Nitrogen, salinity, substrates and growth of gloxinia and chrysanthemum

The study was on the harmful effect of salinity on N utilization in the flower crops gloxinia (a salt-sensitive mesophytic semi-shade plant) and chrysanthemum (a salt- tolerant sun plant). For solid substrates (trials 2 and 3) the specific conductivity of the saturation extract (EC. in mmho per cm at 25°C) was used as measure of salinity (Richards et al., 1954). In water culture (Trial 1), the specific conductivity of the nutrient solution (EC in mmho per cm at 25 °C) was used. The specific conductivity of the substrate also indicated osmotic suction S s .In solid substrates, the availability of water is dependent not only on S s but also on the matric suction S m . To eliminate the influence of S m , attempts were made to keep it constant.In Trial 1, the yield of dry matter was studied at 4 nitrate concentrations and with 4 EC e values, the latter obtained by adding NaCl. Increases in EC e depressed yield increment per unit nitrogen. This reduction in N effect, being much larger for gloxinia than for chrysanthemum, represented a decrease in N utilization, attributable to a disturbance in nitrogen metabolism at rising suction tension ( S l or DPD) in the leaf (Barnette & Naylor, 1966). This rise in S l with EC of the medium could be deduced from a decrease in percentage transpiration. Brouwer (1963) has shown that S l increases with NaCl concentration.There was a practically linear negative relationship between yield and EC. After extrapolating EC to zero, the nitrogen curves coincided into one typical yield curve. Therefore the osmotic factor seemed dominant.The large influence of NaCI on ionic balance in the plant showed that specific ion effects could not be neglected. Notable specific effects were for the cations, the antagonism of Na +to uptake of K +, Ca 2+and Mg 2+, and for the anions the antagonism of Cl -to NO3-. The organic salts (C-A) decreased appreciably with rising Cl -concentrations in the medium.The two plants deviated in pattern of ion uptake. Chrysanthemum selectively absorbed K +, and could to some degree control the entry of ions. Gloxinia showed no selectivity and could not prevent the entry of ions. This difference must partly account for the difference in salt tolerance between the species. Certainly some investigators (Bernstein & Ayers, 1953; Sutcliffe, 1962) looked upon selective uptake of K +, as an indication of a species' salt tolerance.Since salinity depresses water balance of plant through S s , salt tolerance must also depend on the genetically determined osmotic characteristics of the plant. According to Slatyer (1963) the osmotic pressure of mesophytic shade plants (e.g. gloxinia), is about 5 bar, for most crop plants (e.g. tomato and chrysanthemum) between 10 and 20 bar, and for halophytes (e.g. Atriplex nummularia ) even 72 bar.In Trial 2, on a solid substrate, the nitrogen effect was depressed by four different types of salt, as by NaCl in Trial 1. The depression seemed to be almost proportional to the increase in EC e caused by addition of salt. Only K 2 SO 4 depressed yield of gloxinia more than could be explained by the increase in EC e caused by addition of the salt. As in Trial 1, the depression of the N effect by any salt could be attributed to a decreased N utilization.The almost linear negative relationship between yield and EC e was clearly influenced by the N rate. At optimum N rate, the reduction in yield by EC e was much larger than at the lowest N rate. In assessing damage by salinity, the nitrogen status of the crop must be considered. Specific ion effects could be detected by correction for EC e . K 2 SO 4 exerted the largest specific harmful effect on growth of gloxinia. Chrysanthemum, which was usually much less affected by specific salt injury, suffered mostly from the specific effect of Na 2 SO 4 . Extremely important for the ionic balance in the plant was the increase in proportion of N as NH4+with N rate (here given as ammonium nitrate). NH4+competes strongly with other cations but according to van Tuil (1965) contributes much less than NO3-to the content of organic salts. Increases in NH4+in the substrate with N rate therefore accounted for the decreases in organic salts in almost all series.For gloxinia, K +, competed markedly with uptake of Ca 2+, for K 2 SO 4 even more so than for KCl. The specific harmfulness of K 2 SO 4 for growth was therefore essentially a Ca 2+deficiency induced by K +, and SO42-together.The specific harmfulness of Na 2 SO 4 for chrysanthemum can be ascribed to a decrease in K +, in the plant by competition from NH4+and Na +together.The difference between species in pattern of ion uptake in Trial 1 was confirmed. Gloxinia seemed to have a high Ca 2+requirement but unlike chrysanthemum's selectivity for K +, could not absorb Ca 2+selectively. According to van den Berg (1952) the salt tolerance of a crop is often associated with a specific Ca 2+requirement. The results of Trial 2 support this opinion.In Trial 3, the influence of different substrates proved to be based entirely on the inverse proportionality between moisture capacity and EC e .The shape of the pF curve and the daily water loss by transpiration indicate that, despite of attempts to standardize the moisture level, the influence of matric suction S m was considerable in the clay-peat substrates of chrysanthemum, although it had been eliminated in the sand-peat substrates of gloxinia.The increase in N effect with increasing peat content of the substrates proved to be an EC e effect, as did also the lower negative effect of NaCl or of excess N with increasing peat content. These results also explained the significant interaction between nitrogen and substrate, reported elsewhere for gloxinia and cyclamen (Arnold Bik, 1962). The relationship between yield and EC e were almost independent of substrate. The usefulness of EC e as a criterion of salinity in trials with different substrates was thus confirmed.For gloxinia, the curves of N rate against yield for each of substrate-NaCl series coincided into one typical yield curve when EC e was adjusted to zero. Therefore at uniform pF and with adequate aeration the substrate effect is actually an EC e effect so long as the substrate components do not exert any particular effect such as fixation of K +.Plant composition again showed that the form of N (NH4+or NO3-) and, by its influence on nitrification, the CaCO 3 content of the substrate governed the ionic balance of the plant. In gloxinia total cations and total inorganic anions in the plant both decreased with increasing peat content, in accordance with the lower concentrations in the substrate. In chrysanthemum, this relationship was confused by the influence of the clay component of the substrate.Trial 3 suggests that the effect of salinity was more an osmotic effect than a specific ion effect.Practical measures for growers are suggested (Chap. 6) to minimize the harmfulness of salinity on the N effect and on the vegetative growth of pot plants and other ornamentals.