U. Kafkafi

Advances in Chloride Nutrition of Plants

The dependence of modern agriculture on irrigation and chemical fertilization emphasizes the problem of chloride accumulation in soils and its adverse effect on plants rather than on its deficiency. This review evaluates chloride behavior in the soil environment and its functions in plants as an essential nutrient, as a disease suppressor, and as an osmoticum in plant cells. The management practice of chloride content in the root zone of irrigated agricultural soil is reviewed. Both the positive and the negative effects of chloride on plant growth and marketable quality under normal and particularly under saline conditions are discussed. Some mechanisms of crop requirement and tolerance to chloride are also evaluated. The close relationship between potassium and chloride in specific plant cells is described. Potassium and nitrate roles in increasing plant tolerance to salinity and in reducing the hazard of using saline water are discussed on the cell and whole plant levels. Chloride influence on reducing the nitrification rate of ammonium fertilizers in the soils is discussed in relation to rice field management. The practical applications of irrigation methods to prevent excessive chloride accumulation in the root zone are also discussed.

Relationship between chloride and nitrate and its effect on growth and mineral composition of avocado and citrus plants

Abstract Two rootstocks of avocado (Persea americana Mill.), the salt‐tolerant ‘Degania‐113’ and the salt‐sensitive ‘Smith’, and two rootstocks of citrus, the salt‐tolerant ‘Cleopatra’ mandarin (Citrus reshni Hort. ex Tan) and the salt‐sensitive ‘Troyer’ citrange (Poncirus trifoliata x Citrus sinensis [L.] Osbeck), were grown in a sandy soil and irrigated daily with nutrient solutions containing various chloride concentrations. Increasing the concentration of chloride resulted in elevated chloride levels in all avocado plant parts, and toxic symptoms that were more pronounced in the ‘Smith’ than in the ‘Degania‐113’ avocado rootstock. When leaves of both rootstocks had accumulated similar chloride levels and showed scorching damage, the leaves of ‘Degania‐113’ abscised, while those of ‘Smith’ did not. High chloride reduced the total dry matter yield of the root more than that of the shoot, decreasing the “root: shoot”; dry weight ratio in both rootstocks. Addition of nitrate to the irrigation water reduce...

Release of carboxylic anions and protons by tomato roots in response to ammonium nitrate ratio and pH in nutrient solution

The exudation of certain organic anions and protons by roots which may affect solubility of metals and P and uptake by plants, is affected by nitrogen form and pH. The objective of this work was to study exudation of carboxylates and H+/OH− by tomato plants in response to NH4/NO3 ratio and pH in nutrient solution. Four NH4/(NH4+NO3) ratios (R= 0, 0.33, 0.67 and 1) and constant vs. variable solution pH treatments were investigated. The sum of the exudation rates of all carboxylates tended to decline with increasing R, particularly tri- and dicarboxylates. The molar fraction of the exuded tri- and dicarboxylates, averaged over all treatments and plant ages, increased in the order tartarate ∼2%), malate (∼6%), succinate (∼15%), citrate (∼26%) and fumarate (∼46%). At R=1 the solution pH dropped from 5.2 to ∼3 and at R=0 increased to ∼8. The R corresponding to the pH stat of tomato plant was ∼0.3. For the constant solution pH treatment, the effect of solution pH on carboxylate exudation rate was small as compared to the effect of R. The exudation of citrate and H+ efflux which were initiated when NO3 and NH4 uptake rates per plant exceeded certain threshold values, increased with plant age.

Phosphate induced carboxylate and proton release by tomato roots

Exudation of carboxylic anions and protons by plant roots plays an important role in mobilizing soil P under P-deficiency conditions. The objective of this work was to quantify short-term (6 h) carboxylate and H+ exudation by tomato roots in response to P concentration (0, 0.1, 0.5 and 1.0 mt M P) in nutrient solution (Cp). The exudation rate of tri- and dicarboxylates decreased exponentially with increasing Cp, from 0.3 to 0.03 μmol plant−1 6h−1. At low Cp the predominant exudates were fumarate, citrate and succinate, while at Cp=0.5 and 1.0 mt M the prevalent anions were succinate and citrate. The solution pH declined sharply as Cp was lowered from 0.1 (pH=4.2) to 0 mt M P (pH=3.7).

Quantitative competition of calcium with sodium or magnesium for sorption sites on plasma membrane vesicles of melon (Cucumis melo L.) root cells

The presence of Ca2+ ions in solution is vital for root growth. The plasma membrane is one of the first sites where competition between Ca2+ and other ions occurs. We studied the competition between Ca2+ and Na+ or Mg2+ for sorption sites on the plasma membrane of melon root cells.Sorption of 45Ca2+ to right-side-out PM vesicles of melon (Cucumis melo L.) roots (prepared by aqueous two-phase partitioning) was studied at various Ca2+ concentrations, in the presence of increasing concentrations of Na+ or Mg2+ chlorides. Experimentally determined amounts of Ca2+ sorbed to the plasma membrane vesicles agreed fairly well with those calculated from a competitive sorption model. The best fit of the model to the experimental data was obtained for an average surface area of 370 Å2 per charge, and binding coefficients for Na+, Mg2+ and Ca2+ of 0.8, 9 and 50 m-1, respectively.Our results suggest that nonphospholipid components in the plasma membrane contribute significantly to Ca2+ binding. The high affinity of Ca2+ binding to the plasma membrane found in this study might explain the specific role of Ca2+ in relieving salt stress in plant roots.

Root temperature, concentration and the ratio NO3−/NH4+ effect on plant development

Abstract The effect of ammonium ion in the solution on plant growth and ionic composition is reviewed.lt is demonstrated that ammonium is taken up in preference to nitrate when its concentration is above 10 % of total nitrogen in the nutrient solution. Ammonium is a safe source at low root temperatures, and is dangerous at high temperatures. Sensitivity of crops to ammonium varies between species. Chinese cabbage may die in the presence of 5 mM ammonium but thrives on 0.05 mM. The response of various crops to various ratios of ammonium in the solution leads to the conclusion that the rate of sugar supply to the roots minus the demand for root respiration is the key factor that controls the ability of the plant to assimilate ammonium in the root.

AMMONIUM ON POTASSIUM INTERACTION IN SWEET PEPPER

Nitrogen (N) metabolism in plants requires adequate potassium (K) content in the cytoplasm. However, the influence of NH4 + in the nutrient solution on plant uptake of K is controversial. To identify the effect of N form on K uptake in sweet pepper (Capsicum annum L. cv. Hazera 1195) two hydroponic experiments containing three K concentrations (0.5, 2.5, and 5.0 mM) combined with four NH4 + −N to NO3 − −N mM ratios (0:6, 0.9:5.1, 1.8:4.2, or 3:3) at a constant total of 6 mM N were conducted during the spring-summer and autumn-winter seasons. Early fruit yield was affected by K concentration but not by N form. Replacing 0.9–1.8 mM (15–30%) of NO3 − −N by NH4 + −N increased total fruit yield, especially when only 0.5 mM K was supplied. No significant interaction of NH4 + −N and K concentration on early and total fruit yield was found in either season. The uptake rate of NH4 + −N was related to its concentration and was independent of K concentration in the culture solution. The uptake rate of K was reduced as the NH4 + −N concentration increased above 1 mM. Compared to sole NO3 − −N supply, adding 0.9–1.8 mM NH4 + −N significantly increased the plant leaf phosphorus (P) and N content, but did not affect the leaf K content in the high yielding spring-summer season. It is concluded that 0.9–1.8 mM NH4 + −N (15–30% of total N) in the nutrient solution could increase both the total fruit yield and fertilizer K use efficiency in sweet pepper.

Sudden Wilt of Melons in Southern Israel: Fungal Agents and Relationship with Plant Development

Fungi belonging to five genera, Monosporascus sp., Pythium aphanidermatum, Rhizoctonia solani, Olpidium sp., Fusarium solani, and F. proliferatum, were the species most frequently isolated from the root systems of wilted melon. Diseased plants were collected from 24 fields in the northern and central Arava region of southern Israel during the fall seasons of 1994 and 1995. In pathogenicity tests conducted under field conditions, in artificially inoculated microplots, the first wilt symptoms were observed at various stages of fruit maturation. High mortality levels (73 to 97%) were recorded for inoculation combinations in which Monosporascus sp. was involved. Inoculations with the other fungi listed resulted in lower incidences of wilt. The combination of F. solani and P. aphanidermatum resulted in higher mortality than that caused by each pathogen alone. Monosporascus sp. seems to be the primary pathogen, although other fungi could also induce wilt. The dry weight of plants grown in naturally infested soil ceased to accumulate 33 days after transplanting, in contrast to plants grown in methyl bromide-treated soil. At this stage, the first wilt symptoms were observed. Fruit load affected wilt incidence. At the end of the growing season, 98% mortality was recorded for plants having the normal fruit load (2.5 fruits per plant) compared with 75 and 12% for plants that had their fruits thinned to one or zero per plant, respectively.

EFFECT OF VARYING NITROGEN FORM AND CONCENTRATION DURING GROWING SEASON ON SWEET PEPPER FLOWERING AND FRUIT YIELD

Flowering and fruit set of sweet pepper (Capsicum annum L.) is sensitive to environmental condition and nitrogen (N) nutrition status. To clarify the N fertilization effect, four levels of total N concentrations and four nitrate-nitrogen (NO3─N) and ammonium-nitrogen (NH4─N) ratios were supplied in three distinct physiological stages of sweet pepper: Stage I—vegetative; Stage II—fruit set during cross-pollination; and Stage III—fruit developing period. The plants were grown hydroponically during two seasons. In the season of short photoperiod progressing from autumn to winter with 10.9 to 10.2 light h day−1, gradually increasing the total N concentration with the progressing physiological stages from 3 to 6 and 9 mM increased the total set of flowers and fruits, and produced the highest total fruit yield (3444 g plant−1). During the increasing day length season from spring to summer (12.3–12.8 h day−1), supply of 9 mMN and 30% NH4─N before fruit set produced the larger amount of early ripening fruits. When the NH4─N percent rose from 0 to 15 and 30 in the autumn-winter season, the highest amount of early ripening fruits was obtained. In both seasons, the highest total fruit yields was obtained when NH4─N was 30% of the total N (6 mM) during the vegetative stage and NO3─N as sole N source during fruit filling stage.

Response of oilseed rape plant to low root temperature and nitrate: Ammonium ratios

Abstract Oilseed rape (Brassica napus L.) response to root temperature regimes (20/20, 16/8 and 12/12°C day/night) at constant 20°C air temperature was studied. At each regime, three NO3 ‐:NH4 + ratios (10:0, 8:2, or 6:4), at constant 10 mM N, in the irrigation solution were tested. Plant growth, transpiration, ionic composition and level of cytokinins and gibberellins in the xylem exudate were monitored. The two low root temperature regimes, 12/12 and 16/8°C, reduced rape shoot growth by 28 and 22%, and increased the accumulation of soluble carbohydrates by 42 and 26% in the roots, respectively, as compared to the 20/20°C regime. Low root temperatures reduced plants transpiration. The NO3 ‐:NH4 + ratios had no effect on rape growth. At low root temperatures NO3 ‐contents increased in the shoot and decreased in the roots. The sum of cations and that of anions at 12/12 and 16/8°C root temperatures decreased significantly as compared to 20/20°C. The presence of NH4 + in the irrigation solution decreased the...