Why does potassium concentration in flue-cured tobacco leaves decrease after apex excision?

Field studies were conducted from 2000 to 2002 to quantify how the accumulation and distribution of dry matter and K+ in tobacco plants (Nicotiana tabacum L. variety K326 in 2000 and 2002, and Nicotiana tabacum L. variety Yun 85 in 2001) affects the K+ concentration in leaves after apex excision. Dry matter accumulation (DMA) in K326 increased after apex excision but then decreased as the leaves began to mature, while DMA in Yun 85 continuously increased after apex excision. As leaves matured, a net decrease of dry matter occurred sequentially in lower leaves, middle leaves, upper leaves, and even in stems and roots for K326 but only in lower leaves for Yun 85. The patterns of K+ uptake by roots and changes in K+ content and concentration in different plant organs were similar for all three years of the study. K+ content was highest 12-20 days after apex excision and then decreased; until the final harvest of leaves, K+ content decreased 15% in 2000, 10% in 2001, and 30% in 2002. Moreover, a net export of K+ occurred from leaves to stem and roots such that the percentage of K+ in leaves vs. the whole plant dropped from 65% at the time of apex excision to 58% at the last harvest, while that of stem plus roots increased from about 35% at the time of apex excision to about 42% at the last harvest. The K+ concentration in all plant organs continuously decreased after apex excision with time. Overall, the results indicate the K+ concentration in leaves decreases after apex excision because (1) plant dry matter increases at a greater rate than K+ uptake (the dilution effect), (2) there is a net decrease in K+ content, and (3) K+ is transported from leaves to stems and roots.