Induction of a high-capacity nitrate-uptake mechanism in barley roots prompted by nitrate uptake through a constitutive low-capacity mechanism

Roots of nitrate-starved and nitrate-pretreated seedlings of Hordeum vulgare were used to investigate the induction of a high-capacity uptake mechanism for nitrate. When exposed to 0.2 mmol·l-1KNO3, nitrate-starved roots took up nitrate at a rate of approx. 1 μmol·(g FW)-1·h-1; K+ was absorbed at a rate ten-times higher. Nitrate uptake accelerated after a lag of about 1 h, until it matched the rate of K+ uptake about 4 h later. p-Fluorophenylalanine (FPA), which prevents the synthesis of functioning proteins, suppressed the development of the high-capacity mechanism. Pretreatment of the roots with 0.2 mmol·l-1 Ca(NO3)2 for 24 h established the high-capacity mechanism. Pretreated roots were able to absorb nitrate at high rates immediately upon exposure to 0.2 mmol·l-1 KNO3, in the absence or presence of FPA. The high-capacity mechanism, once established, appeared to have a protein turnover as slow as that of the low-capacity mechanism or that of the mechanism involved in the uptake of K+. In contrast, the mechanisms for the transport of nitrate and K+ into the xylem vessels were completely blocked by FPA within 1 h of application, confirming earlier evidence for a rapid turnover of the transport proteins in the xylem parenchyma. Nitrate reduction proceeded at rates which were roughly one-tenth as large as the rates of the respective nitrate-uptake processes, indicating that nitrate-reductase activity was determined by the rate of nitrate uptake and not vice versa. We conclude that the formation of a high-capacity nitrate-uptake mechanism in barley roots occurs in response to nitrate uptake through a constitutive mechanism of low capacity which appears to function as a sensing mechanism for nitrate in the environment of the roots.