Data on leaf shape of six wheat, one barley and three maize cultivars from different growing conditions and years were analysed. Differences were found in shape and a different pattern of the overall form factor ft for juvenile and adult leaves in the wheat cultivar ‘Soissons’ and the barley cultivar ‘Barke’. In the adult phase of barley, values for the overall form factor ft decreased with increasing leaf rank (Fig. ), which could mainly be attributed to an increase in the relative length of the distal leaf segment (swmax). This effect was observed as well in the climate chamber with constant temperature and relatively low radiation input as under natural field conditions. In both cases, the shape of leaves of comparable ontogenetic stage was similar. However, only three to four juvenile leaves grew in the experimental set-ups. It would be interesting to compare spring barley with winter barley that has a longer juvenile phase. This would allow assessing the impact of growth condition in winter – notably temperature and photoperiod – on leaf growth and to verify further observed ontogenetic differences in barley leaf shape.
In wheat, ft of juvenile leaves decreased with rank, whereas adult leaves were of similar shape (value of ft = 0·8; Fig. ). In all cultivars, the flag leaf had a lower value of ft, which can mainly be attributed to a decrease in relative length of the basal leaf segment. There were no clear differences in the shape of adult leaves between the studied cultivars that were grown in the same field experiment. In this experiment (Wheat III) data for juvenile leaves was not available. As discussed for barley, growth conditions did not notably influence leaf shape of a given rank.
Following the present data, ontogenetic stage had more impact on leaf shape than growth conditions or genotypic variation in wheat and barley, but this needs to be validated on a larger range of cultivars. In maize, data were not sufficient (only one experimental year) to derive a clear pattern of model parameter values related to ontogeny, environment or genotype. There were also more difficulties in the acquisition of w(l) from images due to undulations in the leaves of maize.
The presented shape model could be fitted to measured w(l) with a small error. The parameter values in this model − compared with the overall form factor ft − allowed a more precise discussion of the differences in leaf shape. No data were found in literature that previously described differences in shape between juvenile and adult cereal leaves. Here, swmax, − the relative length of the distal leaf segment (see Fig. ) − was identified as the parameter that mainly changes between juvenile and adult leaves. Interestingly for each species, the values of the form factors f1 and f2 specified for the two leaf segments were relatively similar for all leaf ranks apart from the first leaf. The ontogenetic trend in ft reflects thus mainly the change in the contribution of each leaf segment to the leaf area.
The shape model could be improved to take more shape characteristics of cereal leaves into account. First, an indent in wheat and barley leaves caused by a mechanical compression of leaves by the ligule of the previous leaf was observed (Lock, 2003
). This effect becomes more marked in the upper wider leaves. Second, it was found that some leaves had a quasi plateau in width in the basal leaf segment, which was mainly found in barley growing under low light conditions in the growth cabinet (see Fig. S5 in Supplementary Data 3
). The value of f2
= 0·95 leads to an almost rectangular shape computed by the model, but it does not fully capture this characteristic. However, adding these two features to the shape model would increase the number of parameters (at least two) and thus increase the degrees of freedom for parameter fitting. The error predicting shape with the present model is already low and the parameters can be well interpreted to detect differences in leaf shape. Therefore we think that adding more complexity to this descriptive model is not useful.
Analysis of leaf shape using the proposed model allowed a more detailed look at differences according to species, ontogeny and environmental conditions, although these differences are relatively small compared with the change in leaf dimensions. An application of the shape model, is the simulation of leaf shape in 3D plant models. The observed ontogenetic pattern of leaf shape for ‘Soissons’ and ‘Barke’ could be sufficiently well described using the proposed set of parameters for swmax, f1, f2 and klig (Table ). The relative stability of the observed parameter values irrespective of environmental conditions suggest that even for other cultivars of the same species similar responses of shape might be expected.