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R. Romero-Aranda and J.P. Syvertsen

The penetration of foliar-applied urea and salt solutions into citrus leaves was investigated using `Duncan' grapefruit and `Valencia' orange seedlings in a greenhouse, and 8-year-old `Ruby Red' grapefruit trees in field tests during the summer and fall. Net gas exchange rates, Cl, nitrogen, and chlorophyll concentrations of singles leaves were measured during or after the period of foliar applications. Foliar-applied salt treatments increased leaf Cl, and visible burn symptoms were observed when Cl levels reached ≈0.4% of leaf dry weight. After 11 weeks, green areas from salt-treated leaves had similar rates of net CO2 assimilation as control plants. Leaf nitrogen and total chlorophyll increased with repeated sprays. Urea sprayed at 15% caused foliar burn symptoms after two to three applications and increased the amount of leaf abscission. Urea sprayed at 6% increased CO2 assimilation rate ≈50% after three foliar applications in 3 weeks. Nitrogen content and net CO2 assimilation of urea and urea + salt leaves were similar.

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R. Romero-Aranda and J.P. Syvertsen

We compared net gas exchange rates of CO2 and H2O vapor of greenhouse-grown `Duncan' grapefruit (Citrus paradisi Macf.) and `Valencia' orange [C. sinensis (L.) Osbeck] leaves after multiple foliar sprays of urea N with and without NaCl: CaCl2 solutions. Highly saline solutions (3.8 dSm-1) caused necrotic burn symptoms after leaf chloride levels reached 7 mmol·m-2. Grapefruit leaves had higher leaf Cl and more burn symptoms than orange leaves. The remaining green areas of all salt-stressed leaves, however, had similar rates of net CO2 assimilation (ACO2) and stomatal conductance (gs) as water-sprayed control leaves. Total leaf N and chlorophyll increased with repeated foliar applications of urea solutions regardless of salinity levels in the spray solution. Thus, salts in solution did not interfere with foliar absorption of N. High urea N solutions (33.6 g·liter-1) without salts caused foliar burn and leaf abscission after one application. Three sprays of urea-N solution (11.2 g·liter-1) increased N concentration of N-deficient leaves about 60% and increased ACO2 rate about 50%. ACO2 did not increase when nitrogen concentration in leaves exceeded a threshold value of about 200 mmol·m-2 so photosynthetic nitrogen use efficiency (PNUE = ACO2/N) decreased with increasing leaf N concentration. Net gas exchange and PNUE was higher for grapefruit than for orange leaves. Leaf Cl levels from foliar-applied salts may not be as detrimental to leaf gas exchange as Cl from salts in soil-applied irrigation water.

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R. Romero-Aranda, T. Soria and J. Cuartero

High salinity levels in irrigation water available in Mediterranean coastal areas induce a significant loss of yield in greenhouse tomato crops. This loss increases during the spring-summer growing season when high irradiance, temperature, and low humidity occur within greenhouses. This study determined whether salt-induced yield losses could be alleviated by increasing humidity by misting the greenhouse atmosphere. Plants of `Daniela' tomato (Lycopersicon esculentum Mill.), were irrigated with 0 or 50 mm NaCl added to the nutrient solution and grown under natural greenhouse conditions or under applications of fine mist every 8 min during the day. During midday hours, misting reduced greenhouse air vapor pressure deficit 1.0 to 1.5 kPa and reduced greenhouse air temperature 5 to 7-°C. Mist reduced root water uptake from the medium by 40% in nonsalinized plants and by 15% in saline conditions. Foliar concentration of Na was lower in misted-salinized plants than in nonmisted salinized plants. Less negative leaf water potential and higher leaf turgor were recorded with mist at midday, in both salinized and nonsalinized plants. Midday stomatal conductances and net CO2 assimilation rates of salinized-misted plants were 3 and 4 times higher, respectively, than those recorded in salinized-nonmisted plants. Misted plants increased instantaneous water use efficiency 84% to 100%, as estimated from the ratio of net CO2 assimilation to transpiration. Nonsalinized plants grown with mist increased total leaf area by 38%, dry matter by 10%, and yield by 18% over nonmisted plants. Salinized plants grown with mist increased total plant leaf area by 50%, dry matter by 80%, and yield by 100%. Greenhouse misting resulted in a saving of total water input of 31 L/plant under nonsaline conditions and in greater yields and fruit size regardless of salinity. Results suggest that greenhouse misting, during the Mediterranean spring-summer growing season, improves tomato crop productivity both under nonsaline and saline growth conditions.

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B.R. Bondada, R Romero-Aranda, J. Syvertsen and L. Albrigo

Foliar applications of urea-nitrogen are widely used to alleviate N deficiencies in citrus; however, improper applications can cause serious foliar burn and loss of active green leaf area. Light (LM), transmission (TEM), and scanning (SEM) electron microscopy were used to characterize anatomical and ultrastructural details of foliar burn in citrus. LM examination of the burned leaf area showed collapsed adaxial and abaxial epidermal cells and plasmolysis of mesophyll cells that created large intercellular spaces. SEM showed wrinkling of both the adaxial and abaxial epidermal cells. TEM revealed cytoplasmic vacuolation, disruption of cellular membrane, degradation of grana, and appearance of large plastoglobuli, implying loss of physiological activity. In contrast, control leaves had turgid adaxial and abaxial epidermal cells and compact mesophyll cells with few intercellular air spaces.