Challenges faced in vegetable production due to high temperatures

Climate models from the Intergovernmental Panel on Climate Change (IPCC) suggest that by the end of this century, regardless of future carbon dioxide emissions, temperatures will reach their highest point since the end of the most recent ice age. It is observed that the temperature has increased in recent years and it is not only humans who suffer from excessive heat caused by these climate changes. In plants, high temperatures can drastically affect development at different stages of crops and especially during germination, causing great losses to farmers. High temperatures, during the sowing of various vegetable species, can reduce or even inhibit seed germination.

The success of vegetable production will depend, among other aspects, on an acceptable establishment of seedlings in the field, a factor directly related to seed germination. The period between sowing and seedling establishment is a crucial phase of vegetable production. Thus, high quality seeds and conditions that allow maximum germination in the shortest possible time, with maximum uniformity of seedlings, is, without a doubt, a constant search for those involved in the vegetable production chain. Using seeds with high germination and vigor, producers will have a greater chance of success in forming the crop; The use of seeds of high physiological quality (germination and vigor) will minimize the risk of losses during seedling establishment, whether in the greenhouse (transplant) or in the field (direct sowing). This implies great importance in the segment, since for a large number of vegetable species, each seed will produce a single commercial product (i.e. lettuce or cabbage "head", carrot root, onion bulb, etc.). Added to this is the high cost of seeds for new vegetable cultivars or hybrids.

Among the factors that affect seedling establishment, temperature may be the most important, since the producer does not always have full control over this factor. Each species has a minimum, maximum and optimum temperature for germination, and within each species, there may be marked differences between cultivars regarding germination at different temperatures. Very low or very high temperatures may alter both the speed and the final percentage of germination. In general, low temperatures reduce, while high temperatures increase, germination speed. In extreme temperature conditions, germination may not occur, and in some cases, the seed may enter dormancy.

For example, in most lettuce cultivars, high temperature conditions (above 30°C) during seed imbibition can lead to a drastic reduction in the initial stand. Under these conditions, two different phenomena can occur: thermoinhibition, where the seeds do not germinate, but will germinate once the temperature returns to an appropriate level, therefore a reversible process; and thermodormancy, in which the seeds, after remaining soaked in high temperatures for a prolonged period, will not germinate even after reducing the temperature. In this case, the seeds require some treatment to overcome this dormancy (also called secondary dormancy).

In carrots, temperatures close to 35°C can reduce germination and seedling establishment in the field. It is worth noting that the establishment of carrot crops must be done through direct sowing, as this species does not accept transplantation. In this species, the establishment of crops has changed in recent years in some carrot-producing regions in the country, where seed expenditure per area has decreased, due to some aspects: a) the use of more modern sowing machines, where the seeds , embedded or not, are distributed more evenly and with greater precision; b) the increasing use of higher-cost hybrid seeds; and c) the higher labor cost to carry out the thinning practice.

Planting at the appropriate time, where the temperature is close to ideal for the germination of the species and cultivar, must also be considered, although, in certain cases, the producer tends to "escape" the best planting time to achieve more rewarding prices. In this sense, the use of seedlings produced in trays under protected cultivation is a highly interesting practice, not only due to the better germination and uniformity of the seedlings obtained, but also the opportunity for sowing at times unsuitable for the species in question. This technology allows seedling producers to place their trays, immediately after sowing, in controlled environments or even in germination chambers with appropriate temperatures to obtain maximum germination potential.

The use of plastic soil cover (mulching) is another cultural practice that has certain advantages. However, care must be taken when sowing certain species that require milder temperatures for germination, since when using dark plastics, they tend to absorb more heat, and in summer conditions, the soil temperature can exceed 40°C, which is detrimental to the germination of most vegetable species.

The osmotic conditioning technique (seed priming) can also be used quite successfully in stressful conditions, such as high temperatures. Osmotic conditioning consists of controlled hydration of the seeds, sufficient to promote pre-metabolic activity, without allowing radicle emission. In general, the treatment consists of soaking the seeds in an osmotic solution, at a certain temperature, for a certain period of time and then drying them to the original degree of humidity. This makes this treatment advantageous, as the seeds can be handled and/or stored; the possibility of storing the seeds on a commercial scale for a certain period after treatment, without losing the benefit, is highly desirable. Osmotic conditioning has been used mainly in vegetable and flower seeds, with the aim of improving germination speed, seedling uniformity and sometimes total germination, especially in adverse soil and climate conditions, such as super-optimal temperatures. In lettuce, for example, this treatment allows seeds to germinate under high temperature conditions (above 30°C), thus avoiding thermoinhibition and thermodormancy. In general, this treatment is not standardized and requires an appropriate methodology for each species, cultivar and even seed lots. Companies specialized in seed technology carry out this type of treatment.

Finally, the use of high quality seeds, combined with greater knowledge of the factors related to the germination of the species in question, will allow the producer to maximize the development of seedlings, whether in the greenhouse or in the field. Maximum germination with greater speed and mainly with greater uniformity of seedlings can guarantee the success of the enterprise.

Figure 1 - Normal (left) and atypical (right) germination of lettuce seeds under high temperature conditions

Figure 2 - Lettuce establishment through direct sowing, using osmoprimed (left) and unprimed (right) seeds

Figure 3 - Germination of carrot seeds at different temperatures

Figure 4 - Production of seedlings in trays under protected cultivation

Check out the article in issue 84 of Cultivar Hortaliças e Frutas magazine.