22 Aug Factors Influencing Nutrient Absorption
Light & Temperature
Under conditions of intensive light exposure, it is noted that leaves contain more epicuticular waxes than those shaded. The seasonal buildup and development of secondary wax structures on the abaxial surface of leaves also positively correspond with increasing light intensity. Generally, it is believed that light favors absorption of mineral nutrients by the leaves. On the other hand, a negative relationship between air temperatures and the amount of epicuticular waxes on a surface unit of plum leaves had been noted.
High air temperatures during rapid leaf expansion may enhance the absorption of mineral nutrients by the leaves due to a lower amount of waxes on unit surface area of a leaf. It is speculated that differences in nutrient absorption rates depend on chemical composition and compound configuration of epicuticular waxes. There is also claims that under conditions of high air temperatures the surface wax components have vertical configuration and the leaf surface coverage decreases which consequently may increase nutrient absorption.
This view is generally accepted in thinking that even slight alterations in the molecular configuration of surface waxes significantly affect nutrient absorption rate.
High air humidity usually stimulates the absorption of leaf applied nutrients. In studies done, it was shown that the uptake of P by bean leaves was doubled when the treated surface was kept moist, compared with similar treatments in which leaf surfaces were allowed to dry.
It is also reported that enhanced absorption of Ca-ions by pear (Pyrus communis) leaves with increasing air humidity within the range of 50 – 90%. According to most believes, the positive influence of air humidity on nutrient absorption by leaves is related to the reduction in drying of droplets. Additionally, high air humidity causes the swelling of the cuticular membrane that loses its components. This change of cuticle structure increases the absorption of hydrophilic compounds. The dynamics of mineral nutrient uptake by leaves is inversely related to air humidity. It was demonstrated in trails that an increased penetration of Ca-ions through the cuticular membrane of apple fruit correlated with decreasing air humidity in the initial period of time. This phenomenon is explained by an increase in droplets’ Ca-ion concentration as a result of their drying which consequently increased the concentration gradient for diffusion. However, despite initial enhanced absorption dynamics at low air humidity, the final uptake rates of nutrients from salts of low hygroscopicity are decreased because of rapid salt crystallization.
Factors related to spray solution
Foliar application of nutrient solutions causes salt concentrations on a leaf surface to be higher than those of soil solutions. Increased tolerance of the epidermis to high spray solution concentrations is caused by the presence of the wax layer and the cuticular membrane. Since most mineral nutrients passively diffuse into the epidermal cells, absorption depends on their concentrations on the leaf surface. There is a strong correlation between nutrient concentration on a leaf surface and the rate of its uptake by the epidermal cells. However, elevated nutrient concentrations cause leaf injury leading to the reduction in nutrient absorption. Absorption by damaged leaves is limited by the destruction of ectodesmata structures. Maximum concentrations of particular mineral nutrients in a spray solution depend on plant species, plant development stage, nutritional plant status, plant healthiness, and weather conditions.
It is commonly believed that the optimal pH values of spray solutions for the maximum uptake of most mineral nutrients are within the range of 3.0 and 5.5. For example, experimental research showed that the maximum Ca-ion absorption from organic chelates by apple leaves is at pH 3.3 – 5.2. It was also demonstrated that the greatest absorption of urea by apple leaves was at pH 5.4 – 6.6 and it was noted that the maximum Ca-ion absorption by sweet cherry (Prunus avium L.) fruit from CaCl2 solutions was at pH 7.
Surfactants are commonly used in pesticide formulations to improve physicochemical characteristics of a spray solution and consequently to increase the efficiency of foliage applied agrochemicals. Surfactants belong to the surface active agents possessing both hydrophilic and lipophilic groups. Such a structure has ability to create “bridges” between the aqueous solution and lipophilic waxes. Thus, surfactants decrease the surface tension between the liquid and leaf which leads to an increase in leaf wetting. Surfactants also eliminate/reduce the air layer between the liquid and leaf surfaces, increase penetration of solutes through the stomata, cuticular membranes and the cell walls, and limit the drying of droplets. The most frequently used surfactants in agricultural applications are ethoxylated alcohols, alkylphenols, sorbitan and alkylamines. The efficiency of nonionic surfactants in improving mineral nutrient absorption by leaves is mostly estimated by the value of hydrophilic-lipophilic balance (HLB). Theoretically, the higher HLB value of a given surfactant, the better penetration of a nutrient through the cuticular membrane. Practically, it is assumed that for nonionic surfactants the optimal HLB values determining a high efficiency of absorption of leaf applied nutrients are within the range of 15 to 17. However, it should be noted that the precise forecasting of the efficiency of nutrient uptake based on HLB values of surfactants may not be successful since their effects are dependent on many environmental and biological factors. It is believed that organosilicone surfactants have a high efficiency in increasing nutrient absorption by plant tissues.
Generally, two mechanisms of enhancement of leaf nutrient absorption by organosilicone surfactants have been reported. First, they may induce mass flow of spray solutions through the stomatal pore. Second, surfactants may directly or indirectly increase the penetration through the cuticular membranes. Stevens and Zabkiewicz (1990) showed that sprays of Calcium Chloride and Calcium Nitrate solutions with organosilicone surfactants such as Silicote, Silwet L77 and Silwet M were more effective in increasing apple fruit Ca-ion content than those without surfactants.
Chelates are complex compounds consisting of a central metal atom linked by a few coordinating bonds with ligand. Metal ability to create chelates lowers with a decreasing electric charge ratio to ion radius. It is believed that chelation facilitates the mobility of nutrients within a plant. The use of chelated nutrient forms usually does not increase absorption in relation to inorganic salts. Moreover, Beavers et al. (1994) found a lower absorption of Ca-ions from Ca-EDTA (ethylenediamine tetraacetic acid) by apple fruit than that of Calcium Chloride, but in the same instance a higher absorption with organic chelates than with Calcium Chloride. Also Kannan and Wittwer (1965) showed a decreased iron absorption by the leaves treated with Fe-EDTA and Fe-EDDHA (ethylenediamine di-Ohydroxyphenylacetic acid) as compared to those treated with FeSO4. Thus, it seems that an effect of chelates on leaf ability to take up nutrients is related to some properties of a chelate such as molecular weight of the complex, dissociation constant, and stability of the complex at various solution pH’s.
Species & Variety
Absorption of mineral nutrients by plants is species dependent. Research showed that the rate of Boron absorption by apple leaves was two to three times higher than that of pear, plum and sweet cherry. The differences in the uptake rates of mineral nutrients by leaves of different plant species result not only from uptake of mineral nutrients from foliar fertilization and the specific cuticular membrane structure, but also from various amounts of ectodesmata on a leaf surface. It is however suggested that the rate of nutrient absorption is related to plant requirements; plants that require more of a specific nutrient have a higher ability to absorb it. Absorption rate of mineral nutrients by above ground plant parts considerably differs not only among plant species but also between varieties within the same species.
Leaf Surface & Leafage
It is well documented that the lower leaf surface takes up mineral nutrients more rapidly than the upper side. For example, for four plant species tested, within the first 24 hours the absorption of Ca-ions by the lower leaf surface was much higher than that of the upper. The high dynamics of nutrient absorption by the lower leaf surface results from the presence of a thin layer of the cuticular membrane and large number of stomata. Both leaf surfaces differ only in the dynamics of nutrient absorption. It is accepted that the absorption rates of most mineral nutrients by young leaves are greater than those of old ones. For example, terminal apple leaves are able to take up almost twice as much nutrients compared to basal leaves. Lower nutrient uptake by basal leaves is attributed to a decreased metabolic activity and/or a lower amount of ectodesmata on the surface of a leaf. Furthermore, the reduced uptake of mineral nutrients along with the leaf age is related to the environmental conditions determining an increase in amounts of the epicuticular waxes.
Nutritional Status & Plant Development Stage
Leaf ability to take up mineral nutrients also is dependent on the nutritional status of a plant. Komosa (1990) reported that under conditions of low nutritional status of tomato plants, absorption rates of leaf applied nutrients were higher as compared to those of plants well supplied with nutrients via the roots; this was particularly pronounced for N, K+ and Mg2+. It is however also true if the amount of any mineral nutrient in the leaves is drastically low, leaf ability to absorb this nutrient is limited because of irreversible changes in the leaf tissues. There is a significant relationship between the ability of leaves to absorb mineral nutrients and the plant development stage.