Arthur Villordon

The sweet potato has a system of roots that allow it not only to obtain soil-based resources like water and nutrients but also store food reserves. This storage capacity is unique to the sweet potato and represents the most economically important biologi­cal activity of the crop. Lateral or branch roots, the main determinants of root architecture, enable plant root systems to perform these functions. The onset of this specialized function is referred to as storage root formation. Roots that fail to undergo storage root formation become lignified, or woody, and do not contrib­ute to productivity. Hence, the knowl­edge of the intrinsic and environmental factors that favor storage root formation is important in developing and testing management practices that contribute to improved agricultural yields.

New research has uncovered a funda­mental association between lateral root branching and the ability of sweet pota­toes to form storage roots. In roots with restricted branching, swelling is delayed or reduced. Thus, understanding the fac­tors that control root branching will lead to developing and testing methods that promote optimum use of soil resourc­es and consistent productivity. However, the study of plant roots has traditional­ly lagged behind studying stems, leaves, flowers or fruits.

Traditional methods for measuring roots grown in soil, such as washing and root tracing, are destructive and time consuming. Alternative approaches such as the use of underground observation windows typically underestimate some root growth factors. Recent advances in imaging technologies have enabled the non-destructive measurement of root development but are currently cost pro­hibitive and generally inaccessible to the broader scientific community. Another method is the use of a technique called aeroponics, where plant roots are fully accessible through the growth cycle and can be fully recovered for harvesting. LSU AgCenter researchers are developing an inexpensive aeroponics growth system as a tool for sweet potato research.

Growth system requirements

The principle of aeroponics is to grow plants with their root systems exposed to a nutrient mist. Plants show optimal growth in aeroponics systems because of an unimpeded oxygen supply to the root system. Unlike roots that are 100 percent immersed in nutrient solutions, roots grown in aeroponics systems typical­ly show optimum lateral root develop­ment, an important consideration for root architecture research.

At minimum, the system is composed of a root chamber in which the mist is intermittently sprayed. The nutrient mist can be produced by mechanical foggers, venturi-type sprayers, ultrasonic fog­gers or pressurized solutions delivered through nozzles. The duration of the spray interval has an important effect on the development of the root system and should be adjusted for each plant species. Infrequent spraying might cause water stress, whereas too frequent or continu­ous misting may lead to leaching essential nutrients from the root system.

The nutrient solution is typically col­lected at the base of the chamber or in an external container and recirculated. Regulating the nutrient solution tempera­ture, composition, concentration and pH can be either automated or performed manually at specified time intervals.

Materials and methods

One of the principal goals of the work was to develop a low-tech system that simulates the timing of storage root for­mation by field-grown plants. Another requirement was simplicity in operation to avoid complex machinery. The use of off-the-shelf components helped ensure flexibility and adaptability in many envi­ronments, including resource-limited locations. Figure 1 shows the basic com­ponents of the growth system: a plas­tic container (A) was used as a misting chamber while an air pump (B) was used in conjunction with a venturi mister (C) to propel the nutrient spray to the root system. The nutrient solution is self-con­tained within the plastic container and refilled when necessary. Sweet potato cuttings were inserted through open­ings on the plastic lids and secured using commercially available foam collars. A commercially available timer was used to determine the spray intervals after initial­ly evaluating different interval settings.

Results and discussion

The multiyear experiments were conducted at the Sweet Potato Research Station. The initial hypothesis was that manipulating nutrient availability over time contributed to storage root forma­tion and enlargement. This was based on measurements from earlier experiments that showed root swelling was accom­panied by the reduction of nitrogen in the growth medium. In addition, studies were conducted in order to synchronize the timing of storage root formation in the aeroponics culture as close as possi­ble with field conditions. Thus, the initial experiments consisted of an initial growth phase (Figure 2) using a nutrient solution with complete macronutrients and micro­nutrients followed by a storage root for­mation phase characterized by removing nitrogen from the nutrient solution.

It was determined that root swell­ing was more consistent when the ini­tial phase was at least 20 days before the imposition of the nitrogen-deprivation treatment. One of the initial responses that were observed following the removal of nitrogen was the pigmentation of roots as early as three to four days. Around this time, the roots also increased in diameter by as much as 25 percent to 50 percent. These observations would not have been possible if the plants were grown on soil or in pots.

Swollen roots were clearly visible after 35 days, a time frame similar to field-grown roots (Figure 3). At the same time, shoots showed nutrient-deficien­cy symptoms, indicating that nutrients were diverted from the leaves to support enlarging the storage roots. On the other hand, plants grown in a continuous com­plete nutrient solution did not manifest root pigmentation, and root swelling was minimal. Long-term evaluation of the system showed that the timing of stor­age root formation also was influenced by variety and non-uniformity of nutri­ent availability in root systems during advanced stages of development. In par­ticular, it was determined that the sweet potato variety Bayou Belle formed storage roots without the nutrient solution having been changed when the developing root system restricted the access of some roots to the nutrient mist.

These findings demonstrate the advantage of the aeroponics system in facilitating non-de­structive, real-time observations of devel­oping root systems compared with roots grown in soil or arti­ficial growth media. In particular, the marked increase in root pigmentation in response to nutrient deprivation would have been easily missed in root sys­tems grown in soil or an artificial growth substrate.

Conclusions and prospects

It has been demonstrated that sweet potato plants can be manipulated to form storage roots in an aeroponics growth system developed from relative­ly inexpensive, off-the-shelf components. Results from ongoing work will increase the understanding of how nutritional cues influence sweet potato root system development and storage root forma­tion. Such knowledge has potential direct agricultural applications through the identification and evaluation of manage­ment practices that optimize fertilizer recommendations.

Other potential research applica­tions include mineral nutrition, disease research, temperature effects, screen­ing for desirable root mutants and root exudates. Root exudates are compounds excreted by root systems in response to stress stimuli in the soil environment and represent an important knowledge gap in sweet potato research. The aero­ponics system is a convenient method to study the effects of nutrient deficiency on storage root formation and associat­ed physiological and molecular process­es. The nutrient delivery system enables the targeted removal of a specific nutrient without interference from biotic and abi­otic variables that are found in agricul­tural soils.

Arthur Villordon is a professor at the Sweet Potato Research Station at Chase.

Aeroponics growth system in greenhouse.

Figure 1. Diagrammatic cutaway view of the aeroponics growth system.

Figure 2. View looking into the aeroponics misting chamber, showing the developing root system of sweet potato plants at five days after planting.

Figure 3. View of swollen storage roots of sweet potato variety Bayou Belle grown in an aeroponics growing system at 57 days after planting.