Texas is currently the third largest citrus producing state behind California and Florida with the Texas fresh commercial fruit market valued at $80.7 million (NASS 2016). Most of the commercial citrus production in Texas is primarily concentrated in the four southernmost counties located in the Lower Rio Grande Valley (LRGV). Texas economists estimate that the total business activity supporting Texas citrus production is almost $200 million annually making citrus production in Texas an extremely profitable business (Brunner et al. 2013).
Citrus root systems basic and most fundamental responsibility is for tree anchorage and support. They also provide a means of collection and transport of water and nutrients essential for tree growth and production. In general, citrus roots flush periodically throughout the months of February to early December with periods of inactivity occurring only during periods of shoot flush (Wells & Eissenstat, 2003). During spring, summer, and fall, alternation of root and shoot growth has been repeatedly observed. The first root flush usually occurs between late February and early April. However, distinct bursts of root growth frequently occur during the periods of May-June and again during August through October (Eissenstat and Duncan, 1992). It should be recognized that during the warm months, active periods of root growth can occur at any time in a series of smaller, reduced flushes or as major peaks of root growth activity (Noling, 2003). Studies of citrus root distribution in Florida have shown the potential for citrus to grow deep into the soil profile due to certain favorable environmental conditions. Much of South Texas citrus production takes place using traditional planting strategies that grow trees on flat lands in soils that are high in clay and pH. The semiarid climate with low annual rainfall leaves this area more vulnerable to drought and subsequent crop loss.
Soil water status is a major component of the root environment, affecting the growth and health of roots (Boman et al., 2002). The general pattern of water absorption by citrus trees suggests that water is depleted first in surface roots located in soils with high water content. As the available water supply decreases in the surface soil there is an increase in absorption from roots at successively greater depths in advance of the drying front (Noling, 2003). The root systems of citrus trees in deep, well-drained soils are 1.2 to 1.5 m, with the main root system reaching only a depth of 0.6 to 0.9 m. Studies have found roots on trees with rough lemon root-stock at a depth of 3.3 m, while roots on sour orange trees have been found at a depth of 2.7 m in well-drained sandy Florida ridge soils. Water depletion studies confirmed that the roots were active at this depth but most of the water extraction came from the top 0.6 m (Morgan et al. 2007).
Texas citrus production is faced with many challenges that can hinder production and yields; some of these include heavy clays, drought, flood irrigation, and high temperatures which increase tree evaporative demand. While most soils in South Texas hold between three and seven inches of water in the upper three feet of depth, waterlogged plants may become an issue for growers if there are high influxes of water throughout the season. One of the requirements for citrus include deep soil that has both good surface and internal drainage. However in South Texas, water penetrates very slowly due to the presence of clay soils which can cause significant problems if not managed properly. Irrigation practices also affect citrus root system development and distribution. Traditional planting in Texas has some disadvantages mainly associated with flood irrigation. The main water source in the LRGV is the Rio Grande River. Flood irrigation is used due to how cost effective it is for the growers and the existing water delivery infrastructure in the LRGV, but it is not the most efficient form of irrigation due to the poor drainage, evaporation, and erosion of soils (Meyer & Barrs 1985). It can also cause crusting of the soil surface following irrigation and can contribute to the degradation of some soil parameters. While most studies have thoroughly explored citrus production and root development in well drained sandy soils, few have been conducted on heavy clays like those found in Texas.
There is an abundance of knowledge on citrus production in different regions around the world and the effects of root growth, however in Texas there is currently a gap of knowledge on how citrus roots are affected by South Texas clay soils and management practices. To examine how different management practices affect root growth and development throughout the year, we periodically took root cores from citrus trees that were grown in flat beds with no groundcover (traditional), flat beds with groundcover, raised beds with no groundcover, and raised beds with groundcover. Roots were separated into depth increments, washed, and scanned for further analysis. Root scans were evaluated for root length, root area, root surface area, width, and diameter.
The goal of this study was to better understand how different management techniques affect citrus root distribution and seasonal growth. To achieve this goal, the objectives were to:
It is believed that citrus is native to the subtropical and tropical areas of Asia, emerging in certain parts of Southeast Asia including China, India, and the Malay Archipelago (Bartholomew and Sinclair 1952; Sinclair 1961; Scora 1975; Ramana et al. 1981; Gmitter and Hu 1990). It is believed that citrus is native to the subtropical and tropical areas of Asia, emerging in certain parts of Southeast Asia including China, India, and the Malay Archipelago (Bartholomew and Sinclair 1952; Sinclair 1961; Scora 1975; Ramana et al. 1981; Gmitter and Hu 1990). They are found in many places throughout the world in more than 140 nations, with most grown in tropical and subtropical areas of the world between 35ºN and 35ºS latitudes. (Ramana et al. 1981; UNCTAD 2004). The most commercially significant citrus varieties are oranges, lemons, limes, grapefruit, and tangerines. Annual global production of citrus fruit has observed solid and fast development over the most recent quite a few years, from roughly 30 million metric tons in the late 1960s (FAO 1967).
U.S. citrus production for the past 3 years was 9.4 million metric tons, 9.1 m.t., and 8.5 m.t for 2014, 2015 and 2016, respectively (NASS USDA 2017). This is the lowest it has been over the past five years due to the substantial loss of production mainly from the impacts of citrus greening disease on the Florida citrus industry. The top citrus producing states in the U.S. are Florida; 4.2 million metric tons, California; 4 million metric tons, Texas; 263,000 metric tons and Arizona; 70,000 metric tons (USDA 2016). California is the main producer for fresh citrus fruit while Florida leads juice production (NASS 2016). Currently Texas is the third largest citrus producer primarily concentrated to Hidalgo, Cameron, and Willacy counties, market is primarily in fresh grapefruit production (Anon, 2014). Texas economists estimate that the total business activity supporting Texas citrus production is almost $200 million annually, making citrus production in Texas an extremely profitable crop (Lopez, 2014). Citrus grown in Texas were first planted by Spaniards who in the eighteenth century planted seven orange trees on the Laguna Seca Ranch north of Edinburg in what is presently Hidalgo County (W. H. Friend, 2017). Oranges and satsumas, numerous from trees imported from Japan, were delivered along the Texas gulf coast in extensive amounts as soon around the 1910. Shortly after two citrus associations were formed which are the Texas Citrus Growers’ Association and the South Texas Citrus Fruit Growers’ Association. The very earliest marketable grove of grapefruit trees was introduced in 1904 (Liu et al. 2012).
Citrus is a perennial evergreen flowering tree belongs to the Rutaceae family (Mabberley, 1997). This family produces numerous edible fruit crops such as oranges, lemons, grapefruit, pomelo and limes. New citrus trees ordinarily start to mature and produce fruits between ages of 2-5 years and continue to produce for over 50 years (Ferguson & Davies 1995). Reproduction in the genus Citrus happens by means of sexual propagation which happens through fertilization, and most commercial cultivars are self-pollinated. Self-fertilization is encouraged by citrus blooms having both male and female parts in the same flower. Cross-fertilization, required just by a few cultivars, happens in tangerines and tangerine mixtures and mandarins. When cross-pollination is necessary, honeybees are naturally the most significant insect pollinators (Sanford 2003). Many citrus species and varieties also do not inherently possess adequate hardiness to cold, resistance to soil-borne diseases, tolerance to salinity or high-water tables, and other desirable qualities that would enable them to endure many conditions in their planted environment as seedlings. Consequently, some are occasionally propagated by some vegetative means such as cuttings, layers, but predominately by grafting or budding onto a rootstock. Specifically of some closely related species to take advantage of the rootstocks adaptability to adverse conditions (Rosa M. Rivero 2003). Citrus shoots grow in cycles, with the numbers of growth flushes during the growing season varying from two to five (Mendel 1968). Citrus fruit is considered a type of berry (Hesperidina) because of its many seeds and its development from a single ovary (Iglesias et al. 2007). Individual fruit growth and size relies heavily on the health of the nearest leaves. The greater the number of leaves, the better the tree’s ability to use water and produce carbohydrates for fruit (Panigrahi et al. 2017). Citrus crops are grown on a variety of soils from sands to clay loams with different water holding capacities, drainage conditions and irrigation needs. Plant aboveground health significantly influences belowground tree health, especially in the formation of roots. Both the below and above ground environments must be taken into account in citrus health due to their interaction and impact on each factor.
Roots can impact the longevity of plants, specifically in perennial plants where food reserves accumulate during the year of vegetative growth. These food reserves allow the root to survive adverse environmental conditions and then produce new vegetative growth in the spring (Wright, 2000). Plant root systems consist of a taproot system, fibrous roots and finer roots having root hairs close to the tip. The plant root system establishes the major part of the plant body, both in terms of function and size. Roots have many functions such as anchoring plants to soil provides stability to above ground parts of the plant. They absorb water and dissolved mineral nutrients from soil and transport them to the stem. Although a plant’s capacity to take up water and nutrients is highly dependent on fine root architecture (Dali Guo, 2006) changes in management strategies are poorly documented on its impact on root distribution and turnover. The development of deep roots provides crucial functions for individual plants such as nutrient and water uptake and the storage of food in root crops (Kell, 2011). The distribution and activity of roots of fruit trees is an important factor for tackling the problems associated with fertilization, tillage, and irrigation.
The development of deep and extended root systems is very important for tree health and production in times of drought. It has been found that drought can be one of the main causes of root death (Deans, 1979; Persson, 1979; Ferrier & Alexander, 1991; Huang & Nobel, 1992). It is during long drought periods that surface roots may be destroyed. Eissenstat & Yanai (1997) showed that root lifespan has great variability from days to years, depending on the plant species. Roots play a vital role of connecting the plant and soil and the soil to the atmosphere. The growth and development of above ground plant parts depends on the acquisition of soil nutrients and water. These directly are associated with root physiology and turnover. Providing a good environment for root growth will in turn provide a better yield and improved tree health. Conversely, a poor soil environment caused by poor drainage or inadequate soil moisture can reduce yields and tree health significantly (Shaxson, 2003). Understanding the interactions with these environmental stresses can provide information on how cultural practices affect tree health or the creation of new sustainable agricultural practices. Research to improve understanding of citrus root distribution and root turnover will be very important for future developments of environmentally beneficial management practices in citrus.
Among Citrus and close relatives there is wide variety in normal width and particular root length of the fine root framework (Pierret et al., 2005). Secondary development ordinarily does not happen in the fibrous root framework, although, fibrous roots can be reduced in diameter on account of shrinking of cortical cells and loss of the epidermis (Schwartz 1994). It has been found in previous studies that trees with identical scions, root diameter and root length show different rates of proliferation (Eissenstat, 1991). In citrus rootstocks, median lifespan of the fibrous roots can exceed 100 days (Eissenstat & Yanai 1997). Throughout this time, fibrous roots change significantly, both because of normal development and to natural conditions. These changes can be biochemical or structural.
The soil environment that is found in South Texas citrus is very unique with heavy, poorly aerated clay soils which have found to be restrictive for root growth. Citrus produced in South Texas have a shallow root system, which concentrates under the tree canopy, at shallow depths with most roots in the 0-15 cm. depth (Castle, 1978) . At these shallow depths tree roots are easily destroyed by tillage or poor water management. Root growth occurs when soil temperatures range from 53.6 to 95°F with most active root growth occurring when the soil temperatures are between 77 to 86°F (E. G. Johnson, 2013). Soils with low organic matter contents are often low in nutrients especially nitrogen and sulfur. A low organic matter content can also impact soil structure by increasing compaction and decreasing porosity (Pagliai et al. 2004). In compacted soils, aeration can be restricted, affecting root growth, nutrient uptake and overall crop development (Unger & Kaspar, 1994). It is well known that trees and other plants growing in poorly drained soil often develop shallow root systems and are concentrated in the surface of the soil (Maeght et al., 2013).
Understanding of the association between root and shoot growth are of great concern especially dealing with root turnover. Fine roots or feeder roots are mainly responsible for water and nutrient uptake by trees(Asaye, 2013). Even though fine roots may comprise less than one percent of the total biomass of a mature tree, it may represent as much as two-thirds of the annual biomass production (Contador et al., 2015). The lifespans of these roots can be turned over two or three times each year (Brunner et al. 2013). This is very important due to the production below ground biomass can direct affect above ground biomass and influence yield in addition to production.
Citrus plants are evergreen and therefore transpire throughout the entire year, and water requirements range from 900 to 1200 mm per year (C. Brouwer, 1986). Roots play a major role in tree health especially in harsher environments where trees are prone to drought (Backlund et al., 2008). The majority of Citrus orchards in the Lower Rio Grande Valley are irrigated via flood irrigation which has been shown to greatly impact roots and overall plant health (Martínez-Cuenca et al., 2015). In regions where the annual rainfall is relatively lower than average during the year and uneven distributions of rainfall, irrigation plays a significant role in root health (Tracy et al., 2011). Climate change could exacerbate drought by increasing the length of dry periods in many regions, which could increase the demand for irrigation, especially in citrus production.
Extensive research over the recent decades has resulted in significant scientific evidence that global climate change threatens the stability of natural ecosystems (Williams et al., 2008). Research has demonstrated that increments in ambient temperatures and changes in related progressions are specifically connected to rising greenhouse gases (GHG) concentrations in the atmosphere (Jonathan A. Patz, Diarmid Campbell-Lendrum 2005). Global climate change is a crucial environmental issue that has presented very difficult challenges to overcome for the start of the 21st century. The agricultural sector is facing significant challenges such as increasing global food production for the purpose of providing security for 9 billion people while also protecting the environment and enhancing function of global ecosystems (Backlund et al., 2008). Temperature, precipitation, atmospheric carbon, incidences of extreme events and sea level rise are the main climate change effects that impact agricultural production.
Higher temperatures affect the growth and development of crops, influencing potential yields (Hatfield & Prueger, 2015). The numbers of days a crop is exposed to temperatures exceeding specific thresholds during critical growth stages of flowering, pollination, or fruiting can significantly reduce the quantity and quality of crop yield (Parthasaranthi et al., 2013). Climate change is expected to disrupt pre-existing rainfall regimes in many regions, resulting in changes in duration and intensity of flooding and episodes of drought (Rosenzweig et al,. 2002). Higher soil temperatures can also alter nutrient and carbon cycling by modifying the habitat of soil, which then affects the diversity and structure of plants and their ability to produce important root proliferation (Pregitzer et al., 2000). Soil water retention capacity can be affected by rising temperatures and a decline in soil organic matter due to both climate change and land-management changes (Reichert et al., 2009). Keeping up water retention capacity is imperative to lessening the effects of exceptional precipitation and dry spells, which are projected to become more frequent and severe. Increased temperature and decreased moisture tend to accelerate the decomposition of organic material in soils, causing a decline in soil organic carbon stocks and an increase in CO2 into the atmosphere (Niles, 2008).
Demand for water has increased due to population increases, industry and other municipal needs. Due to brackish groundwater in the LRGV, surface water from the Rio Grande is the primary source of water for the region (Meyer et al., 2014). Conservation of water in the agricultural sector is essential since water is necessary for the growth of plants and crops. In areas where rainfall is low and water is scarce such as the LRGV, growers are aware of water conservation techniques but have not implemented them due to infrastructure demands and cost. Severe droughts can cost growers and farmers an extensive amount of money a year, more than damages caused by floods or hurricanes (Mabberley, 2007). Therefore, by improving the efficiency of water use, and by reducing water loss due to evaporation, we can reduce water usage and demand. The stress of potential droughts, will continue to make efficient water management practices a necessary tool for growers who wish to remain competitive in today’s market. Efficient agricultural water conservation practices are essential to ensure the viability of Texas’ agricultural industry.
Agriculture consumes more than 85 percent of the surface water and groundwater withdrawals in the Rio Grande Basin (TWRI 2012). Droughts are already a serious threat to agriculture in the region, and climate change combined with population growth will stress irrigation supply even further. Higher temperatures due to climate change can seriously affect the agriculture industry and eventually the production of crops in Texas. Use of new sustainable management techniques and drip irrigation can help enormously to manage limited water supplies and become more stable during future periods of drought if they become more severe. Soil management techniques such as reduced tillage and management can be used to conserve water, reduce erosion, and increase soil productivity (Bailey et al., 1914). With extreme heat and greater need for water on the part of farmers possibility of future water shortages could become an often occurrence.
As climate change starts to influence drought, temperatures, weather patterns and stress on water resources will be seen throughout the environment, requiring more water conservation friendly irrigation practices.
South Texas farmers depend greatly on water from the Rio Grande and other tributaries but they are sometimes subject to restrictions and low flow during droughts. Growers depend greatly on surface water due to our brackish groundwater and the Rio Grande is the main source of water through canal systems. Rainfall alone is often insufficient and supplemental irrigation is required for tree growth and production. The most common irrigation practice in the LRGV is flood irrigation, a method in which water travels through the eventually filling the field with a certain amount of volume of water. Flood irrigation generally is considered less efficient than other methods due to runoff, deep percolation, and evaporative losses. Most orchards and crops in South Texas that are currently in production employ this standard irrigation technique (Nelson, 2013). It is mainly used due to its cost effectiveness and usually drip and microjet requires the construction of a holding pond. This system requires a large amount of water because it is watering more than the target area which is the effective root zone (Zandalinas et al., 2016). Water requirements of citrus estimated from evapotranspiration (ET) may range from 76 to 124 cm (30 to 49 inches) annually (Munns, 2002).
Flood irrigation in citrus is mainly used in South Texas and can cause reduced crop yield, growth and root death due to factors that affect the root and soil environment (Deans 1979; Persson 1979; Ferrier & Alexander 1991; Huang & Nobel 1992). Flood conditions can lead to a lack of oxygen in the root zone and excessive water loss through evaporation (Uckoo, 2005). This may cause a poor soil environment caused by poor drainage or inadequate soil moisture and can reduce yields and tree health significantly (Unger & Kaspar, 1994). Improvements in irrigation techniques are likely to improve the profitability of orchards as a result of better plant growth and fruit quality (Pires et al. 2005; Bremer Neto et al. 2013). Flood irrigation methods have low water use efficiency and can negatively affect production. Irrigation conservation and efficient use of available water supplies will likely be critical in the future due to increased water demand from cities and industries and other municipal users.
Water scarcity in Texas is also another major constraint that influences management strategies every year for not just citrus production but many other crops as well. In addition to wasting water, flood irrigation can possibly result in the deterioration of root environments, decrease water holding capacity and increase soil compaction (Fahong et al., 2004). Effective water management strategies could also provide more benefits than older practices such as sustained moisture availability for crops to prevent drought stress. Alleviating these issues for growers is important in order to sustain the supply and demand of crops. Attempts to modify the environment in which crops are grown was first made with the development of a wide variety of techniques. The implementation of raised bed farming with the addition of plastic cover has become a globally applied agricultural practice for its economic benefits such as higher yields, earlier harvests, improved crop quality and increased water-use efficiency (Franquera & Mabesa 2016). Wittwer et al. (1993) stated that the driving forces for the use of plastics in agriculture was to increase total crop productivity and enable producers attain greater returns. These practices may offer many benefits, for example, the ability to protect seedlings and shoots through insulation and evaporation prevention, which maintains soil temperature and moisture (Jiang et al., 2014). The effective root zone is impacted greatly by distributing water to the concentrated areas of the root zone. By keeping a constant amount of water in the root zone, this system is able to maintain water near fibrous roots which helps maintain growth and yield. Black plastic covering has been used in crop production for its ability to absorb the most of ultraviolet, visible and infrared wavelengths of incoming solar radiation, effectively solarizing soil, increasing temperatures and reducing water usage (Alexandrov & Hoogenboom, 2000). Plastic mulches and groundcovers are also effective in controlling weeds which are effective competitors for nutrients and water.
Studies conducted by southern California growers have reported that plastic covered strawberry beds have less fruit rot, cleaner fruit, and in many instances earlier production when compared with uncovered beds. The positive effects of raised beds cropping systems on crop performance, yield, and water use have been demonstrated globally. For example McHugh et al. (2009) in Australia and Verhulst et al. (2011) in Mexico indicated that planting on permanent beds increased soil available water capacity, improved water infiltration, and aggregate stability, when compared with conventional tilled crops without beds. Kukal et al. (2010) also determined that raised beds were effective in increasing soil water content, reducing irrigation water requirement, and improving water use efficiency in rice.
Due to the vast majority of literature currently published, using raised beds can increase soil drainage and prevent evaporation. This study evaluated the impact of raised beds and plastic mesh groundcovers on root growth and other dynamics throughout the growing season.
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