Larry R. Parsons

Salinity tolerance of citrus rootstocks: Effects of salt on root and leaf mineral concentrations

The effects of three concentrations of sodium chloride (NaCl) on seven citrus rootstocks were studied under greenhouse conditions. Leaf and root mineral concentrations and seedling growth were measured. Sodium chloride was added to the nutrient solution to achieve final osmotic potentials of −0.10, −0.20, and −0.35 MPa. Increasing the concentration of NaCl in the nutrition solution reduced growth proportionally and altered leaf and root mineral concentrations of all rootstocks. Significant differences in leaf and root mineral concentration among rootstocks were also found under stressed and non-stressed conditions. Salinity caused the greatest growth reduction in Milam lemon and trifoliate orange and the least reduction in sour orange and Cleopatra mandarin. No specific nutrient deficiency was the sole factor reducing growth and causing injury to citrus rootstocks. Sodium chloride sensitivity of citrus rootstocks in terms of leaf burn symptoms and growth reduction could be attributed more to Cl than to Na. Sodium and Cl concentrations were greater in the leaves than in the roots, particularly at the medium and high salinity levels. Root Cl was not useful for assessing injury because no differences were found in root Cl concentrations among rootstocks. Increasing salinity level did not affect the level of N and Ca in the roots but did reduce N and Ca levels in the leaves. No relationship in mineral concentration or accumulation seemed to exist between citrus leaves and roots. At the −0.10 MPa salinity level, sour orange, rough lemon, and Milam were not able to exclude either Na or Cl from their leaves. Trifoliate orange and its two hybrids (Swingle citrumelo and Carrizo citrange) excluded Na at the lowest salt level used, but were unable to exclude Na at the higher salinity levels. Similarly, Cleopatra mandarin excluded Cl at the lowest salt level, but was not able to exclude Cl at higher salt concentrations. Hence, the ability of citrus rootstocks to exclude Na or Cl breaks down at higher salt concentrations.

Comparison of laboratory- and field-derived soil water retention curves for a fine sand soil using tensiometric, resistance and capacitance methods

The approximate range from 100 to 50% of plant-available water in Apopka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult) is 0.08–0.04 cm3 cm−3 soil water content (θ) or −5 to −15 kPa of soil water matric potential (φ). This narrow range of plant-available soil water is extremely dry for most soil water sensors. Knowledge of the soil water retention curves for these soils is important for effective irrigation of crops in fine sand soils of subtropical and tropical regions of the world. The primary objective of this study was to compare sandy soil water retention curves in the field as measured by tensiometer and resistance block φ values and capacitance sensor θ. The second objective was to compare these curves to one developed on a Florida fine sand soil using a pressure plate apparatus. Tensiometer and resistance block φ values were compared to θ values from capacitance sensors calibrated gravimetrically. The effective range of both tensiometers and resistance sensors in fine sand soils is between −5 and −20 kPa φ. Soil water potential values for both sensors were within 2 kPa of the mean for each sensor. Change in φ was similar over the range of 0.04–0.08 cm3 cm−3 θ. Curves for the two sensors were different by 4 kPa at 0.04 cm3 cm−3. The relationship between φ and θ were similar at 10–20, 20–30 and 40–50 cm depths. This was not true for a laboratory determined soil water retention curve for the same soil type. These differences are significant in soils with very low water holding capacities. Differences between laboratory- and field-determined retention curves could be due to a combination of entrapped air in the field soil and/or alteration in bulk density in the laboratory samples.

Comparative effects of NaCl and polyethylene glycol on root distribution, growth, and stomatal conductance of sour orange seedlings

An experiment was conducted to study sour orange (Citrus aurantium L.) seedling root density, distribution, and morphological development under NaCl and polyethylene glycol (PEG) stresses in relation to shoot growth and stomatal conductance. Plants were treated with 2 stress levels (− 0.12 and − 0.24 MPa) of NaCl and PEG 4000 for 7 months. Root observation chambers were used to monitor root growth and distribution under stressed and non-stressed conditions. Seedlings receiving NaCl or PEG treatments produced fewer roots and shallower root systems with 46 to 65% of the roots occurring in the top portion of the soil. Fibrous root weight per unit length was increased by 24 to 30% by PEG but was not significantly increased by NaCl.Root growth rate usually alternated with shoot growth in a 2-month cycle. This alternating pattern was not shifted by NaCl and PEG stresses. In all NaCl and PEG treatments, growth was depressed and stomatal conductance was reduced. Compared to controls, plants that received NaCl or PEG had smaller shoot and root dry weights, fewer leaves, shorter height, and fewer roots. Sodium chloride usually caused less damage than PEG to sour orange seedlings suggesting that NaCl and PEG acted through different mechanisms.

Remotely-Piloted Helicopter Citrus Yield Map Estimation

A remotely-piloted Gas Xcell mini-helicopter with GPS, wireless Ethernet, and a machinevision system was flown over orange trees to take overhead images. The individual tree images were processed to estimate the number of orange pixels in the images. The fruit yield of individualtrees at harvest a month later had an r-squared value of 0.373 with the number of orange pixels onthe irrigated tree images. While it was demonstrated that a remotely-piloted mini-helicopter could beused to acquire useful overhead images of citrus trees, substantial further research is needed toimprove the accuracy of estimates of individual tree yields or health.

Citrus root distribution under water stress grown in sandy soil of central Florida

O experimento foi conduzido na Universidade da Florida (Centro de Ensino e Pesquisa em Citrus), em Lake Alfred, Polk County, Florida-EUA. O objetivo deste estudo foi avaliar os efeitos do estresse hidrico na distribuicao de raizes de laranjeira Valencia enxertada em citrumelo Swingle, em um pomar de 11 anos de implantacao. Tres tratamentos foram aplicados: 1) plantas irrigadas por microaspersao no periodo seco (Nov-Mar); 2) plantas nao irrigadas no periodo seco, e 3) plantas nao irrigadas no periodo seco e tambem sem chuva no periodo umido. As plantas do tratamento 3 nao receberam chuva durante o periodo umido, e os tratamentos 1 e 2 receberam chuva normalmente durantes 3 anos consecutivos (2003-2006). A irrigacao normal foi recomecada em todos os tratamentos, em meados de marco. Amostras de raizes foram coletadas utilizando um trado de boca (0,09 m de diâmetro e 0,25 m de altura) nos dois lados da linha de plantio, em 3 distâncias horizontais do tronco da planta (1, 2 e 3 m) e 4 profundidades (0,0-0,15; 0,15-0,30; 0,3-0,60 e 0,6-0,9 m). Os resultados mostraram que uma significante diferenca na distribuicao de raizes entre as plantas irrigadas e nao irrigadas.

Status of Microsprinkler System Design, Operation, and Maintenance in 2010

Microsprinkler irrigation is often the preferred method of irrigation for tree and vine crops since it provides a greater degree of freeze protection than drip irrigation and provides water and energy savings over sprinkler and flood irrigation methods. With chemigation, microirrigation also provides an economical method of applying fertilizer and other agricultural chemicals on a timely basis. However, microsprinkler systems generally require more maintenance than drip or overhead systems, and they require a higher level of management expertise than sprinkler or flood irrigation methods. This paper discusses the current status of design, operation, and maintenance of microsprinkler systems and the benefits and limitations of these systems for several agricultural crops. Types and characteristics of available microsprinklers are discussed in relation to appropriate application considering crops, soils, and management philosophy. Topics include uniformity, clogging, insect problems, wetting patterns, emitter maintenance, chemigation, system evaluation, management for both young and mature trees, crop response, and freeze protection.