Aggregation: Due to larger size of a plant cell, it is capable of withstanding tensile strain but is sensitive to shear stress. Aggregation is common, largely due to failure of the cells to separate after division. The secretion of extracellular polysaccharides, particularly in the later stages of growth, may further contribute to increased adhesion. This tendency of plant cells to grow in clumps results in sedimentation, insufficient mixing and diffusion-limited biochemical reaction. Even the fine suspension culture consists of micro-to sub-macroscopic colonies made up of around 5-200 cells and such degree of cell aggregation is acceptable. Cultures consisting of larger aggregates like, 0.5-1.0 mm in diameter, are more readily attainable, grow perfectly well and depending on the aim of the research are often sufficient to meet all requirements. This so called cell-cell contact is desirable for the biosynthesis of many secondary metabolites by the plant cells in suspension cultures. Therefore, controlled aggregation of plant cells may be of interest from process engineering point of view.

 

Oxygen and aeration effect: Oxygen requirements of plant cells are comparatively lower than that of microbial cells due to their low growth rates. In some cases high oxygen concentration is even toxic to the cell's metabolic activities and may strip nutrients such as carbon dioxide from the culture broth. Carbon dioxide is often considered as essential nutrient in the culture of plant cells and has a positive effect on cell growth. Moderate shaking speed like, 90-120 rpm is ideal for standard aeration. As the plant cells are shear sensitive and the immediate effects of high agitation are the cell damage, reduction in cell viability, release of intracellular compounds while low agitation (<90rpm) results in cell aggregation.

 

4. Plant cell cultures vs. microbial cultures

Although basic equipment-and process-related requirements are same for both plant and microbial cell suspension cultures but some of the features suitable for microbial cells cannot be used for plant cells due to prominent differences in the nature and growth pattern of the two types of cells:

1.  Plant cells are sensitive to shear stresses because of the relatively inflexible cell wall and their large size (50-100 μm) compared to microbial cells (2-10 μm).

2.  Plant cell cultures show relatively long growth cycles. Typical specific growth rates (increase in cell mass per unit time) may range from 0.12d ^-1 to 0.05d ^-1; thus, the typical doubling time of plant cell cultures is measured in days, as compared to hours for microbial systems.

3.  In plant cells, the vacuole is usually the site of product accumulation and extracellular product secretion is rare. While in microbial cells product accumulation often extracellular.

4.  The plant cells mostly grow as aggregates while microbial cells grow as single cells..

5.  The plant cells in suspension cultures often undergo spontaneous genetic variation in terms of accumulation of secondary metabolites, which leads to heterogeneous population of cells in a suspension culture. Compared to this microbial cells are genetically stable.