Low external pH prevents cell elongation but not multiplication of embryogenic carrot cells

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Abstract
Embryogenic cultures of cultivated carrot (Daucus crota cv. Scarlet Nantes) were initiated from seedling hypocotyls on hormone‐containing nutrient medium and from wounded zygotic embryos on hormone‐free medium. Both of these cultures were maintained with continuous multiplication as unorganized, embryogenic cell masses on hormone‐free medium at pH 4.0, containing NH+4 as the sole nitrogen source. When grown on hormone‐free medium at pH 4.0, neither culture contained any elongated cells. Virtually all cells were densely cytoplasmic and nearly spherical. Some cells were enlarged, not densely cytoplasmic, but always spherical. When either culture was transferred to an auxin‐containing medium at pH 5.8, numerous elongated cells were produced. Elongated cells were observed when either naphthaleneacetic acid or 2,4‐dichlorophenoxyacetic acid was used, and whether the nitrogen source was NH+4 alone or a combination of NH+4 and NO−3. Elongated cells were more abundant when a combined nitrogen source was used. When cultures containing elongated cells were transferred to and multiplied on hormone‐free or hormone‐containing medium buffered at pH 4.0, all elongated cells disappeared after 2 weeks. No elongated cells were observed in any of the lines tested at pH 4.0. These results clearly show that it was the pH of the culture medium and not the presence or absence of an auxin or the nitrogen source(s) that permitted or prevented cell elongation in the embryogenic cultures tested.