Time course of physiological responses in kiwifruit induced by bicarbonate

Abstract

KEY MESSAGE: Ionic imbalance is one adaptive strategy of kiwifruit to bicarbonate, as indicated mainly by NH₄⁺ accumulation and increased NH₄⁺/NO₃⁻ in bicarbonate-treated roots as well as decreased NH₄⁺/NO₃⁻ in bicarbonate-treated leaves. Bicarbonate-induced leaf chlorosis is frequently observed in kiwifruit orchards. A time-course experiment was conducted to investigate the effects of bicarbonate stress on the growth, root acidification, and organic acid and nutrient concentrations of kiwifruit plants treated for 7, 14, 21, 28, and 42 days in hydroponics. After 21 days of bicarbonate treatment, the kiwifruit vines exhibited leaf chlorosis, as indicated by decreased SPAD, chlorophyll a and b as well as carotenoid. Moreover, bicarbonate treatment induced NH₄⁺ accumulation and NO₃⁻ reduction in roots from day 21 onward, thereby increasing the ratio of NH₄⁺ to NO₃⁻ in roots, and the opposite was true for leaves that seemed to respond to bicarbonate earlier than roots but in an inconsistent manner. Before leaf chlorosis, bicarbonate imposition induced succinic acid accumulation and K reduction in roots from day 14 onward. However, the K concentration increased in bicarbonate-treated leaves from day 28 onward. Bicarbonate treatment also reduced P in all plant parts and Fe in leaves from day 21 onward, lowered Zn and enhanced Ca and Mg in roots from day 28 onward. In addition, bicarbonate treatment increased citric acid and ferric chelate reductase (FCR) activity in roots at days 7 and 14, but decreased citric acid and H⁺ extrusion at day 28 and decreased FCR activity at day 42, respectively, indicating that root acidification is duration dependent. These results suggest that, except for succinic acid accumulation, ionic imbalance in the whole kiwifruit plants (particularly N form’s shift) might be an alternative strategy to adapt to bicarbonate stress.