Nitrogen Nutrition of Greenhouse Pepper. I. Effects of Nitrogen Concentration and NO3 : NH4 Ratio on Yield, Fruit Shape, and the Incidence of Blossom-end Rot in Relation to Plant Mineral Composition

Blossom-end rot (BER) is one of the major physiological disorders of greenhouse bell pepper (Capsicum annuum L.). The objective of the present work was to study the effects of the solution N concentration and N-NO3 : N-NH4 ratio on fruit yield and the incidence of BER and other fruit-quality traits of greenhouse-grown bell pepper in a Mediterranean climate. Three experiments were conducted: Expt. 1 included five total N concentrations (0.25 to 14 mmol·L, with a constant N-NO3 : N-NH4 ratio of 4); Expt. 2 included five treatments of different NO3 : NH4 molar ratios (0.25 to 4, with a constant N concentration of 7 mmol·L–1); and Expt. 3 included three treatments of different NO3 : NH4 molar ratios (1.0, 3.0 and 9.0, with a constant N concentration of 7 mmol·L). Plants were grown in an aero-hydroponics system in Expts. 1 and 2 and in tuff medium in Expt. 3, in greenhouses in Israel. The optimal values of N concentration for total fruit yield and for high fruit quality (marketable) were 9.3 and 8.3 mmol·L–1, respectively. The total and high-quality fruit yields both increased with increasing N-NO3 : N-NH4 ratio in the range studied. The total and high-quality fruit yields both decreased sharply as the NH4 concentration in the solution increased above 2 mmol·L. The increase in the NH4 concentration in the solution is the main cause of the suppression of Ca concentration in the leaves and fruits and the increased incidence of BER. The occurrence of flat fruits also increased with increasing NH4 concentration in the solution. Bell pepper fruit quality in Israeli commercial and experimental greenhouses decreases during spring and summer, because of blossom-end rot (BER) and shape deformation of fruits (Israel Ministry of Agriculture, unpublished data). A N-NO3 : NNH4 ratio of 1:1 was found to be optimal for growth of young tomato plants under a wide range of root temperatures (GanmoreNeumann and Kafkafi, 1980a). Feigin et al. (1980) reported that the solution NH4 fraction in the range of 0% to 30% had no signifiespecially during the springtime and onward. One of the deformations is that the fruits are flat instead of having the desired blocky shape, with a length/diameter ratio of 1.0. Flat fruits are defined here as fruits with a length/diameter ratio below 0.90 but above 0.8. This disorder is distinct from the disorder which causes very flat seedless fruits; the flat fruits described here have a normal number of seeds; therefore this physiological disorder is not related to the absence of seeds. It is well known that the final shape of the fruit is strongly affected by pre-anthesis temperature conditions (Ali and Kelly, 1993; Aloni et al., 1999; Polowick and Sawhney, 1985; Rylski, 1973; Rylski and Spigelman, 1982). Rylski and Spigelman (1982) reported that a low night temperature (12 °C) caused deformed, blunt pepper fruits. Aloni et al. (1999) reported that conditions that enhance carbohydrate accumulation in the flower increase the size of the ovary and the incidence of flat fruits in bell pepper. There is no information on the effect of plant nutrition on the incidence of flat fruits. The objective of the present investigation was to study the effects of N concentration and the N-NO3 : N-NH4 ratio on fruit yield and quality, in relation to the mineral composition of the organs of greenhouse-grown bell peppers. Materials and Methods Plant growth and experimental design. Expts. 1 and 2. The experiments were conducted at Bet Dagan, Israel (35°E, 31°N; 50-m altitude), in a climate-controlled greenhouse. Minimum and maximum air temperatures were 18 and 30 °C, respectively. Bell pepper plants, cv. Mazurka, were grown in an aero-hydroponics system, as described by Feigin et al. (1984), consisting of two separate 50 × 29 × 20-cm deep polystyrene boxes mounted on a 140-L covered container (one plot). Roots were exposed continuously to the nutrient solutions that were circulated by means of a pump and plastic tube system with small holes through which the solution was injected. The solution was leached to the bottom of the 140-L container and recirculated. Expt. 1 included five total N levels, 0.25, 3.50, 7.0, 10.5, and 14.0 mmol·L (with constant N-NO3 : N-NH4 ratio of 4:1) and Expt. 2 included five N-NO3 : N-NH4 ratios, 0.25, 0.50, 1.00, 2.00, and 4.00 (at a constant N concentration of 7.0 mmol·L) (Table 1). The treatment with N-NO3 : N-NH4 ratio of 4.00 was also included in Expt. 1. The only ions that varied among treatments, in addition to NH4 and NO3, were Cl, SO4 and protons (Table 1). In both experiments the nutrient solutions were prepared with tap water containing about (in mmol·L) 0.2 NO3, 4 Cl, 1.5 SO4, 1 HCO3, 2 Ca, 1 Mg, and 3.5 Na. The initial pH of the solution was 6.5 and it was monitored daily; when it increased above 7.0, sulfuric acid was added and when it fell below 6.0, sodium hydroxide was added to adjust the pH back to 6.5. The electrical conductivity (EC) was in the 2.0–2.5 dS·m cant effect on fruit yield of tomato, but the yield decreased as the NH4 fraction increased from 30% to 50%. In the Netherlands, the recommended N-NO3 : N-NH4 ratio for greenhouse bell pepper is much higher, ≈6:1 (Roorda van Eysinga and van der Meijs, 1981). The N-NO3 : N-NH4 ratio affects BER, probably because of its effects on Ca uptake and concentration in tomato (Wilcox et al., 1973; Wojciechowski et al., 1969) and bell pepper (Marti and Mills, 1991; Morley et al., 1993) fruit. The early stage of fruit development is the period of greatest sensitivity to Ca supply (Marcelis and Ho, 1999; Marti and Mills, 1991). Recently, Marcelis and Ho (1999) showed that the incidence of BER in pepper correlated well with the Ca concentration in the fruit. Ho et al. (1993) reported that high radiation combined with NH4 nutrition increased the incidence of BER in tomato. There is a lack of quantitative information on the effect of NH4 concentration on BER in greenhouse-grown pepper under spring and summer conditions in a Mediterranean climate. Fruit deformation is another widespread disorder affecting bell pepper production, 6443, p. 1244-1251 12/19/01, 1:00 PM 1244 1245 HORTSCIENCE, VOL. 36(7), DECEMBER 2001 Table 2. Total and high quality fruit yield, BER incidence and the fruit length to diameter ratio as affected by the N-NO3 : N-NH4 ratio in the nutrient solution, Expt. 3. Dec. 1996 to June 1997.