Establishment of a plastochron index and analysis of the sink-to-source transition in leaves of Moricandia arvensis (L) DC (Brassicaceae)

A plastochron index was established for Moricandia arvensis to describe the course of the sink-to-source transition in leaves 7 and 8. Use of the plastochron index required the verification of three assumptions: (1) young leaves grew exponentially, (2) leaf elongation rates were similar, and (3) equal intervals of time occurred between initiation of successive leaves. These three criteria were evaluated for expanding leaves of 40 growth chamber-grown M. arvensis plants in semilogarithmic plots of leaf length versus time and by linear regression analysis of In-transformed leaf lengths. Quantitative analysis indicated that leaves 4-11 grow exponentially (r(2) values between 0.89 and 0.99). Elongation rates of leaves 6-8 (average value 1.41 mm d(-1) +/- 0.003) showed no significant variation by two-way ANOVA and Tukey's studentized range test. Successive leaves did not have similar plastochron durations, but a stabilization was seen for leaves 6-11 (average plastochron duration of 2.23 d +/- 0.11). In addition, the plastochron index had a definite linear relationship with time (r(2) = 0.99). Therefore, only leaves 6-8 of M. arvensis fulfilled all three criteria of the plastochron index. The sink-to-source transition in expanding M. arvensis leaves was analyzed by (CO2)-C-14 labeling of source leaves and analysis by autoradiography of C-14-photoassimilate import into sink leaves. The transition occurred in a basipetal fashion and had an approximate duration of 2.5 plastochrons or 5.6 d. Four successive leaves were found to be simultaneously in transition. The sink-to-source transition was shown to be linearly related both to leaf plastochron index and to leaf length for leaves 7 and 8.