h1, and about 15 h1. In comparison, the sun tracking Melilotus indicus had maximum rates of 65 h1 [34] and Lupinus succulentus had maximum rates of 13 h1 [41]; mallow leaves were thus intermediate in the maximum tracking rates. By contrast, Phaseolus vulgaris plants grown a PFD of 200 mmol (photons) m2 s1 also responded to the incident PFD, with tracking rates ranging from 50 to 150 h1 with increasing PFDs from 400 to 2000 mmol (photons) m2 s1 [25]. However, the PFDs were directed specifically at the pulvinule of the bean leaves and hence were not directly comparable with the mallow response since the PFD was directed at the leaf blade where the photosensitive region is probably located [24]. Comparable changes in leaf inclination over the course of a day were reported by Schwartz et al. [34] for M. indicus and also for L. succulentus (recalculated from [41]), with that shown for mallow leaves. Furthermore, the Melilotus responded asymptotically to PFD in a similar pattern to that shown here. However, there was a distinct difference in that the light response for Melilotus was saturated in PFDs of 50–100 mmol (photons) m2 s1, markedly below (ca. 8%) that for mallow leaves. By contrast, for the related
950
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Fig. 8. The modelled consequences of tracking for net carbon exchange. The diel rhythm of net photosynthesis (Ps), responding (as in Fig. 2) only to light, with intensity and angle given by a clear-sky radiation model [6], are shown for a horizontal leaf (dotted) and a solar-tracking leaf (solid line), along with solar zenith angle (dashed line), for latitudes A: 15 and B: 60 . The modelled annual net photosynthesis for tracking and horizontal leaves (C: evergreen and D: annual leaf-durations), and the benefit of tracking, are shown as a function of latitude. For modelling purposes, the annual leaf season was defined as May 1 to September 30.
M. neglecta, the rate of change, (recalculated from [16]), was 0.13 0.01 Cos(q) h1 and well in keeping with maximum rates measured for M. parviflora. However, the rate for M. neglecta was determined at a PFD of 100 mmol (photons) m2 s1; the comparable rate for M. parviflora at the same PFD was considerably slower at about 0.025 Cos(q) h1 (Fig. 7). Unfortunately, the tracking response to PFD for M. neglecta appears not to have been determined. For L. succulentus, maximum rates of tracking at 95 mmol (photons) m2 s1 were 0.006 Cos(q) h1 (equivalent to 13 h1, (recalculated from [41]). Although the maximum tracking rate of mallow leaves was similar to those of many other species, their tracking response is apparently unique in saturating at a much higher light intensity (close to 75% full sunlight). One beneficial consequence of this high saturation irradiance is that the mallow leaves will track very slowly if shaded (for example during sunflecks or scattered clouds), and will not be far from the appropriate angle when exposed again to the solar beam. Despite reorientation of the mallow leaves towards a light source, which would have increased the intercepted PFD per unit leaf surface area [15,31], there were few differences in the photosynthetic capacity and only slight (15%) decreases in photon yield. Thus, the photosynthetic response was not affected by changes in light history caused by leaf movement. Conforming to this result, photosynthetic response was not affected by large shifts in leaf angle in several other solar tracking species [32]. However, for mallow, both the saturated photosynthetic rate of about 25 mmol (CO2) m2 s1 and the saturation PFD of 1200–1400 mmol (photons) m2 s1, were much higher than those parameters for M. neglecta (7.5 mmol (CO2) m2 s1 and 500–600 mmol (photons) m2 s1 [16]). The relatively low-light saturation of the tracking response of M. neglecta, compared to M. parviflora, was, therefore, probably
a consequence of growth conditions; M. neglecta a shade-grown plant and M. parviflora, a sun-grown plant. M. neglecta photosynthesis does not, however, usually exhibit shade characteristics in natural growth conditions [3,40]. The high rates of photosynthesis and relatively high-light saturation for photosynthesis in the mallow leaves, nevertheless, occur in other high-light demanding sun tracking species such as alfalfa [38], cotton [12], soybean [23] and the weedy species Abutilon theophrasti [22]. Prior to tracking when the leaves were still in low light, the responses of mallow leaves to PFD for photochemical yield, electron transport rate, proportions of open PSII reaction centres and nonphotochemical quenching were similar to those reported for mallow by Bjo¨rkman and Demmig-Adams [8] and also for many other species [2,18,21]. However, after mallow leaves tracked towards the light, the photochemical yield, ETR and NPQ were all affected whereas the oxidation state of PSII was unchanged. Proportions of functional PSII reaction centres hence appeared to be unaffected by the tracking and consequent exposure to high PFDs. By contrast, the reductions in photochemical yield when the leaves tracked towards the light source could be attributed almost entirely to increased non-radiative energy dissipation, presumably manifested by an increase in xanthophyll-cycle activity [8,29]. Consistent with a high NPQ capacity, Bilger and Bjo¨rkman [7] showed that M. parviflora leaves had higher rates of de-epoxidation of violaxanthin than cotton. This suggests that mallow leaves have a high capacity for xanthophyll-mediated thermal dissipation, as well as a high rate of carbon assimilation which saturates at an unusually high irradiance. Having established that in mallow leaves there is little photoinhibition even if the intercepted PFD was enhanced by solar tracking, we calculated the benefit of tracking for this plant’s
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carbon exchange on both daily and annual bases. We considered a wide range of latitudes, since mallow has been reported to grow in virtually all latitudes up to 65 (http://data.gbif.org) and, as our objective was merely to explore the consequences of tracking for plants with the measured photosynthetic light response (Fig. 2), we chose not to consider other important environmental factors such as leaf temperature, which would be essential for understanding the role of tracking in the species’ growth. Because photosynthetic rate approaches saturation as PFD increases, a horizontal leaf is saturated for most of the day at low latitudes where the sun remains nearer than about 20 to the zenith, and the carbon benefit of tracking is only about 13%. In fact, plants in the field are more likely to be shaded at such times when the sun is nearer the horizon and so the benefit of tracking will tend to be lower than the model suggested. At higher latitudes where the sun is away from the zenith for a larger fraction of the day, the benefit rises to about 25% at 60 latitude. The annual carbon exchange is obviously greater for an evergreen plant, as leaves are active for the most of the year (Fig. 8), but the benefit from leaf tracking is much greater at high latitudes, due to photosynthesis when the sun is nearer the horizon and horizontal leaves are poorly illuminated. Interestingly, at low latitudes, the benefit of tracking is virtually identical, at 15%, for both the evergreen and annual leaf habit. But at 60 latitude, the benefit rises to 42% for evergreen and 25% for annual plants. Of course, these benefits may well be counterbalanced by other factors such as the lower temperatures of high latitudes. These benefits in carbon gain are, nevertheless, comparable with those measured in solar tracking compared with horizontally fixed leaves of the desert annuals Lupinus arizonicas and Malvastrum rotundifolium [17]. In conclusion, M. parviflora leaves have a high capacity to track towards a light source and the rate of tracking is extremely PFDdependent. Significantly higher PFDs are required to saturate the tracking rate in this species compared with other sun tracking species. Furthermore, the pattern of tracking response conforms closely with the photosynthetic response to light, suggesting mallow leaves are high-light demanding. Consistent with this, few changes in intrinsic photochemistry, photon yield or light-saturated photosynthesis occurred as the leaf angle shifted to maximize its absorption of photons. In other words, mallow leaves were relatively resistant to high-light induced photoinhibition. The modelling also suggested the annual carbon gain was increased about 25% as a consequence of the solar tracking habit. These various attributes may well account for the success of this species as a weed in many locations. 4. Materials and methods 4.1. Plant material and growth conditions Mallow (M. parviflora L) seeds were collected from plants growing in the wild. The seeds were germinated in a gravel/ vermiculite/pumice (70/15/15% v/v) growing medium in 1.2 L pots and raised in greenhouse conditions of natural summer photon flux densities (PFD, see [19]) and average day/night temperatures of 25 and 15 C, respectively. The plants were supplied with a modified Hoaglands nutrient solution twice daily. 4.2. Tracking capacity in greenhouse conditions The angle of mallow leaves was measured at regular intervals across several days for plants growing in the greenhouse conditions. Leaf angles were measured relative to the horizontal along the main plane of the leaf with a purpose-built protractor. This was comprised of two stainless steel arms, 200 20 mm each
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with a slot towards one end. The slot measured 80 6 mm and the two arms were attached with a wing-nut through their respective slots. One arm was fixed in a vertical position to a wooden platform. This enabled the angle of any leaf plane from below the horizontal to near vertical to be measured. Leaves at the start of the day were typically near zero, i.e. close to horizontal. On each measurement occasion, four leaves each of two separate plants were measured. 4.3. Leaf gas exchange responses during tracking Light responses of individual mallow leaves that were tracking in natural light were measured with an open gas exchange system with a 3 2 cm leaf chamber and a LED lighting system (LI-6400, Licor, Lincoln, NE, USA). Leaves were measured at selected times of the day between 0900 and 1600 h, with the leaf constrained in the chamber for less than 20 min at the natural leaf angle while determining light responses to photon flux densities from 0 to 2000 mmol (photons) m2 (leaf area) s1 using the Licor light source. All measurements were conducted at 23 C, a leaf vapour pressure deficit of 1.2 kPa and at 350 ppm CO2. Three separate leaves on each of two plants at each time were measured and conducted over three separate days. Leaf angle was measured prior to each measurement. 4.4. Photon yield determinations The photon yield of individual mallow leaves was measured using detached 10 cm2 leaf disks using a leaf disk oxygen electrode (LDE, Hansatech, Kings Lynn, UK). PFDs were provided with a halogen lamp (LS2, Hansatech, Kings Lynn, UK) and neutral density filters and the photon yield determined on an absorbed light basis following the procedure of Greer and Halligan [18]. Each leaf disk was collected from a single leaf at selected times through the day and the leaf angle determined beforehand. Measurements were conducted on different leaves over three separate days. 4.5. Treatment system for leaf tracking response to different PFDs On each occasion, a plant was removed from the greenhouse at the start of the day and placed in a laboratory-based treatment system. This was comprised of a 75 W, 10 Halogen lamp (Hi Spot 120, Sylvania, Drummondville, Quebec, Canada) which illuminated the plant with a horizontal beam, and a vertical water-screen placed between the lamp and the plant. An additional fan was directed at the lamp for further heat dissipation. Horizontal photon flux densities at each target leaf were measured with a lightsensitive diode calibrated against a quantum sensor (LI-190SA, Licor Inc, Lincoln, NB, USA). To alter PFD, plants were positioned at different distances from the light source. Each plant was maintained at the same position over a whole day (typically 6–7 h). Before and during the measurement day for each PFD, the angles of three leaves on each of at least two plants were measured with the protractor at intervals of about 40 min. 4.6. Measurements by chlorophyll fluorescence Immediately prior to and after about 450 min exposure in the treatment system to specific PFDs, fluorescence light responses for selected leaves were determined using a fluorometer (PAM 2000, Walz, Effeltrich, Germany) and an external controlled lamp (2030B, Walz) according to the methods described by [21]. Chlorophyll fluorescence parameters, i.e. photochemical yield of electron flow to PSII (Fs/Fm’), oxidation state of PSII (qP), electron transport rate (ETR) and non-photochemical quenching (NPQ),
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were determined following van Kooten and Snel [39]. For the ETR determinations, leaf absorptance was measured in a laboratorydesigned integrating sphere as described by [18]. Each leaf was measured in situ within the treatment system. Each fluorescence response was recorded on at least three leaves on separate plants for each PFD. 4.7. Data analyses Leaf angle, q, was transformed to its cosine, Cos q, since this showed an approximately linear change with time. Denoting
dðcos qÞ dt
g ¼
(1)
Then
dq dt
g ¼ ðsin qÞ
(2)
To infer the rate of angle-tracking from the g, the cosine-transformed angle rate, the leaf angle is important:
g dq ¼ dt sin q
(3)
All data were analysed statistically using general linear models [33] for which least squares means and standard errors were determined. Curve fitting used the non-linear functions of Origin [28]. Photosynthetic light response data were fitted by a rectangular hyperbola from which we derived light saturation attributes according to Greer and Halligan [18]. 4.8. Modelling the effects of solar tracking on carbon gain A model of clear-sky irradiance [6] was combined with our fitted photosynthetic light response to calculate the photosynthetic rate of tracking and horizontal leaves, and the percentage benefit from tracking in comparison with a permanently horizontal habit. The clear-sky model accounted for atmospheric absorption and scattering of the extraterrestrial beam, giving direct and diffuse solar radiation (QB, QD: W m2) for a given latitude and time. The corresponding irradiances for the photosynthetic spectrum were calculated as 1.68QB and 2.88QD [37]. For simplicity, diffuse radiation was taken to be hemispherically isotropic, so that incident radiation driving the photosynthesis of tracking and horizontal leaves, respectively, was
PFDs ¼ 1:68B cosðZÞ þ 2:88QD PFDH ¼ 1:68QB þ 2:88QD
mmol m2 s1
mmol m2 s1
(4) (5)
The annual carbon exchange, AT, AH, for each scenario was calculated as the integral of the leaf photosynthetic rate, both where leaves remained active throughout the year, i.e. ‘‘evergreen’’, and also for a leaf duration from spring to autumn, i.e. ‘‘annual’’ (1 May to 31 September for northern latitudes). We used a 30 min timestep and thus
A ¼ 1:8 106
X
PFD
mol m2
(6)
The benefit of tracking, B, was calculated as a percentage:
As B ¼ 100 1 AH
(7)
Acknowledgments We thank Wayne Scott for technical assistance for this project. The study was also technically supported by Evert Van Thoor while on an Internship from Wageningen University and Research Centre for a Masters degree in Agricultural Engineering. We also thank SAS Australia for their generous support to the senior author for supplying software.
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