Abstract
Continuous light (CL) or a predominant nitrogen supply as ammonium (NH4+) can induce leaf chlorosis and inhibit plant growth. The similarity in injuries caused by CL and NH4+ suggests involvement of overlapping mechanisms in plant responses to these conditions; however, these mechanisms are poorly understood. We addressed this topic by conducting full factorial experiments with tomato plants to investigate the effects of NO3− or NH4+ supply under diurnal light (DL) or CL. We used plants at ages of 26 and 15 days after sowing to initiate the treatments, and we modulated the intensity of the stress induced by CL and an exclusive NH4+ supply from mild to strong. Under DL, we also studied the effect of nitrogen (N) deficiency and mixed application of NO3− and NH4+. Under strong stress, CL and exclusive NH4+ supply synergistically inhibited plant growth and reduced chlorophyll content. Under mild stress, when no synergetic effect between CL and NH4+ was apparent on plant growth and chlorophyll content, we found a synergetic effect of CL and NH4+ on the accumulation of several plant stress hormones, with an especially strong effect for jasmonic acid (JA) and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, in xylem sap. This modulation of the hormonal composition suggests a potential role for these plant hormones in plant growth responses to the combined application of CL and NH4+. No synergetic effect was observed between CL and NH4+ for the accumulation of soluble carbohydrates or of mineral ions, indicating that these plant traits are less sensitive than the modulation of hormonal composition in xylem sap to the combined CL and NH4+ application. Under diurnal light, NH4+ did not affect the hormonal composition of xylem sap; however, N deficiency strongly increased the concentrations of phaseic acid (PA), JA, and salicylic acid (SA), indicating that decreased N concentration rather than the presence of NO3− or NH4+ in the nutrient solution drives the hormone composition of the xylem sap. In conclusion, N deficiency or a combined application of CL and NH4+ induced the accumulation of JA in xylem sap. This accumulation, in combination with other plant hormones, defines the specific plant response to stress conditions.
Highlights
Continuous light (CL) and a predominant NH4+ supply are considered prospective treatments to increase crop yield
The increased daily light integral (DLI) directly contributes to enhanced plant growth; at the same DLI, plants exposed to CL and lower intensities of photosynthetically active radiation (PAR) grow more quickly than plants grown under periodic light with higher PAR [1,2]
We analyzed the effect of light and N form on major plant growth traits of both genotypes by destructive analysis of plants sampled after 11–12 days light treatment and 14–15 days N treatment
Summary
Continuous light (CL) and a predominant NH4+ supply are considered prospective treatments to increase crop yield. CL can maximize the daily light integral (DLI), defined as the photosynthetically active light received within a 24 h window, at the same light intensity of an installed lighting setup. The increased DLI directly contributes to enhanced plant growth; at the same DLI, plants exposed to CL and lower intensities of photosynthetically active radiation (PAR) grow more quickly than plants grown under periodic light with higher PAR [1,2]. Plants supplied with NH4+ can, in theory, show positive growth responses because NH4+ nutrition is more energetically favorable than nitrate nutrition. Nitrate assimilation in plants initially involves a reduction of NO3− to nitrite (NO2−) by nitrate reductase, followed by reduction of NO2− to NH4+ by nitrite reductase [3], and these steps are highly energy demanding [4]. Supplying N as NH4+ bypass this high demand for reductant required for conversion of NO3− to NH4+
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