Chapter 5: Effects of stem training on physiology, growth, and yield responses of
5.2 Introduction
Tomato production under protected cultivation has gained popularity in South Africa over the past decade (Maboko et al., 2011). This increase has been intensified by the high market returns even in areas of limited resources, such as poor soils and shortage of land. Producing high yield and good quality of this crop in an open field is very challenging due to unfavorable environmental conditions and a high incidence of pests and diseases. However, the planting of tomatoes under protected cultivation provides a certain degree of control and allows farmers to produce even when the crop is out of season because it is easier to control temperature under such systems.
Despite the widespread adoption of protected cultivation, the use of such systems remains a challenge, including reduced yield and difficulty of the plant to grow upright due to limited height which is provided by the dome shape tunnel. The yield of tomato in protected cultivation does not always reach its full production potential due to poor management which is caused by the highly intensive nature of these systems. Accordingly, several management practices aiming at improving yield by enhancing fruit number, size as well as quality (Maboko and Plooy, 2008) have been
122
developed. This includes horticultural practices such as fruit thinning, management of plant population, cultivar selection and stem training (Maboko and Plooy, 2008). Stem training has been identified as one of the most important horticultural practices used to increase yield and improve fruit quality (Ara et al., 2007). Stem training is defined as the number of stems allowed to grow as a leader during plant growth. There are different types of stem training methods used for tomato production in tunnels and this includes; single stem, two plants per pot and double stem. Single stem training is achieved by removing all sucker stem to allow plants to grow as a single leader.
Two plants per pot stem training method is achieved by planting two seedlings in one pot while removing all sucker stems to allow each seedling to grow as a single stem. On the other hand, double stem training is achieved by leaving sucker at the bottom to grow as the second main stem resulting to double leader stems growing.
Stem training methods have a different impact on plant physiology as well as yield. Different stem training methods may exhibit different leaf area index, and percentage of leaves exposed to sunlight. In addition, stem training also impacts the root density of tomato plants, for example, a method of growing two plants per pot at a later stage form root balls and also reduce light interception leading to down-regulation of photosynthetic capacity (Shi et al., 2008). Furthermore, the stem training method may also impact other numerous variables such as water use efficiency, transpiration and fruit formation.
The most commonly used stem training method for the production of indeterminate tomato in South Africa is a single stem method (Snyder, 2007). It has been reported that tomato fruit produced with this method are not only large but have high fruit mass (Snyder, 2007). However,
123
tomatoes produced using this training method have been reported to have a low marketable fruit and very susceptible to fruit cracking (Maboko and Du Plooy, 2009). Furthermore, this training method produces a minimal number of fruit per plant and large fruit which might be the main cause of fruit cracking, thus, resulting in a reduction of marketable fruits (Maboko et al., 2011). Most farmers are trying to optimize their yield by shifting from single stem method to two plants per pot method. Amundson et al. (2012) reported that this method results in a slight increase in yield per unit area, however, it has no impact on farmers’ profit. This steadiness in profit margin is hypothesized to be related to the additional cost incurred when increasing the number of seedlings.
These costs includes, inputs costs such as seedling, fertigation, maintenance and labour cost.
Growing tomatoes as a double stem has been identified as an alternative method that can increase yield and reduce production cost compared to two plants per pot method because maintenance costs are similar (Amundson et al., 2012). Alam et al. (2016) found that tomato of BARI hybrid produced with double stem had high fruit mass compared to a single stem. Maboko et al. (2011) reported that tomato (FA593) produced in a double stem method had high yield and a high number of marketable fruit compared to a single stem. Similarly, Amundson et al. (2012) found that the two plants per pot method had a high yield of tomato during summer compared to a double stem method, whereas, there was no significant difference in winter. The findings reported by these authors necessitate more research since there is no clear information or consensus on yield of two plants per pot stem training compared to double stem training. Thus, the study aims at identifying the best stem training method that can increase yield and improve quality at the same time providing information on the horticultural performance of these training methods
124