Reduced Nitrogen Fertility Accelerates the Stimulation of Senescence Caused By Chronic Ozone In Nicotiana sylvestris.

Tropospheric ozone (O₃) is a damaging air pollutant that has been estimated to cost U.S. growers over $1.7 to 3.8 billion per year. Growth in chronic elevated ozone leads to reductions in chlorophyll, photosynthetic capacity and leaf longevity, all of which have a negative impact on yield.  Here, we compared sequential cohorts of leaves in tobacco (Nicotiana sylvestris) that when grown in elevated O₃ with limiting nitrogen (N), revealed major decreases in photosynthetic parameters for the older leaf, including a 55% decrease in CO₂ assimilation, 57% decrease in stomatal conductance, 49% decrease in maximum carboxylation rate and 41% decrease in photosynthetic electron transport. Unexpectedly, an increase in shoot N content was observed in these leaves. As no change in root N content was observed between plants grown in limiting N and either ambient or elevated O₃, this indicates that N recycling to young leaves was induced by O₃-stimulated senescence. Up-regulation of senescence-related transcripts, as determined by qRT-PCR, confirmed that elevated O₃ caused the older cohort of leaves to enter senescence sooner and had progressed through senescence at a faster rate. While sufficient N was able to prevent the onset of senescence in elevated O₃, root N content was decreased, implying that N was diverted from root development in order to sustain normal metabolic rates at the leaf level. This works suggests that as O₃ levels continue to increase, crops will become more sensitive in fields with poor N fertility.