CHALLENGES IN TEACHING PLANT SCIENCES

by Phyllis L. Coates-Beckford & Annmarie Hosang,
Department of Life Sciences

Those of us who are responsible for the delivery of courses in Plant Sciences in the Department of Life Sciences are quite familiar with the usual ‘human-driven’ challenges of power outages during laboratory periods, malfunctioning of equipment at critical times, loss of field data due to praedial larceny, etc.

These we can try to overcome. But what if nature itself throws us a curve ball at the most unexpected of times, sending our best-laid plans in the most undesirable of directions?

This misfortune occurred during the 2002/2003 academic year to the field and greenhouse experiments set up to demonstrate factors affecting plant health during a six-week-long course. The following example is that of the field plot experiment set up almost six weeks before the beginning of the course to demonstrate the beneficial effects of soil solarization on plant health.

Soil solarization is a hydrothermal process whereby wet soil is incubated under a plastic film exposed to the sun’s rays. This process is conducted only during the hot months in temperate zones, but may be carried out throughout the year in the tropics.

The process is a simple one, but the mode of action is quite complex, and many of the resulting activities in the soil are yet to be explained.

One obvious effect is a physical one – i.e., the raising of soil temperatures to levels lethal to pathogenic, soil-inhabiting microorganisms.

A second effect is a biological one. Not only are the populations of microbial pathogens (mainly mesophiles) killed, but the populations of beneficial thermophilic microorganisms increase, resulting in an increased rate of attack of the pathogens, an increased rate of recycling of minerals from organic matter for plant growth, and the release of various metabolites in the soil which may kill pathogens. The latter effect is essentially a chemical one.

Therefore, a single experiment that evaluates the effects of soil solarization on plant vigour will expose students to a variety of factors that influence plant health.

From the 1999/2000 academic year to the present one, six soil solarization experiments have been set up for students taking courses dealing with soil and plant health.

For each experiment, plants have been grown in soil that has been (a) untreated, (b) solarized, (c) solarized and fertilized, and (d) fertilized. Each treatment has been replicated five times. The same planting locations, batch of cucumber seeds, and fertilizer rates have been used on each occasion (see Table 1). Planting has taken place immediately after the plastic films have been removed from the plots.

Differences in the experiments have been the time of year that solarization has been conducted, and the duration of solarization, the shortest being 5.5 weeks for Crop 4 (1.5 week longer than the generally accepted practical length of time) and the longest being about 8 weeks for Crops 1, 5 and 6. For Crops 1 and 5, soil was solarized during August to October, and for Crops 4 and 6 this was done during December to February. Crops 2 and 3 were planted immediately after removing Crops 1 and 2, respectively; therefore, there was no pre-plant period of solarization for these two crops.

Each plot was sown with many seeds and later thinned to leave five or six seedlings in each. The shoot lengths were measured at intervals, and the fresh weights were taken after cutting shoots to soil level. Length and weight were previously shown to be reliable indicators of plant vigour and fruit production. However, in all six experiments, plants were harvested before the fruits had matured as the length of each course did not permit enough time for maturation to occur.

Figures 1 to 4 show that for Crops 1, 2, 5 and 6 the values of shoot lengths and weights for plants from soil that was solarized or solarized and fertilized were greater than those from plants that were grown in untreated soil, and were also greater than or similar to values for plants from soil that was fertilized but not solarized.

It was exciting to note that similar results were obtained for Crop 2, despite this crop being grown in soil from which a crop had just been removed. This suggested that the beneficial effects of solarization were relatively long-lived. However, the benefits apparently did not extend to a third crop, as shown by the results for Crop 3, where all treatments yielded similar shoot lengths and weights.

So what about Crop 4? It was an unseasonably cold and wet period when soil solarization was conducted in December 2002 to January 2003, in time for the launch of the Plant Health course and the planting of Crop 4. Seeds were duly planted, and students, technical assistants and lecturers waited with bated breath for the plants to ‘do their thing,’ as expected. Then nature threw the curve ball. Few of the seeds germinated. The seedlings that did emerge were reluctant to extend themselves and embrace life. They all remained extremely ‘wingie,’ (to use the Jamaican parlance) regardless of soil treatment.

Needless to say, the students were unimpressed. They remained unimpressed when another unrelated experiment failed to develop a foliar disease naturally, as is typical for that usually dry season of the year (the weather that year was unseasonably wet), and a four-week-old greenhouse experiment failed to demonstrate differences in root growth after infection by microorganisms (the temperatures were unseasonably low).

And what is the consequence, after weeks of careful preparation, to deliver exciting practical classes to such eager students? An average rating of 5 when the course is assessed? We will leave you to ponder on the life of instructors of the plant sciences.