The role of apical development around the time of leaf initiation in determining leaf width at maturity in wheat seedlings (Triticum aestivum L.) with impeded roots

High soil resistance to root penetration (measured as penetrometer resistance, R(s)) slows down leaf growth and reduces mature leaf size in wheat seedlings (Triticum aestivum L.). Underlying changes in the kinetics of cell partitioning and expansion and in the size and organization of mature cells were reported in companion papers (Beemster and Masle, 1996; Beemster et al., 1996). In the present study, the relationships between apex growth, primordium initiation and expansion were analysed for plants grown at contrasting R(s), focusing on a leaf whose whole development proceeded after the onset of root impedance (leaf 5). High R(s) reduced the rates of apex and leaf development, but did not appear to have immediate effects on the pattern of development of the newly initiated phytomers. During an initial short period, the rate of development of a leaf primordium and associated node were related to plastochronic age, according to similar relationships (slopes) at the two R,. Effects on developmental patterns were first detected on phytomer radial expansion during plastochron 2. The ontogenetic pattern of leaf elongation was affected later, during the next few plastochrons preceding leaf emergence ('post-primordial stage'). It is concluded that a reduction in the number of formative divisions and in the number of proliferative cells along the intercalary meristem reported earlier (Beemster and Masle, 1996; Beemster et al., 1996) is not related to the size of the apical dome at leaf initiation nor to the size and number of meristematic cells initially recruited to the leaf primordium, which were all unaffected by R(s). Rather they are generated at the primordial and post-primordial stages.