Differential responses of leaf water-use efficiency and photosynthetic nitrogen-use efficiency to fertilization in Bt-introduced and conventional rice lines

Leaf stomatal conductance (g (s)), transpiration rate (E), and light-saturated net photosynthetic rate (P (Nmax)) at three developmental stages (tillering stage, jointing-booting stage, and milking stage) and leaf total nitrogen concentration (LTNC) and delta(13)C value at milking stage were measured for a conventional rice line (Minghui 63) and its corresponding Bacillus thuringiensis (Bt)-gene [cry1A (b and c)] introduced line (Bt line) under three fertilizer levels. Compared to conventional line, Bt line showed lower g (s), which was associated with lower P (Nmax) and E, but instantaneous water-use efficiency (WUE), measured as the ratio of P (Nmax) to E, was higher in the Bt line than in the conventional line, particularly in the jointing-booting stage. However, delta(13)C values were not significantly different across treatments, suggesting that intrinsic water-use efficiency (WUE(in)) might be indistinguishable between Bt and conventional lines. LTNC was higher but P (Nmax) was lower in Bt line compared to conventional line, resulting in significantly lower photosynthetic nitrogen-use efficiency (PNUE). This might result from the additional cost of producing Bt protein in the Bt line due to the effect of competing nitrogen with photosynthetic machinery. Bt-gene introduction and expression does not significantly change WUE(in) but may significantly decrease leaf PNUE. Thus we suggest that Bt rice should be carefully examined in relation to environmental risks (e.g. water-body pollution) before planting commercially.