Journal of Experimental Botany, Vol. 52, No. 359, pp. 1259-1268,
June 1, 2001
© 2001 Oxford University Press
Original Papers |
The elongation rate at the base of a maize leaf shows an invariant pattern during both the steady-state elongation and the establishment of the elongation zone
Laboratoire d'Ecophysiologie des Plantes sous Stress Environnementaux, INRA, ENSAM, 34060 Montpellier, France
Received 14 August 2000; Accepted 3 January 2001
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Abstract |
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Spatial and temporal analyses of elongation and cell length of monocotyledon leaves have most often been performed during the period when leaves are visible and elongate at a constant rate (steady-state). In the present study, the focus was on the earlier stages, during the establishment of the elongation zone. Regardless of leaf development stage, the segment located between 0 and 35 mm from the leaf insertion point had a relative elongation rate that increased with distance from insertion point (‘accelerating zone’) while the segment located further than 35 mm had a relative elongation rate that decreased (‘decelerating zone’). This stable pattern held for both young, non-emerged leaves, where it was restricted to the portion corresponding to the length of the blade, and for leaves during steady-state elongation. In the same way, the profile of cell length was essentially the same during early development and during steady-state elongation. The results of a temporal analysis of whole-leaf elongation rate, carried out in the field and in the greenhouse at different light intensities were consistent with a time-invariant pattern of elongation. Whole-leaf relative elongation rate increased with time until the leaf reached 30–40 mm length (although at different leaf ages depending on conditions), and declined afterwards. These results suggest that the patterns governing the elongation rate of a sector of a maize leaf are independent of the leaf developmental stage but depend on sector position only.
Key words: Leaf elongation, spatial pattern, relative elongation rate, cell length, monocotyledons.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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Growth of monocotyledon leaves has most often been studied during the linear phase of elongation which provides an easy system to analyse the responses of leaf growth to environmental conditions (Passioura and Gardner, 1990
; Ben Haj Salah and Tardieu, 1997). During this phase, the spatial distributions of relative elongation rate and of cell length at the base of leaf (the growing region) are in steady-state for several days (Schnyder et al., 1990; Bernstein et al., 1995; Ben Haj Salah and Tardieu, 1995). As in roots (Erickson and Sax, 1956; Muller et al., 1998), these steady-state pattern allow the calculation of local cell division (Tardieu et al., 2000), and the analysis of the impact of environmental conditions on tissue expansion and/or cell division (Schnyder et al., 1990; Ben Haj Salah and Tardieu, 1995; Beemster and Masle, 1996; Schuppler et al., 1998). However, this linear phase is short compared to the duration of the whole leaf development (Lafarge, 1998; Durand et al., 1999) and little is known of the phase that precedes this linear phase. Elongation is believed to be exponential since it is usually reported that the logarithm of leaf length increases almost linearly with time (Gallagher, 1979; Skinner and Nelson, 1995; Tardieu et al., 1999). Any stress experienced by the leaf during this early phase may reduce final leaf area as shown in sunflower where water shortage or light deficit during this phase permanently affects leaf expansion rate and can halve the final leaf area (Granier and Tardieu, 1999a, b).
The study reported here aimed to identify the temporal and spatial patterns of elongation during the early, exponential-like phase of leaf elongation in maize. In order to test the generality of patterns, the spatial pattern of leaf elongation was analysed in leaves of different length and of different positions on the stem and the temporal pattern in a series of experiments in the field and in the greenhouse, for two genotypes with contrasting precocity and for different leaf positions on the stem. Two considerations allowed simplifying this approach. First, expansion was analysed in one dimension (elongation only). Clonal analysis has shown that cell lineage follows leaf veins and that the leaf has a base–tip organization even during early development (Poethig and Szymkowiak, 1995). This one-dimensional approach made it possible to compare spatial patterns during early development with those already published for later periods. The adequacy of a one-dimensional analysis was partially tested in this study. Second, time was expressed in thermal time in order to unify the results of experiments carried out in different thermal conditions (field versus greenhouse) and at different times (leaf 6 versus leaf 8). Both cell division rate and tissue expansion rate have linear responses to temperature with a common x-intercept (10 °C in maize), so these rates can be expressed in thermal time (Ben Haj Salah and Tardieu, 1995). It has recently been shown that, expressed per unit thermal time, relative expansion rate at the base of the maize leaf has a spatial distribution which is common to several experiments, thereby providing a reference distribution characteristic of the studied genotype (Tardieu et al., 2000). All observed distributions were therefore compared to this reference distribution.
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Materials and methods |
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Growth of plants in the field and in the greenhouse
Maize (Zea mays L.) plants were grown in Montpellier (southern France) in a field experiment (June 1998) and in three greenhouse experiments referred hereafter as GH1 (April 1998), GH2 (November 1998) and GH3 (November 1999). The field experiment comprised two genotypes with markedly contrasting precocity (F1 hybrids Déa and Volga, with 16 and 19 leaves, respectively). The greenhouse experiments involved cv. Déa only. In the greenhouse, seeds were placed at 25 mm depth in columns (0.08 m diameter, 0.6 m in height) containing a 1:3 mixture (v/v) of a loamy soil and an organic compost. Two seeds were sown per column and 50–100 columns were used in each experiment. The soil was maintained at water retention capacity by automatic irrigation (four times a day) with a modified one-tenth strength Hoagland solution corrected for micronutrients. In the field, 10 seeds were sown m-2 and plants were irrigated twice a week.
Recording of the environmental conditions
Air temperature and relative humidity were measured every 30 s at plant height (HMP35A, Vaisala Oy, Helsinki, Finland). The temperature of the growing zone was measured with a set of 3–4 fine (0.5 mm diameter) copper-constantan thermocouples inserted close to the base of leaf 6. PPFD was measured every 30 s at plant height (LI-190SB, Li-Cor, Lincoln, USA). All data were averaged and stored every 15 min in a data logger (CR10X, Campbell Sci, Crewe, UK). Environmental conditions during the four experiments are presented in Table 1. Mean meristem temperature was slightly higher in experiment GH1 and in the field than in experiments GH2 and GH3 (range 22–25 °C). Daily PPFD was double in the field compared with experiments GH2 and GH3, and experiment GH1 was intermediate. Leaf to air VPD was very low in all greenhouse experiments, and was higher but with a high day-to-day variability in the field. In experiment GH1, 50 plants were shaded for 4 d during the beginning of leaf 6 development, in such a way that incident PPFD was decreased to 8 mol m-2 d-1. Shading slightly reduced meristem temperature (1 °C) and leaf to air VPD. Thermal time was calculated from mean meristem temperature with a 10 °C basis (Ben Hah Salah and Tardieu, 1995; Tardieu et al., 2000).
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Temporal analysis of elongation
Temporal change in leaf length was followed by destructive sampling from leaf initiation to the end of leaf growth. The number of visible leaves and the length of the youngest visible leaf were measured non-destructively every second day in a set of 10 reference plants selected for homogeneity at emergence of leaf 3. Other plants were sampled on the same days in such a way that each sampled plant had the same leaf number and similar lengths of the youngest leaf as reference plants (10 and 6 sampled plants in the field and experiment GH1, respectively). They were immediately stored at 4 °C until dissection. Sixth and eighth leaves were freed from enclosing leaves and measured with a stereomicroscope (Leica, wild F8Z, Wetzlar, Germany) coupled with an image analyser (Bioscan-Optimas V 4.10, Edmonds, USA) when shorter than 20 mm, and with a ruler later on. Leaf length was the distance from leaf insertion point to the leaf tip. Relative elongation rate of whole leaves (RERleaf, mm mm-1 h-1) was calculated as the slope of the relationship between the logarithm of leaf length (L) and thermal time (t). It was estimated on day j by linear regression on the three coupled values of leaf length and thermal time corresponding to days j, j-2 and j+2.
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During periods when leaf relative elongation rate rapidly changed with thermal time (for leaves 1–100 mm), it was estimated from two consecutive harvest only (at days j-1 and j+1) and attributed to day j.
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Absolute elongation rate (ER) was calculated with the same method as the slope of the relationship between leaf length and thermal time. The thermal time at leaf initiation was interpolated on the linear regression between number of initiated leaves and thermal time. Leaf length was plotted as a function of thermal time from leaf initiation. Relative and absolute elongation rates were plotted both as a function of thermal time from leaf initiation and of leaf length.
Spatial analysis of elongation
Spatial distribution of relative elongation rate was analysed in leaves 6 and 8–11 in experiments GH2 and GH3 in one genotype (cv. Déa). This analysis was performed by following the displacement of holes pierced with fine needles in the basal part of the leaf as presented earlier (Ben Haj Salah and Tardieu, 1995) but extended here for very young leaves. Briefly, the leaf elongation zones of 4–6 plants were marked at 20.00 h with 40 needle holes (0.2 mm diameter) 4 mm apart. After 9 h (corresponding to 3 °Cd at 18 °C, night temperature), plants were harvested and stored at 4 °C until dissection. Elongation of each segment between two neighbouring holes was obtained by subtracting the initial from the final distances between holes. Initial distance was estimated by measuring, with the image analyser, the distance between holes on the sheath of youngest non-growing leaf. Growing leaves were then freed from enclosing leaves, and final positions of holes were recorded with the image analyser. This analysis was carried out on leaf 6 during its development, and also on leaves 8–11 on two occasions during experiment GH3.
Relative elongation rate of the ith segment on day j (RERij, mm mm-1 h-1) was calculated as:
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t is the duration of the experiment, Li,0 and Li,f (mm) are the initial and final distances between holes i and i+1, and LERnp and LERp (mm h-1) are the mean elongation rates (n=4) of a leaf in the phase of constant elongation rate, in non-pierced plants and pierced plants, respectively. The ratio LERnp/LERp corrects local relative elongation rate for the effect of piercing injury (this ratio was 1.4–1.7 in these experiments). LERnp and LERp were measured on leaves 4–8, depending on the developmental stage of the plant.
At one occasion (experiment GH2), spatial distribution of elongation was assessed by following the displacement of ink marks drawn directly onto a dissected leaf 6 when it was 10–20 mm. At the end of the photoperiod (20.00 h), plants were transferred from the greenhouse to a growth chamber with no light and very low VPD (lower than 0.6 kPa). They were then dissected in situ (under light lower than 10 µmol m-2 s-1) in such a way that only leaf 6 and leaves of higher ranks were left attached to the meristem. One-mm apart ink marks were then carefully drawn along the blade and an image of the leaf was taken with a video camera at the beginning of measurement, 5 h and 12 h later. The displacement of ink marks was measured by image analysis and RERij was calculated as above (equation 3)
. Correction for the injury effect was obtained by comparing the absolute elongation rate of these leaves with that of the same leaf in non-dissected plants, estimated destructively in experiment GH1. RERij were thus corrected by the ratio of elongation rate in dissected versus non-dissected plants (this ratio was 1.5 on average).
Modelling the time-course of leaf relative elongation rate from the spatial distribution of relative elongation rate
It has been previously shown that the spatial distribution of relative elongation rate in leaf 6 is unique (called below ‘reference distribution’) during steady-state elongation (Tardieu et al., 2000). It was assumed that any leaf sector was elongating at a rate predicted by this ‘reference distribution’ at any developmental stage (including leaves shorter than 80 mm). Then, a modelled 
was calculated as a function of leaf length as:
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is the relative elongation rate in the reference distribution in the segment i whose boundaries are at positions xi and xi-1 from the leaf insertion point. For that calculation, 
was interpolated to 2.5 mm and 
was calculated for each 2.5 mm length increment (120 ‘data points’ to describe leaf length increase from 2.5–300 mm). In order to account for the bias induced by the sampling interval (which was 30 °Cd on average during experiment GH1), was calculated from a set of modelled data points separated by 30 °Cd intervals. For that purpose, the thermal time corresponding to each 2.5 mm leaf length increment was first calculated using:
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The leaf length at 30 °Cd interval was then recorded (starting for L=2.5 mm). Eight ‘data points’ were then generated for 2.5–300 mm leaf length. The corresponding was then calculated as in equations 1 or 2. It was named ‘modelled-smoothed’ later in the text by difference with ‘modelled’ calculated in equation 4.
Epidermal cell length
Epidermal cell length was measured throughout leaf 6 development in experiments GH1, GH2 and GH3. Leaf six was freed from older, enclosing leaves. When shorter than 6 mm, it was gently unfolded under the stereomicroscope and placed in a droplet of water between two glass slides. The length of 60 epidermal cells was then measured at 1–3 mm intervals under a microscope (LEICA-Leitz DM RB, Wetzlar, Germany) coupled to the image analyser. When the leaf was longer than 6 mm, it was carefully unfolded on a two-sided scotch tape and a transparent negative film of the adaxial epidermis was obtained after evaporation of a varnish spread on the upper surface of the leaf. Films were removed from the leaf with a strip of scotch tape and fixed on a glass slide. It was then analysed under the microscope coupled to the image analyser. Twenty-five to thirty cells were measured at 5 mm intervals at 2–4 locations in transects perpendicular to the midrib. Most of the measurements were done on cell files parallel to the midrib that contained no stomata. However, at two occasions, cell files not parallel to the midrib in the first cm close to the leaf insertion point were chosen and the angle between the cell file and the midrib was measured together with mean cell length.
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Results |
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The spatial pattern of relative elongation rate is essentially time-invariant during early leaf development
The spatial distribution of relative elongation rate was examined at different developmental stages of the leaf, either by analysing the sixth leaf on different days or by analysing leaves 8–11 on a given day (Fig. 1
). Observed distributions were systematically compared to the distribution observed during the period with steady-state elongation in leaf 6 (Fig. 1d). The latter was similar in leaf 8 at the same stage and close to the mean distribution observed for leaf 6 in a series of 10 experiments where plants were grown without appreciable stresses (Tardieu et al., 2000), represented by a dotted line in Fig. 1. In all cases, the relative elongation rate observed during the period with steady-state elongation increased with distance between 0 and 35 mm from the leaf insertion point (‘accelerating zone’, see Tomos and Pritchard, 1994) and it decreased with distance in the zone between 35 and 80 mm (‘decelerating zone’).
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), 10 (b, 
), 9 (c, •), and 8 (d, 
). All data were obtained from pining experiments except in the inset in (a) where RER in 20 mm long leaves (
) was estimated by drawing ink marks on leaf 6 after it had been freed from older, enclosing leaves. In each graph, a dashed line corresponds to the ‘reference distribution’ of RER of leaf 6 during steady-state elongation (Tardieu et al., 2000Overall the results presented in Fig. 1
show that this spatial pattern was essentially time-invariant during early leaf development, although it was restricted to the portion corresponding to the length of the blade.
Six days after leaf initiation (80 °Cd), the blade of leaf 6 was 20 mm long and therefore shorter than the ‘accelerating zone’ observed during steady-state (Fig. 1a). Consistently, the whole blade was elongating and the relative elongation rate increased with distance from the leaf insertion point until the leaf tip. The same was observed in leaf 11 at the same age. The distribution of relative elongation rate followed the distribution observed during steady-state elongation, although at slightly lower values. It is noteworthy that the distribution of relative elongation rate was similar whether it was measured using either the pinning method or the ink method (Fig. 1a, inset). Thus, the pinning method did not appreciably bias the spatial distribution of elongation.
Seven days after leaf initiation (95 °Cd), the blade length of leaf 6 was longer than the ‘accelerating zone’ but shorter than the whole elongation zone (Fig. 1b). Consistently, the whole blade was elongating, but the ‘accelerating zone’ was restricted to the first 30 mm of the blade, as during steady-state. The distribution of relative elongation rate followed that observed during steady-state with slightly lower values. Leaf 10 of the same age essentially followed the same pattern, although the ‘accelerating zone’ was longer than in leaf 6.
During further stages of leaf development, the blade length exceeded the length of the elongation zone observed during steady-state, so elongation was restricted to the base of the leaf. Nine days after leaf initiation (120 °Cd, Fig. 1c), leaves were 120 mm long, the elongation zone was 70 mm long in both leaves 6 and 9. The length of the ‘accelerating zone’ was similar to that during steady-state elongation. Finally, the distribution of relative elongation rate became similar to that observed in all experiments when the leaf reached steady-state elongation (Fig. 1d).
The distribution of the projection of cell length along the mid-rib direction is time-invariant
The analysis of the spatial pattern of epidermal cell length was complicated by the fact that this length was highly variable in a direction perpendicular to that of the midrib. In the first centimetres beyond the leaf insertion point, cell lines close to the midrib had a mean cell length of 20 µm, while those close to the edge of the leaf reached 40 µm (Fig. 2b). This variability in cell length was linked to the angle of the considered cell line in relation to the direction of the midrib. In both 30 mm and 300 mm long leaves, cell lines close to the midrib were nearly parallel to it, while cell lines closest to the leaf edge had an angle up to 60° with the midrib (Fig. 2a). It can be seen in Fig. 2b that the longest cells were those with the greater angle with the midrib (Fig. 2b), so cell length closely correlated with the cosine of the angle of the considered cell file. As a consequence, the projection of cell length in the direction of the midrib was close to 20 µm regardless of the considered cell file (Fig. 2c). In further analyses, this projection was only considered to analyse the distribution of cell length as a function to the distance to leaf insertion point. For simplicity, it will just be called ‘cell length’.
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) of the considered cell file with the midrib direction. In (a) is shown a view of typical leaves analysed (negative varnish film of the adaxial epidermis). Arrows and numbers show typical cell files with directions parallel (3) or with increasing angle (2 and 1) to the midrib. (b) Relationship between the cosine of the angle The spatial distribution of cell length was analysed throughout the early development of leaf 6 (Fig. 3
), with the same approach as that presented above for relative elongation rate. The distribution of cell length during early development was compared to that observed during steady-state elongation. During the latter period, the spatial distribution of cell length was similar in different leaves regardless of their length from 200–400 mm (Fig. 3c). Mean cell length was 20 µm close to the leaf insertion point, increased slowly in the first 20 mm and reached a plateau at 80 mm from the leaf insertion point, i.e. at the end of the elongation zone.
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) mm long. Leaves were also analysed when they were 30 (
) mm long (a), or 120 mm long (b, Cells in very young leaves (2–6 mm long) had a length ranging from 10–18 µm (Fig. 3a
, inset). They had therefore the same length as the shortest cells located at 2–6 mm from the leaf insertion point in older leaves (see Fig. 5 in Tardieu et al., 2000). In 15 mm long leaves, the tip cells began to be longer than basal cells (Inset in Fig. 3a). In 30 mm long leaves (Fig. 3a), the distribution of cell length closely followed the distribution observed during steady-state elongation, including cells located at the tip. This strongly suggests that cells continued to elongate at the tip in these leaves. In 60 and 120 mm long leaves (Fig. 3b), the distribution of cell length continued to follow the reference distribution except at the tip where cells were consistently shorter than cells located at the same distance from the leaf insertion point in older leaves. This indicates that the leaf tip probably ceased growing when the leaf was 60–120 mm long. Overall, these results suggest that the pattern of cell elongation in the basal region of the maize leaf is essentially time-invariant.
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Is the temporal pattern of whole-leaf elongation rate consistent with time-invariance of the spatial pattern of elongation?
If the spatial patterns described above are correct, it is expected that whole-leaf relative elongation rate should increase with time during the period when the whole leaf blade is shorter than the ‘accelerating zone’ (case in Fig. 1a), and decrease afterwards (Fig. 1b–d). In addition, the absolute elongation rate should remain stable during the period when the length and the distribution of elongation rate do not change with time (Fig. 1d). These hypotheses were tested in a field experiment with two genotypes, in leaves 6 and 8, and in a greenhouse experiment in which the steady-state period was delayed because of a severe shading.
It was confirmed in these experiments that the period with constant leaf elongation rate is considerably shorter than the total leaf development duration. The length of these periods was 4–6 d versus 21–26 d, respectively in leaf 6 of both studied genotypes in the field and in the greenhouse (Fig. 4). The period with constant elongation rate was 1 d longer in leaf 8 than in leaf 6 in both genotypes. A severe reduction in incident PPFD for a short period (4 d at 1/5 of PPFD of control plants) during early leaf development delayed the onset of linear elongation (Fig. 4i). Although the period with constant absolute elongation rate began at different times according to environmental conditions (Fig. 4g–i), the beginning of linear increase in length always began when the leaf was 200 mm long in the two studied genotypes (Figs 4, 5).
During the first half of leaf development, elongation was slow and the logarithm of leaf length increased almost linearly with thermal time, suggesting an exponential elongation (Insets in Fig. 4a–c). However, leaf relative elongation rate was not constant. It tended in all cases to decrease for a short time, then to increase and to decrease again (Fig. 4d–f). This tendency was observed in leaves 6 and 8 of the two studied genotypes, in the field as well as in the greenhouse. The duration of the near-exponential period, ending when maximum leaf relative elongation rate was observed, was longer in the field than in the greenhouse, and was longer in leaf 8 than in leaf 6 (Fig. 4d–f). It was lengthened when plants were subjected to a short shading period during early leaf development. Leaf relative elongation rate was reduced during shading but recovered afterwards to a value higher than that in control plants. Its maximum value was observed later than in control plants.
The temporal pattern of leaf relative elongation rate was unified when it was presented as a function of leaf length and not of thermal time (Fig. 5). In all studied cases, leaf relative elongation rate began to increase when the leaf was about 1 mm long and reached its maximum when the leaf was 30–40 mm long, i.e. the length of the ‘accelerating zone’ (Fig. 1) in accordance with the hypothesis formulated above. It was always declining when the leaf was more than 70 mm long. This pattern applied to leaves 6 and 8 of both genotypes, in the field as well as in the greenhouse. It also applied to leaves of shaded plants, although maximum leaf relative elongation rate was observed later (in time) than in control plants.
To link the temporal and the spatial pattern of elongation in young maize leaf further, a modelled leaf elongation rate was calculated assuming that each segment of the leaf was elongating at the relative rate predicted by the reference distribution (presented in Fig. 1) at any developmental stage of the leaf. Whole-leaf relative elongation rate was calculated by two different methods leading to either a ‘modelled leaf relative elongation rate’ or to a ‘modelled-smoothed leaf relative elongation rate’ (Fig. 6). The difference between both calculations was based on the sampling density (see Materials and methods). Modelled leaf relative elongation rate (bold line in Fig. 6) increased until the leaf was 35 mm long because leaf segments newly recruited in the elongation zone had increasing local elongation rates. It was maximum when the leaf was 30–40 mm long, corresponding to the peak of the spatial distribution of local relative elongation rate, and decreased at further leaf lengths because zones newly involved in the elongation zone had lower and lower local relative elongation rates. The time-course of the ‘modelled-smoothed relative elongation rate’ followed the same pattern with a much lower amplitude (Fig. 6). In the region 20–80 mm, it was below the modelled leaf relative elongation rate while in the region 150–300 mm it was above. Overall, the whole-leaf relative elongation rates calculated assuming a time-invariant spatial pattern of elongation were consistent with experimental results, although with markedly different elongation rates.
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The fact that these two curves (Fig. 6
) were computed from the same data set shows how a sampling procedure can dramatically affect the aspect of a time-course. The ‘modelled-smoothed’ leaf relative elongation rate was calculated from eight data points picked when the leaf was 2.5–300 mm, i.e. with the same sampling density as experimental data. In contrast, the non-smoothed modelled elongation rate was computed from 120 points in the same region. The measured time-course of leaf relative elongation rate (estimated in experiment GH1 redrawn from Fig. 5c) was close to the model established on eight points, and considerably lower than the model established on 120 points. The remaining difference between experimental elongation rates and modelled-smoothed elongation rates were due to the fact that observed distributions of local relative elongation rates were lower than the reference distribution when leaves were 20–80 mm long (Fig. 1a–c).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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The period with steady-state elongation is short compared to whole leaf development
The phase with constant elongation rate began when the leaf was 200 mm long in all experiments (Fig. 5
). The spatial analysis shows that this occurred when local relative elongation rate began to follow the reference distribution observed during steady-state elongation (Fig. 1d). During the latter period, the distributions of relative elongation rate and cell length were invariant as previously pointed out (Schnyder et al., 1990; Bernstein et al., 1995; Ben Haj Salah and Tardieu, 1995). However, this period was short (4–6 d), and followed by a longer (5–8 d) phase with gradual decrease in elongation rate that might be caused by a decrease in cell production at the meristem (Palmer and Davies, 1996), causing in the long term a decline in wall plasticity.
The ‘accelerating’ and the ‘decelerating zones’ of elongation are spatially fixed and are defined very early in leaf development
At all stages of leaf development, the leaf segment located between 0 and 35 mm from the leaf insertion point had a relative elongation rate that increased with distance (‘accelerating zone’; Tomos and Pritchard, 1994) while a segment located in the 35–80 mm region had a relative elongation rate that decreased with distance from the leaf insertion point (‘decelerating zone’). This was particularly clear for the leaf tip which travelled through both zones in Fig. 1a, b, respectively. The most striking feature is that the boundary between both zones was located at the same distance from the leaf insertion point (35 mm) at any stage of leaf development. The consequence is that the spatial distribution of elongation always followed that found during steady-state along most of the blade length.
The above conclusion is strengthened by the fact that cell length was largely independent of the leaf developmental stage (Fig. 3). Because local cell length can be considered as the result of the balance between local tissue expansion and local cell division (Green, 1976), these data suggest that the spatial pattern of both elongation and cell division rate are stable with time during early development. This would confirm the highly co-regulated nature of tissue expansion and cell division processes (Granier et al., 2000).
The temporal pattern of elongation confirms the occurrence of ‘accelerating’ and ‘decelerating zones’ in young leaves
These results suggest that, in young maize leaves, the temporal pattern of elongation is determined by the length of the leaf, not by its age (either calendar or thermal time). After an initial decrease, the leaf relative elongation rate increased when the leaf was 1 mm until it was 30–40 mm long and it decreased afterwards. This pattern was consistently observed for two different leaf numbers (6 and 8), two hybrids (genetically distant) in experiments performed either in the greenhouse or in the field. This conclusion was reinforced by the fact that in shaded plants, the decline in leaf relative elongation rate occurred later (in time) but for the same leaf length than in non-shaded plants. These results converge with those obtained from the analysis of the spatial distribution of relative elongation rate which shows that the boundary between ‘accelerating’ and ‘decelerating’ zones is located at 30–40 mm from the leaf insertion zone. In contrast, the initial decrease in relative elongation rate cannot be explained.
The results presented here somewhat diverge from a view which distinguishes a period with exponential elongation from a period with linear elongation (Gallagher, 1979, on wheat and barley; Skinner and Nelson, 1994, 1995, on tall fescue; Lafarge, 1998, on sorghum). In all these studies, the slope of the relationship between logarithm of leaf length and time was nearly constant during a first stage of development, and decreased afterward while absolute elongation rate became approximately constant. In contrast, it is shown here that leaf relative elongation rate is never constant with time, so elongation is never strictly exponential. A strictly exponential increase in area or in length would require that the leaf is formed of elemental zones which all have the same relative expansion rate that remains constant with time. This was observed for instance during early development of sunflower leaves (Granier and Tardieu, 1998), but was not seen during early development of maize in the present study.
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Conclusion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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The results presented here show that the spatial pattern of elongation rate and of cell length at the base of a maize leaf are largely invariant not only during the steady-state elongation but also during the establishment of the elongation zone. This suggests that the ‘steady form from changing cells’ theory (Silk, 1992
) can be extended to the early phases of leaf development. Because ‘accelerating’ and ‘decelerating zones’ are fixed and defined very early in development, the biochemical and/or biophysical causes responsible for this bell-shape pattern (gradients of cell wall enzyme activities; Wu and Cosgrove, 2000; cell cycle related enzyme activities; Granier et al., 2000; or of other hypothetical inductive signal, Freeling, 1992; Green, 1994) are probably already set up during the establishment of the elongation zone and may be unaffected by the age of the leaf.
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Acknowledgments |
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Philippe Hamard helped us to prepare the field experiment and expertly built the needle holder used for the spatial analysis of elongation. Christine Paysant-Le Roux is warmly acknowledged for the quality of the data obtained on cv. Volga. Christine Granier helped in the experiment GH3 and provided helpful comments on the manuscript. Tanguy Lafarge discussed unpublished results from his thesis in sorghum.
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Notes |
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To whom correspondence should be addressed. Fax: +33 4 67 52 21 16. muller{at}ensam.inra.fr
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Abbreviations |
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RERleaf and RER as defined in the Materials and methods section refer to relative elongation rate of the whole leaf and of a given sector of the leaf (characterized by its position from the leaf insertion point), respectively. Both are in units of increase in length per unit leaf length per (thermal) time. In the text, ‘leaf relative elongation rate’ and ‘local relative elongation rate’ are used for RERleaf and RER, respectively.
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References |
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