Journal of Experimental Botany, Vol. 51, No. 349, pp. 1467-1470,
August 2000
© 2000 Oxford University Press
A simple method to determine leaf angles of grass species
Laboratory of Plant and Vegetation Ecology, Department of Biology, University of Antwerp UIA, Universiteitsplein 1, 2610 Wilrijk, Belgium
Received 22 December 1999; Accepted 4 April 2000
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Abstract |
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There are several very accurate methods to determine leaf angles in closed canopies. However, these are generally very time-consuming or require special equipment. Average canopy leaf angles were derived from simple height and blade length measurements. An exponential relationship between the height/length ratio and the average blade leaf angle was used. The method was tested for two grass species, Dactylis glomerata and Festuca arundinacea, grown under different UV-B levels. The results clearly show that the method is reasonably accurate and able to identify UV-B induced changes in leaf angle. To get these results only 50 measurements of leaf blade height and length were necessary to calculate the allometric relationship, after which 10 length and height measurements from a canopy were used to calculate the average canopy leaf angle.
Key words: Leaf angles, height/length ratio, grasses.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The determination of leaf angles is important in various kinds of studies. Many kinds of stresses and environmental factors induce changes in leaf angles, including changes in UV-B irradiation (Tosserams et al., 1997
). These changes are important since leaf angles influence canopy light absorption and therefore photosynthesis. Consequently, in many photosynthesis models, average canopy leaf angle is used as a parameter (Aries et al., 1993; Thornley and Johnson, 1993).
However, most techniques for determining leaf angles are either very time-consuming or require specialized equipment. Initially, an apparatus was designed for point-quadrat measurements (Warren-Wilson, 1963). The system is based on a fine needle pushed into the vegetation in a fixed direction, while the experimenter counts the number of contacts. Recently, similar methods using laser beams have been described (Sinoquet et al., 1993). Another sophisticated method is the use of a 3D digitizing device, where a magnetic field is induced and the exact position of any point can be accurately determined (Sinoquet and Rivet, 1997). For many species, where each leaf has a constant leaf angle, and is easily distinguishable and reachable, use of mechanical or digital clinometers is obviously no problem.
However, grass canopies have a typical structure with long upright (erectophile) leaves that droop when they are very long. Even the determination of the leaf angle of one blade is therefore not easy. However, within one species there is a typical curvature, blades tend to droop at a specific length, leaf angles of short blades are quite homogeneous. Therefore, it was postulated that there would be a relationship between the height (relative to the soil, as the blade stands in the canopy) to blade length (soil to leaf tip) ratio and the average leaf blade angle. This relationship could then be used to determine average canopy leaf angles from simple leaf height and length measurements. The purpose of this study was to determine such a relationship and to investigate its accuracy for different species and for plants of different ages and grown under different UV-B doses.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Plant material and growth conditions
Plants from an ongoing experiment on UV-B effects on two grass species, Dactylis glomerata (DG) and Festuca arundinacea (FA) were used. The plants were grown in large (20x30 cm) pots, at a density of 40 plants/pot. The pots were placed in six greenhouses, covered with different kinds of plexi-glass, allowing 0, 82 and 88% UV-B penetration (measured with an Optronic OL754 spectroradiometer). The plants were cut every 4 weeks and allowed to regrow. After each cut, fertilizer was added (N, 8.33 g m-2; P, 6.25 g m-2; K, 9.75 g m-2).
Determination of the relationship
Leaf height (highest point the leaf reaches relative to the soil) and blade length (soil to leaf tip, including ligule) of 50 leaves (15–18 from different pots of each treatment) were measured 21 d after cutting. On the same leaves, leaf angle 
(relative to the horizontal) was measured at 5 cm length intervals with a clinometer. An exponential relationship between the length/height ratio R and the average leaf angle was fitted to the data by non-linear regression (Fig. 1):
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The results for the two different species are summarized in Table 1
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Testing of the relationship
To test the relationship, measurements of leaf length and height, and leaf angle were made on 10 leaves from each treatment, at 12 d after cutting, (for the 82% UV-B DG plants and the 88% UV-B FA plants) and 21 d after cutting, (for all other plants) a month after the relationships were determined. Thus measurements on plants with different canopy heights and at different ontogenic stages were made, to investigate whether the relationship could be used in all cases. The leaves were selected as the 10 nearest leaves to a random point at the base of the canopy (so both short and long leaves were measured). Using the relations from Table 1
, the leaf angles were calculated. Average canopy leaf angle of each treatment was determined from the leaf angles of the selected leaves weighted according to the leaf length (a long leaf has greater impact on the average canopy angle). For the 88% UV-B treatment of Dactylis glomerata a second random point was used as well, to have an idea of the variability between canopy sections of the same treatment. The significance of the treatment effects was analysed with ANOVA in combination with a Student–Newman–Keuls test (P<0.05).
The calculated leaf angles were compared to the measured leaf angles by linear regression, and the difference between the methods analysed with split-plot ANOVA (P<0.05, statistical package S+).
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Results |
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The variation in leaf angles was quite high, and there was quite a large error term on the calculation of single leaf angles (Fig. 2
). This resulted in low r2 values (0.59 and 0.77 for DG and FA, respectively) when comparing measured to calculated leaf angles. The slopes were significantly different from 1 (0.61±0.06 and 0.77±0.11 for FA and DG, respectively).
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The calculated average canopy angles were always close to the measured values, and well within the standard errors of the measured values (Tables 2
, 3) for both FA and DG. There were significant differences in canopy height and average leaf angle between the treatments. These differences were detected both by the measured and by the calculated canopy values. The values from the two points in the high UV-B DG plots were quite close to each other, and not significantly different (66.8 and 68.7).
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The overall correlation between measured and calculated canopy values (Fig. 3
) yielded an r2 of 0.72. The results of the two methods were not significantly different (analysed with split-plot ANOVA, P<0.05)
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) and Festuca arundinacea (
).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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DG is a very erectophile grass with short leaf lengths but longer leaves droop, while for FA even short leaves show a large variation in leaf angles, but there is less drooping. The relationship was slightly more accurate for DG than for FA, but for FA a wider range of leaf angles was covered. For both species the leaf angles were quite low, compared to what one would expect of an erectophile canopy.
Although there were quite large differences between individual measurements and calculations of leaf angle (as indicated by the low r2 and the slope different from 1), the measured and calculated canopy averages were quite close to each other (r2=0.89), and never significantly different. The method is therefore unsuitable for single leaf measurements, but accurate enough for the determination of canopy averages.
UV-B had a significant effect on leaf angle. These differences were detected both by direct measurement of the leaf angles and by calculated leaf angles in both FA and DG. These effects were probably partially due to the large differences in plant height. The data show that the method can be used for canopies over a wide range of heights and ages.
The relations were determined using pooled data from the different treatments. The different treatments did not affect the relationship however, as can be seen in Fig. 1, where data from the different treatments are plotted with different symbols.
The given data were only used to check the adequacy of the method, not to detect treatment effects. Obviously, for good average canopy values, it would be better to choose three random points in each canopy, and measure the 10 nearest leaves from each point. However, the two points in the 88% UV-B treatment of DG did yield very similar results, implying that having three points would probably not have affected the outcome greatly.
In conclusion, this method appears to be reasonably accurate and very easy to perform. Disturbance of the canopy is minimal. Since changes in water content or turgor, and leaf thickness probably influence the relationship, it has to be calculated for every experiment (however, even that is not too time-consuming). The results show that it can be used over a wide range of canopy heights or stand age. This method is therefore particularly useful when the purpose is to follow changes in leaf angle during an entire growing season.
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Acknowledgments |
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G Deckmyn is indebted to the Flemish Foundation for Scientific Research (FWO-Vlaanderen) for her position as post-doctoral researcher.
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Notes |
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1 To whom correspondence should be addressed. Fax: +32 3 820 22 71. E-mail: gdeckmyn{at}uia.ua.ac.be
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References |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Aries F, Prevot L, Monestiez P.1993. Geometrical canopy modelling in radiation simulation studies. In: Varlet-Grancher C, Bonhomme R, Sinoquet H, eds. Crop structure and light microclimate. Paris: INRA, 159–173.
Sinoquet H, Valmorin M, Cabo X, Bonhomme R.1993. DALI: an automated laser distance system for measuring profiles of vegetation. Agricultural and Forest Meteorology 67, 43–64.
Sinoquet H, Rivet P.1997. Measurements and visualisation of the architecture of an adult tree based on a three-dimensional digitising device. Trees 11, 265–270.
Thornley JHM, Johnson IR.1990. Plant and crop modelling. Oxford: Clarendon Press.
Tosserams M, Magendans E, Rozema J.1997. Differential effects of elevated ultraviolet-B radiation on plant species of a dune grassland ecosystem. Plant Ecology 128, 266–281.
Warren-Wilson J.1963. Estimation of foliage denseness and foliage angle by inclined point quadrats. Australian Journal of Botany 11, 95–105.
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