Effect of Nitrogen Fertilization on the Photosynthetic Activity, Growth and Yield of Virginia Tobacco (Nicotianatabacum L.)

A field experiment was carried out in 1996 in Central Italy in order to evaluate the effects of nitrogen fertilization (0, 60, 120 kg h^-1 N) of Virginia tobacco (cv. K394) in terms of net assimilation, growth and yield. Measurements of net leaf assimilation of CO₂ were taken at midday under conditions of strong sunlight (An_high) using a portable infra-red gas analyzer and at decreasing photosynthetic photon flux density by shading the leaves with filters (An vs PPFD). For An vs PPFD measurements the An_max value was calculated as the asymptote of the function proposed by CONNOR et al. (1). Growth was analysed by taking weekly samples to determine the fresh and dry weight and LAI (Leaf Area Index).

Before topping, An_high (average assimilation found in lower, middle and upper leaves) rose in proportion to increased levels of N (14.7, 17.2, 20.2 μmolm^-2 s^-1, for 0, 60 and 120 kg ha^-1 N respectively). After topping, An_high also grew with increased levels of N, but at a lower rate (6.8, 7.2 and 8.2μmol m^-2 s^-1, respectively).

Similarly, for each of the three fertilization methods (N0, N60 and N120) the increase in An_max in relation to N levels was greater before topping than afterwards (i.e. 18.7, 23.3, 26.3 μmolm^-2 s^-1 before topping compared to 7.5, 14.0 and 17.8 μmol m^-2 s^-1 after topping for each treatment respectively). The decrease of CO₂ assimilation after topping was probably caused by the accumulation of soluble photo assimilates in the leaf which could have led to a feed-back control on leaf photosynthesis. In N60 and N120 treatments, which had the same leaf expansion rate of 0.14 m² m^-2 d^-1 (m² of leaf on m² of soil per day), the rapid leaf expansion phase started about 40 days after transplanting, while it started some days later in N0 (rate of 0.11 m² m^-2 d^-1). That phase ended at topping in N60 and N120, while in N0 it ended some days before topping. Biomass accumulation followed the same pattern of LAI. The rapid biomass accumulation phase was characterised by growth rates of 9, 16 and 19 g d.m. m^-2 d^-1, respectively for the N0, N60 and N120 treatments but did not end at topping. This confirms that assimilate accumulation occurred after topping but was not accompanied by leaf expansion. At fertilization rates of 0, 60 and 120 kg N ha-1 the yields of cured leaf tobacco were 3226, 4202 and 4839 kg ha-1 respectively.