Leaf expansion and biomass allocation in wild wheat (Aegilops) species

Fast expansion of leaf area is an important trait to select for in cereal crop species, especially in arid environments. It is associated with higher crop water-use efficiency, higher above-ground biomass production and yield, and increased competitive ability. This thesis examined the physiological basis of species-specific differences in leaf expansion rate, starting at a cellular level and scaling up to the whole plant level. The species that were used for this study are from the genus Aegilops L. (Poaceae), the wild progenitors and relatives of wheat. Species of this genus show a wide variation in traits relevant to survival in harsh environments and can be crossed with the current wheat cultivars, making them good candidates for wheat improvement. The cellular processes in growing leaves of Aegilops caudata, a species with slow-elongating leaves, with those in growing leaves of Ae. tauschii, a species with fast-elongating leaves, were compared by means of a kinematic analysis. This analysis showed that the faster leaf elongation rate of the third leaf on the main stem of Ae. tauschii compared with that of Ae. caudata was associated with a higher meristematic activity in the leaf growth zone. The meristem of Ae. tauschii produced more cells per unit time than that of Ae. caudata, because it had a larger number of cycling cells, and not because the cells divided faster. The maximum cell elongation rate and cell elongation duration were not associated with differences in leaf growth between the species, nor was mature cell size. Although leaf growth can only occur as a result of cell expansion and not cell division, these data suggest that differences in the number of dividing cells can bring about differences in the number simultaneously elongating cells and hence leaf elongation rate. A linear increase in length of individual grass leaves can only lead to an exponential increase in leaf area of the whole shoot, by increasing (i) leaf elongation rate of successive leaves, (ii) leaf appearance rate, (iii) leaf elongation duration of successive leaves, and /or (iv) leaf width of successive leaves. In grasses, the main cause of the exponential increase in leaf area is an increase in the rate of leaf and tiller production. However, in a comparison of two wheat (Triticum) species and three Aegilops species, it has been shown that differences in relative leaf area expansion rate are not necessarily associated with differences in leaf and tiller production. Rather, the species with the fastest relative leaf area expansion rate showed the fastest increase in leaf width and leaf elongation rate in successively growing leaves. Moreover, the fast increase in leaf width and leaf elongation rate in successive leaves of these species was associated with more biomass allocated to the shoots at the cost of the roots. The inherent differences in cell production rate, leaf elongation rate, biomass allocation and relative leaf area expansion rate between Ae. caudata and Ae. tauschii were largely reduced by exogenous supply of either gibberellic acid to the slow-elongating Ae. caudata or an inhibitor of the gibberellin biosynthesis to the fast-elongating Ae. tauschii. These results suggest that gibberellins play a key role in the observed variation in leaf growth and biomass allocation between the Aegilops species studied in this thesis