Figure 1.

Flowering as a Condition for Hypocotyl Xylem Expansion in Plants with Rosette Growth Habit.

(A) Progression of xylem expansion in Arabidopsis illustrated by transverse sections, stained for lignin with phloroglucinol to highlight xylem vessels and fibers (yellow line indicates xylem diameter). Xylem area compared with total hypocotyl area increases slowly during the vegetative phase but accelerates considerably once flowering occurs.

(B) Schematic illustration of the approximate position of sections (red lines) sampled to measure the relative xylem area in the genotypes analyzed in this study. Sections were taken from the hypocotyl center, except in the case of grafting experiments. For the latter, scion and stock sections were taken about halfway between the apical and basal ends of the hypocotyls, respectively, and the silicon tubing collar of the grafts.

(C) Relative xylem area as determined from serial sections along individual Arabidopsis hypocotyls before and after flowering.

(D) to (K) Hypocotyl cross sections of species with nonrosette growth habit (i.e., the stem internodes elongate already during vegetative growth) ([D] to [G]) and with rosette growth habit (i.e., the stem internodes elongate only once flowering has been induced) ([H] to [K]), demonstrating that in nonrosette plants, xylem expansion already starts during vegetative growth. S. lycopersicum (D), A. alpina (E), Aster alpinus (F), N. benthamiana (G), B. verna (H), Arabidopsis (I), C. hirsute (J), T. officinalis (K). dag, days after germination; veg., vegetative growth phase; flo., flowering, at appearance of inflorescence meristem. Bars = 200 μm.