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 penetrometerresistance, R_s slows down leaf growth and reduces mature leafsize in wheat seedlings {Triticum aestivum L.). Underlying changesin the kinetics of cell partitioning and expansion and in thesize and organization of mature cells were reported in companionpapers (Beemster and Masle, 1996; Beemster et al., 1996). Inthe present study, the relationships between apex growth, primordiuminitiation and expansion were analysed for plants grown at contrastingR_s, focusing on a leaf whose whole development proceeded afterthe onset of root impedance (leaf 5). High R_s reduced the rates of apex and leaf development, butdid not appear to have immediate effects on the pattern of developmentof the newly initiated phytomers. During an initial short period,the rate of development of a leaf primordium and associatednode were related to plastochronic age, according to similarrelationships (slopes) at the two R_s. Effects on developmentalpatterns were first detected on phytomer radial expansion duringplastochron 2. The ontogenetic pattern of leaf elongation wasaffected later, during the next few plastochrons preceding leafemergence (‘post-primordial stage’). It is concludedthat a reduction in the number of formative divisions and inthe number of proliferative cells along the intercalary mer-istemreported earlier (Beemster and Masle, 1996; Beemster et al.,1996) is not related to the size of the apical dome at leafinitiation nor to the size and number of meristematic cellsinitially recruited to the leaf primordium, which were all unaffectedby R_s. Rather they are generated at the primordial and post-primordialstages. Key words: Wheat, apex development, leaf primodium development, mature leaf width, root impedance