Local nitrogen variability alters root architecture and influences storage root formation in the Beauregard sweet potato

Linda Benedict, Smith, Tara, Villordon, Arthur O., Labonte, Don R.  |  9/12/2013 8:04:02 PM

Figure 1. Schematic of key features of sweet potato adventitious root architecture associated with nutrient and water uptake and storage root formation. (a) Adventitious roots develop on nodes and cut ends of cuttings as early as three to five days after planting. Lateral root primordia develop from deep within the main adventitious root axis. (b) As the mainaxis of adventitious roots grows, lateral root primordia emerge through the surface of the main root axis. (c) Emerged lateral roots continue to develop and can produce secondary branches. Storage root formation occurs in adventitious roots after lateral root primordia emerge and develop.

Figure 2. Schematic of the split-root system used for simulating local nitrogen variability in the vertical plane.

Arthur Villordon, right, professor at the Sweet Potato Research Station, explains his research during a field day at the station. (Photo by Linda Foster Benedict.)

Arthur Villordon, Don LaBonte and Tara P. Smith

In most agricultural soils, the distribution of water and plant nutrients is not homogeneous across space and time. Most plants respond to the nonuniformity of these soil-based resources by modifying the root system architecture. Root system architecture refers to lateral root initiation and development. Lateral, or branch, roots are mainly responsible for determining water and nutrient uptake efficiency.

In sweet potatoes, the main root axes of adventitious roots have five to six rows of lateral roots, corresponding to the number of protoxylem poles (Figure 1a), which range from five to six in the Beauregard variety. Lateral roots are derived from a layer of founder cells that surround the stele, or core tissue, deep within the main axis of the adventitious root (Figure 1a). These founder cells produce the earliest stage of lateral root development, called primordia, that progressively emerge as the main axis of the adventitious root grows (Figures 1a, b).

Recent evidence has shown that lateral root emergence and development are fundamentally associated with the capacity of a sweet potato adventitious root to become a storage root. If the growing environment is favorable, lateral roots progressively and repeatedly emerge as the main root axis grows through the soil, resulting in 10 to 20 lateral roots per inch. Adventitious roots that meet these lateral root development growth parameters are able to become storage roots, if the growing conditions are favorable (Figure 1c).

If the soil environment is unfavorable, some lateral roots fail to emerge. This failure has been associated with the development of woody tissue in the adjacent stele tissue, causing this section to quit growing. Secondary growth is a necessary step in storage root formation and involves the formation of a cambium ring within the stele tissue (Figure 1b). The cambium ring is a zone of active cell growth responsible for storage root development. If the lateral root primordia fail to emerge from the main root axis, the adventitious root will fail to become a storage root (Figure 1c). Thus, understanding the internal and external cues that determine root architecture will lead to development of management practices that optimize root development and increase production efficiency.

This research project investigated the influence of local nitrogen availability on root architecture development and its influence on storage root formation in the Beauregard sweet potato. Among plant nutrients, nitrogen is considered the most critical for growth because of its low availability in the soil and because nitrogen fertilizers can easily be lost by volatilization, leaching or denitrification. Knowledge of how fertilizer nitrogen influences sweet potato root architecture development during the critical storage root formation period can be used to improve management practices to increase nutrient use efficiency and promote consistent yields.

Recent developments in scanning and digital image analysis have increased the accuracy of the measurement of lateral root development attributes in response to experimental treatments. These attributes include lateral root length, number, diameter, volume and surface area.

Specialized scanning equipment was used to measure advanced root architecture. The scanned images are analyzed by specialized software that differentiates root types based on thickness. This method was used in greenhouse experiments to describe how varying the availability of nitrogen influenced sweet potato root architecture and the nutrient content of leaf tissue. The experiments were timed to coincide with the onset of storage root formation.

In the Beauregard variety, this critical yield-determining physiological process can occur as early as 13 days in the field and 20 days in the greenhouse. Two greenhouse studies were carried out to simulate variation in local nitrogen availability in fields.

First, a split-root culture system was developed to replicate variation of nitrogen distribution in the vertical plane (Figure 2). This simulates the localized presence of fertilizer nitrogen on one side of a ridge in agricultural fields, especially with fertilizers applied with an injector. Experimental controls included split pots with or without nitrogen fertilizer in both partitions.

Second, fertilizer placement experiments were conducted to simulate variation of nitrogen in the horizontal plane. This simulates the presence of nitrogen at a specific depth in the plow layer in agricultural fields. The experimental treatments include premixing nitrogen fertilizer in the growth substrate prior to filling the pot – top placement about 2 inches from the surface of the substrate, bottom placement about 2 inches from the bottom of the pot and an unfertilized control. In all experiments, nitrogen was added as urea at the rate of 45 pounds per acre.

In split-root experiments, the lateral root volume increased by 161 percent and surface area increased by 172 percent among root samples grown in the fertilized compartment compared with the untreated compartment. There were no differences in root architecture attributes between compartments with similar experimental treatments.

In the nitrogen-placement experiments with root samples of plants grown in the substrate with premixed nitrogen, lateral root volume increased by 123 percent and surface area increased by 400 percent when compared with plants grown in the substrate with the fertilizer nitrogen placed at the bottom. The differences between the unfertilized controls and bottom placement were significantly less. A similar result was observed between premixed and top placement of nitrogen fertilizer. This evidence is consistent with data from model systems that indicate localized nitrogen presence is necessary for lateral root initiation and development.

These results suggest the broadcast application and incorporation of nitrogen prior to forming ridges is the application method that helps ensure optimum sweet potato root architecture development during the critical period of storage root growth.

In the nitrogen-placement experiments, leaf nutrient analysis data from the premixed and bottom fertilizer placement treatments showed that the former treatment had 75 percent greater nitrogen, 86 percent greater phosphorus, 60 percent greater potassium and 70 percent greater calcium content. The evidence suggests that nitrogen uptake can be increased by as much as 75 percent by simply altering the preplant placement of fertilizer nitrogen. Phosphorus, potassium and calcium are relatively immobile in the soil, and optimum root architecture development is necessary for the efficient use of these and other nonmobile nutrients.

Results indicate that localized nitrogen deprivation can directly alter root architecture development, which in turn diminishes the plant’s ability to acquire relatively immobile soil nutrients. These results underscore the importance of gaining an understanding of nutrient effects on root architecture, especially in a crop where lateral development is associated with yield. These findings are being used to develop and evaluate management practices that seek to improve nitrogen use efficiency and optimize adventitious root architecture development during the critical storage root formation period in commercially grown sweet potatoes in Louisiana.

Arthur Villordon is a professor at the Sweet Potato Research Station in Chase; Don LaBonte is a professor in the School of Plant, Environmental & Soil Sciences; Tara Smith is director of the LSU AgCenter Northeast Region.

Acknowledgements
Part of the funding for this research was provided by the Louisiana Sweet Potato Commission and the U.S. Department of Agriculture.

(This article was published in the summer 2013 issue of Louisiana Agriculture magazine.)

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