Effects of ammonium to nitrate ratio and salinity on yield and fruit quality of large and small tomato fruit hybrids

Tomato cultivars respond differently to nitrogen (N) sources and to saline conditions, in terms of both yield and fruit quality. Interactions between salinity and NH4(+)/NO3(-) ratios with tomato genotypes were studied. The effect of four ammonium levels (0, 1, 2, 4 mM of total 8 mM N) and two salinity levels (0 and 45 mM NaCl) on cv. "R-144", and the effect of two salinity levels (0 and 45 mM NaCl) on four tomato hybrids ("R-144", "R-175", "FA612", and "FA624") were studied in two greenhouse experiments. The effects of two NH4(+) levels, (0 and 2 mM out of total N at 8 mM), and two salinity levels (0 and 20 mM NaCl) on large-fruit ("R-144") and small-fruit ("FA612") tomato hybrids were also studied in a net-house. The NaCl at 45 mM resulted in a smaller leaf area index (LAI), lower plant dry matter and lower fruit yield than controls. Addition of 1 mM NH4(+) to the nutrient solution contributed to improved growth. Genotypes bearing large fruits were more prone to suffer yield losses under saline conditions than those with small fruits. In the second experiment, salinity treatment resulted in improved fruit TSS, EC and titratable acidity in all of the hybrids except of "FA612". Fruit TSS was inversely correlated with yield. However, the ratio in TSS decline versus yield varied among the hybrids. At mild salinity (20 mM NaCl), fruit TSS and titratable acidity were significantly increased in "R-144" while yield was slightly decreased. Transpiration rate decreased with the presence of ammonium in the nutrient solution in both hybrids. The highest and lowest values were obtained for "FA612" and "R-144", respectively. The detrimental effect of salinity is mainly attributed to the decrease in LAI and the subsequent reduction in water uptake, resulting in low fruit weight. Addition of 1 mM ammonium to 7 mM nitrate in the nutrient solution had an ameliorating effect on tomato fruit yield under salinity.