Two maize cultivars of contrasting leaf size show different leaf elongation rates with identical patterns of extension dynamics and coordination

Abstract

Simulating leaf development from initiation to maturity opens new possibilities to model plant-environment interactions and the plasticity of plant architecture. This study analyses the dynamics of leaf production and extension along a maize (Zea mays) shoot to assess important modelling choices. Maize plants from two cultivars originating from the same inbred line, yet differing in the length of mature leaves were used in this study. We characterized the dynamics of the blade and sheath lengths of all phytomers by dissecting plants every 2-3 days. We analysed how differences in leaf size were built up and we examined the coordination between the emergence of organs and phases of their extension. Leaf extension rates were higher in the cultivar with longer leaves than in the cultivar with shorter leaves; no differences were found in other aspects. We found that (i) first post-embryonic leaves were initiated at a markedly higher rate than upper leaves; (ii) below ear position, sheaths were initiated at a time intermediate between tip emergence and appearance, while above the ear position, sheaths were initiated at a high rate, such that the time interval between the blade and sheath initiations decreased for these leaves; and (iii) ear position also marked a change in the correlation in size between successive phytomers with little correlation of size between upper and lower leaves. Our results identified leaf extension rate as the reason for the difference in size between the two cultivars. The two cultivars shared the same pattern for the timing of initiation events, which was more complex than previously thought. The differences described here may explain some inaccuracies reported in functional-structural plant models. We speculate that genotypic variation in behaviour for leaf and sheath initiation exists, which has been little documented in former studies.

Keywords: Blade; Zea mays; coordination; emergence; extension rate; initiation; leaf; phyllochron; phytomer; sheath.

Figure 1.

Architecture of maize cultivars M40 and M52. (A) Plants that have completed their vegetative development; M40 is taller and its female flowering is delayed of a few days compared with M52. (B) The non-destructive measurements. (C) A plant at successive stages of dissection. Bn and Sn refer, respectively, to the blade and the sheath of leaf n, Pan refers to the panicle. The horizontal dotted line shows approximately the height of the sheath tube. Leaves 1–10 are liguled, leaves 11–14 are visible, leaves 15–18 have emerged from the sheath tube but are not visible yet and are not exposed to direct light.

Figure 2.

Distribution of the length of mature maize leaves. Red triangles correspond to cultivar M40, and black circles to cultivar M52. Empty symbols and dashed lines correspond to destructive data. Filled symbols with continuous lines correspond to non-destructive measurement data. Vertical grey lines indicate the position of the ear.

Figure 3.

Time of initiation of blades and sheaths in maize cultivars M40 and M52. Symbols correspond to the time when each organ reached a reference length of 0.025 cm for blades and 0.1 cm for sheaths. Pale colours show blades of phytomer 6 and 7, for which calculating the time when they reached the reference length implied some extrapolation. The lines correspond to linear regressions fitted on the data. The horizontal grey line indicates the position of the ear. The dashed vertical line indicates the time of tassel initiation. Confidence intervals (95%) are shown as vertical bars and are smaller than the size of the symbols.

Figure 4.

Measured and fitted dynamics of extension of (A) blades and (B) sheaths vs. thermal time for five phytomers along the stem of cultivar M52. Circles represent experimental data and lines show the adjustment with the three-phase model, from which RERs and LERs were estimated. Blade extension was fitted considering slope continuity between the experimental and linear phase.

Figure 5.

Blade relative (A) and linear (B) elongation rates for maize cultivars M40 and M52. Parameter values were estimated using a three-phase extension model with a slope continuity between the exponential and linear phases. Black circles correspond to M52, and red triangles indicate M40. The vertical grey line shows the position of the ear.

Figure 6.

Sheath relative (A) and linear (B) elongation rates for maize cultivars M40 and M52. Black circles correspond to M52, and red triangles correspond to M40.

Figure 7.

Emergence events: tip emergence and appearance, collar emergence, beginning of blade linear extension and sheath initiation. The dotted vertical line shows the time of tassel initiation. (A and B) Upwards-facing triangle: leaf initiation (blade length = 0.025 cm); cross: tip emergence; square: tip appearance; circles: collar emergence. Lines correspond to linear models adjusted on points in (A) and (B). (C and D) Downwards-facing triangles: sheath initiation (sheath length = 0.1 cm); filled diamonds: linear extension of blades. Lines correspond to the tip emergence and appearance as fitted on data shown in (A) and (B).

Figure 8.

Correlations between the final length of blades and sheaths along the maize shoot. The source of variation is plant-to-plant variability within two sets of 30 plants from cultivars M52 (left) and M40 (right). For phytomer positions given by the coordinate of the pixel, the colour of a pixel represents the r² between the length of blades and/or sheaths of a same shoot. Colour chart is at the bottom of the figure, positive values of r are associated with green colour and negative values of r are associated with red colours, grey pixels mean that less than 10 entities were available and the correlation was not calculated. The horizontal and vertical discontinuous lines show the position of the ear.