High K+ does not affect potato (Solanum tuberosum L.) tuber induction, but represses its development in vitro

The role of K+ in potato (Solanum tuberosum L.) tuberization, based on the effects of K fertilizer and soil exchangeable K+, appears to be mostly contradictory. Here, we provide evidence that K+ at high concentrations is detrimental to tuber development in vitro once induction has taken place. An experimental system using in vitro-cultured single-node cuttings showed that K+ at ≥30.0 mM significantly reduced tuber fresh mass concomitant with a corresponding decline in starch content. However, high K+ did not affect tuber induction in terms of number of tubers developed per cutting. High K+-induced inhibitory effect on tuber development was attributed to a reduced rate of assimilate partitioning. 86Rb(K) transport to stolons, and tubers that acted as strong sinks in vitro were proportional to exogenous K+ levels; however, 86Rb accumulation and K+ deposition were markedly reduced in tubers as compared with that in stolons, especially at higher K+ levels. The results indicated a diminishing sink strength developed by tubers with increasing K nutrition. Highly significant negative correlations between 86Rb accumulation/K+ deposition in both the sink organs and tuber fresh mass reinforced the inhibitory effect of high K+ on tuber development. The rate of tuber K removal in vitro was similar to that of crop K removal reported in vivo, suggesting highly conserved K uptake and transport mechanisms during tuberization process. The results have been discussed in the context of possible effects of high K+ on impairing sucrose uptake and metabolism.