Assessment of the rehydration procedure has been consistently overlooked in evaluation of factors contributing to successful cryostorage of partially dehydrated, embryonic axes excised from recalcitrant seeds. Conventional rehydration of Trichilia dregeana (Sond.) axes in distilled water resulted in the lack of root pole gravitropism after culture on medium in vitro. In comparison, a strong gravitropic response was observed in axes that had not been dehydrated, and by those rehydrated in a solution containing 1 M CaCl2 and 1 mM MgCl2. However, no marked loss of either cation from the tissue could be detected, whether axes were rehydrated in distilled water or the Ca2+/Mg2+ solution. Starch-packed statoliths differentiated rapidly in both non-dehydrated axes and those rehydrated in the divalent cation solution, but these organelles failed to develop or accumulate much starch following axis rehydration in water, as was the case for the amyloplasts of root cells generally. After rehydration in the Ca2+/Mg2+ solution and 48 h in culture, axis root-cap columella cells accumulated Ca2+, whereas axes rehydrated in water or solutions containing either Ca2+ or Mg2+ alone did not take up Ca2+. Rehydration with the Ca2+/Mg2+ solution also altered distribution of the actin component of the cytoskeleton. When rehydrated in the divalent cation solution, actin was associated with the nucleus and with the statoliths, which were located distally in statocytes of axes. In contrast, actin was largely confined to the perinuclear area in root-cap columella cells of the agravitropic, water-rehydrated axes. The present results indicate a definitive primary role for Ca2+ with Mg2+ in graviperception, via starch metabolism, and in the determination of statolith morphology, which appears to be linked with maintenance of the actin component of the cytoskeleton in root-cap statocytes.
Root cap size and shape influence responses to the physical strength of the growth medium in Arabidopsis thaliana primary roots