Contributed
Environmental scientist Mark Harris inspects growth of sweet potato cuttings in humidifier after three days.
A study at Northern Caribbean University (NCU) may help to increase yields of sweet potato in some drought-prone locations in western Jamaica. Research led by environmental scientist Dr. Mark Harris at the Mandeville-based Northern Caribbean University, has culminated in a portable field humidifier for treating stem cuttings, resulting in more vigorous growth in dry soils.
In addition to portability, Dr. Harris identifies the main advantages of the humidifier as "self-sustaining, requiring no human intervention after the commencement of the treatment, and self-powered by natural forces."
The sweet potato has long been an important staple crop in Jamaica, for national and international markets. It is high in vitamins A, C and D; the yellow-fleshed variety, in particular, contains significant levels of beta carotene. In some Asian countries, the steamed leaves are a source of dietary protein.
Though a drought-resisting crop, freshly planted cuttings, having not developed enough, often fail to survive in the first few days. In dry conditions, a good start for the crops and higher yields require a potentially well-developed root system at the transplanting stage. The main methods of propagation of sweet potato are shoots developed from the root as well as stem cuttings. The latter are advantageous in that they do not transfer soil-borne diseases and nematodes as do sprouts with roots.
A strong, early rooting system with its wide, deep network speeds up plant development by greatly increasing the absorptive surface area in the soil, thereby enhancing the capability to withstand prolonged periods without water. Rapid root extension after planting is therefore important in drought-prone
locations.
As hot, low-humidity environments are associated with premature drying of stored cuttings, strategies to forestall drying, and, in the process, to stimulate root growth have been practised in several tropical locations. Such practices insulate the cuttings and raise the relative humidity of the pile.
For example, in Jamaica, cuttings are often stored under grass clippings in the field. However, this strategy is not always feasible, or totally successful, due, often to
A lack of adequate amounts of grass for cover in the dry season
Rotting through decreased oxygen at ground level, which can cause loss of cuttings through rotting
Soil-borne diseases, which sometimes infect cuttings lying on the ground
Hand watering, which is time-consuming and thus not practical to keep pace with the transpiration requirements of the cuttings.
Based on the above observations, it was hypothesised that, compared with traditional methods, an affordable fog system for rural farmers, which artificially maintained a high relative humidity in the air would produce a higher proportion of viable cuttings and improved root initiation.
It is well known that plant cuttings initiate roots more readily under highly-humid conditions, but, according to Dr. Harris, "the challenge in this case lay in packaging that humidity (at low cost) such that farmers in remote farm locations could carry it around, without the need for electrical power."
Humidity chamber
Heavy cotton cloth or denim was snugly draped over the mesh frame —a cylinder of 1 cm diameter square aperture galvanised iron wire. With the bottom of the whole assembly placed upright in a container of water (about four litres, recyclable), a polythene shield around the outside served both as a temperature and evaporative loss insulator. Cuttings were suspended on wire shelves in the cage which facilitated a constantly high humidity, at 100 per cent, by capillary action or rise in the water surface.
"The extra push upwards was supplied by the tendency of water particles to stick together by inter-particle electrical binding forces," Harris explains. By first soaking the whole cloth, water particles were then attracted upwards by not one, but two forces - adhesion to the cloth, and cohesion with particles from above - effectively jump-starting the system and maintaining upward capillary movement indefinitely. This process produced "a fog-like 100 per cent humidity inside the chamber continuously", the researcher added.
Results
After just three days in the chamber, all 50 of the forced-humidified cuttings had initiated roots and visible scar tissue, which induces rooting. In contrast, in under grass storage, as practised currently on farms, only two cuttings had any visible roots and there was no visible scar tissue after three days. Cuttings left out on the ground without cover were all withered dry and without roots.
Furthermore, for the grass covered treatment, rooting occurred only on a portion of those cuttings actually touching the ground.
Compared to cuttings of other treatments, and also fresh cuttings, forced-humidified growth after planting was more vigorous, and leaves and stems were more turgid and open at day three after planting. This increased vigour of the forced humidified cuttings was attributed to a greater ability to collect moisture from dry soil.
Forced-humidified cuttings were still the greenest in colour after six days, and with the highest levels of turgor pressure (firmness) evident by eye and touch, without having actively photosynthesised, being in the closed chamber for several days. All others were limp and shrivelled, despite having had significantly more light. Compensation for decreased light was attributed to increased carbon dioxide and water vapour pressure in the chamber.
"The chief factor is, therefore, water vapour. In this study, shade per se did not trigger root initiation as popularly assumed," reports Harris. "The results of this study show that ambient water vapour, and not shade, is the single most critical factor for root initiation on sweet potato cuttings."
Application of forced humidification
One hundred per cent efficiency with no losses of vine cuttings.
Avoids the wasteful practice of spraying large amounts of liquid water on hot, dry days.
All cuttings are suspended in the air high above ground level in protected chambers, thus preventing infection by soil-borne pathogens and removal by wind or flooding.
Shorter cuttings can be used after forced humidification, hence, the "transplant stock" is accordingly increased. More robust transplants are produced more quickly than from traditional curing.
