Significant variations in mineral composition among agar sources: Implications in nutrition and abiotic stress studies that use in vitro culture

Gelling agents such as agar are commonly used to solidify the nutrient media for micropropagation. Agar is always treated as a fixed component in the culture media. Mineral composition is not generally considered when selecting an agar source. We tested two popular agar sources namely Acros Organics and Fisher Scientific. There were dramatic differences in the mineral composition among these two brands. For example the Acros agar had about two times Na and ten times P as compared to Fisher agar. On the other hand Fisher agar had about 4 times Ca, Mg and B as compared to Acros agar. We tested the influence of these mineral contents on the growth and development of potato shoot cultures in presence of calcium (Ca) sufficient and deficient media. Nodal cuttings of five potato cultivars were grown in standard MS media with different calcium concentrations. Calcium deficiency symptoms (shoot tip necrosis, formation of axillary shoots) more pronounced in the shoots of plants grown in Acros agar as compared to Fisher agar, although the media was supplemented with same calcium concentration. As pointed out above the Fisher agar tested over 2000 ppm Ca and Acros agar tested only about 560 ppm Ca. Thus it appears that the inherent higher (4X) Ca concentration of Fisher agar was responsible in mitigating injury due to calcium deficiency. Our results suggests that studies utilizing in vitro cultures to either screen breeding populations for abiotic stresses (salinity, Na, osmotic) or study plant nutrition should consider inherent mineral composition of agar used.