ON THE VALIDITY OF PLASMODESMOGRAMS

Because the operation of plasmodesmata is of paramount importance for the integrative action of tissues and organs, it is important to quant ify the symplastic continuity. An attempt to visualize the intercellul ar communication pathways in plant tissues is the plasmodesmogram, a t wo-dimensional diagram of plasmodesmatal frequencies. Plasmodesmograms assume that the greater the frequency of plasmodesmata, the greater i s the potential for symplasmic communication. In fact, however, the va lidity of plasmodesmograms hinges on the premise that plasmodesmata ar e uniform in structure and functioning. In view of recent findings tha t plasmodesmata are gatable channels with different functional diamete rs, a correlation between plasmodesmatal frequency and the rate of int ercellular communication - including transport - is disputable and the concept of the plasmodesmogram needs to be re-examined. Its validity appears to vary with the developmental stage and the nature of the pla nt tissue. Our over-all conclusion is that plasmodesmograms are an acc eptable device for coarse (qualitative) assessment of intercellular tr ansport pathways in mature phloem tissues. Pathways identified by plas modesmograms often correlate with the physiologically determined modes of photosynthate transport. Whether this applies to differentiated ti ssues, in general, remains to be elucidated. The value of plasmodesmog rams for developmental phyisology seems to be limited, as developmenta l processes coincide with temporary or permanent closure of plasmodesm ata.