Interactive effect of salt (NaCl) and nitrogen form on growth, water relations and photosynthetic capacity of sunflower (Helianthus annuus L.)

The effects of the ammonium (NH4+) and nitrate (NO3-) forms of nitrogen and NaCl on the growth, water relations and photosynthesis performance of sunflower (Helianthus annuus L.) were examined under glasshouse conditions. Eight-day-old plants of cv. Hisun 33 were subjected for 21 days to Hoagland's nutrient solution containing 8 mol m(-3) N as NH4+ or NO3-, and salinised with 0, 60, or 120 mol m(-3) NaCl. Fresh weights of shoots and roots, and leaf area of NO3--supplied non-salinised plants were significantly greater than those of NH4+-supplied non-salinised plants. But addition of NaCl to the rooting medium of these plants had more inhibitory effect on the growth of NO3--supplied plants than on NH4+-supplied plants. Both leaf water and osmotic potentials of plants grown with NH4+ were lower than those of plants given NO3- under both non-saline and saline conditions. Chlorophylls a and b concentrations were higher in plants grown with NH4+ than NO3--supplied plants at the lower two levels of salinisation. The rate of photosynthesis in plants was considerably higher in non-salinised plants grown with NO3- than with NH4+, but with increase in salinisation the photosynthesis rate decreased in NO3--supplied plants, but not in those given NH4+. The rate of transpiration was increased significantly by salinisation in NO3--supplied plants, but not consistently so in NH4+-suplplied plants. The stomatal conductances were much higher in plants given NO3- than with NH4+ when grown under non-saline conditions, but not when salinised. As a consequence, water-use efficiency in NO3--supplied control plants was better than in NH4+-supplied under non-saline conditions, but worse under saline conditions. The different forms of nitrogen and the addition of NaCl to the growing medium did not affect the relative intercellular concentrations of CO2 (C-i/C-a). Overall, the NH4+ form of nitrogen inhibited the growth of sunflowers under non-saline conditions, but NO3- and NaCl interacted to inhibit growth more than did NH4+ under saline conditions.