Nitrogen assimilation
Nitrogen assimilation was examined using elemental N content analysis and 15N-nitrate labelling to measure nitrate absorption and distribution in shoots and roots (Fig. 3). In all tissues, N elemental content was significantly lower at low K, showing the general inhibition of N assimilation under low K availability (Fig. 3a). Low Ca conditions resulted in slightly lower N content in old leaves under high K. Putrescine was beneficial to the N content in young leaves under low K. N absorption measured via15N-labelling was much lower under low K, and this was alleviated to some extent by putrescine. Interestingly, low Ca was beneficial to N absorption at low K but detrimental at high K, suggesting that the balance between cation species was essential for nitrate absorption by root cells.