We studied inherent variation in final leaf size among four Poa spp. that live at different elevations. The average final length of leaf 7 of the main stem of the smallest species (Poa alpina) was only one-half that of the largest species (Poa trivialis); it was correlated with leaf elongation rate, but not with the duration of leaf elongation. A faster rate of leaf elongation rate was associated with (a) larger size of the zone of cell expansion, and (b) faster rates of cell production (per cell file) in the meristem, which in turn were due to greater numbers of dividing cells, whereas average cell division rates were very similar for all species (except Poa annua). Also we found that the proliferative fraction equaled 1 throughout the meristem in all species. It was remarkable that rates of cell expansion tended to be somewhat higher in the species with slower growing leaves. We discuss the results by comparing the spatial and material viewpoints, which lead to different interpretations of the role of cell division. Although the presented data do not strictly prove it, they strongly suggest a regulatory role for cell division in determining differences in growth rate among the present four Poa spp.

Aegilops tauschii was the only wild species with rapid leaf expansion rates similar to those of wheat, and it achieved the highest photosynthetic rates, making it an interesting species for further study.

The 2-fold difference in final length of leaf number three on the main stem between the fast-growing Aegilops tauschii L. and the slow-growing Aegilops caudata L. is correlated with a difference in leaf elongation rate (LER), and not in duration of leaf elongation. In this paper the cellular basis of inherent differences in LER between these species was investigated.The dynamics of abaxial epidermal cells along the growth zone of leaf number three on the main stem of both species was analysed by means of a kinematic analysis. The faster LER of Ae. tauschii compared with that of Ae.�caudata was associated with (i) a larger leaf basal meristem and cell elongation-only zone, and (ii) a faster cell production rate owing to a larger number of dividing cells. Cell division rate, mature cell size and cell elongation rate did not differ between the two species. The lack of variation in cell expansion rate between the species was supported by a similar capacity of both species to extend their isolated cell walls upon acidification.These data suggest that differences in the number of dividing cells can bring about differences in the number of simultaneously elongating cells, and hence in LER.

Plants vary widely in their relative growth rate (RGR), be it dependent on environmental conditions or due to their genetic background. In a comparison of the RGR of grasses growing under different environmental conditions, variation in RGR tends to correlate with that in the leaf elongation rate (LER). When different species or genotypes thereof are compared under identical growing conditions, variation in LER may or may not correlate with that in RGR, depending on the comparison. However, since RGR is described by an exponential equation, whereas LER is mainly a linear process, we conclude that any correlation between RGR and LER must be fortuitous. That is, exponential growth must be due to increases with time in plant traits such as 1) leaf dry mass per unit leaf length invested per unit time, and/or 2)E̊, i.e., the total LER of all the growing leaves at one point in time. The latter can be achieved as follows: 1) each subsequent leaf has a higher LER than the preceding one; 2) leaves appear at an increasing rate; 3) the duration of the process of leaf elongation increases for subsequent leaves. In this review, we only explore possible factors that account for changes in E̊ with time, in different genotypes and under different environmental conditions. Inherent variation in LER of individual leaves and variation due to environmental factors may reflect variation in the rate of cell division and/or in cell elongation.