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 describ e the course of the sink-to-source transition in leaves 7 and 8. Use o f 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 o f successive leaves. These three criteria were evaluated for expanding leaves of 40 growth chamber-grown M. arvensis plants in semilogarithm ic plots of leaf length versus time and by linear regression analysis of In-transformed leaf lengths. Quantitative analysis indicated that l eaves 4-11 grow exponentially (r(2) values between 0.89 and 0.99). Elo ngation rates of leaves 6-8 (average value 1.41 mm d(-1) +/- 0.003) sh owed no significant variation by two-way ANOVA and Tukey's studentized range test. Successive leaves did not have similar plastochron durati ons, 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, onl y leaves 6-8 of M. arvensis fulfilled all three criteria of the plasto chron index. The sink-to-source transition in expanding M. arvensis le aves was analyzed by (CO2)-C-14 labeling of source leaves and analysis by autoradiography of C-14-photoassimilate import into sink leaves. T he transition occurred in a basipetal fashion and had an approximate d uration of 2.5 plastochrons or 5.6 d. Four successive leaves were foun d to be simultaneously in transition. The sink-to-source transition wa s shown to be linearly related both to leaf plastochron index and to l eaf length for leaves 7 and 8.