| Summary: | Most tomato cultivars used for commercial food production are imported into South
Africa. Optimal growing conditions for these specific cultivars need to be determined, as
wrong cultivar choices can lead to great financial losses. Lack of information on selecting
well-performing cultivars may lead to lower yield or unacceptable fruit quality. Information
on the performance of tomato cultivars under South African conditions, utilizing plastic
tunnels or shadenet structures under soilless cultivation is still very limited. Soilless
cultivation of vegetables is becoming a preferable over in-soil cultivation due to the
improved yield and quality of produce, efficient water and nutrients usage by the crop;
furthermore, the grower can regulate nutrient solution, electrical conductivity and pH of the
nutrient solution.
To identify the optimal system for growing tomatoes hydroponically, the performance
of four tomato cultivars (‘FA593’, ‘Miramar’, ‘FiveOFive’ and ‘Malory’) under different
growing conditions was evaluated: directly planted in soil under 40% shadenet with drip
irrigation, a closed hydroponic system under 40% shadenet, an open bag system under 40%
shadenet, or an open-bag system in a temperature controlled as well as a non-temperature
controlled tunnel. The study revealed that ‘Miramar’ performed better than the other cultivars
in all production systems, with the exception of soil cultivation where there were no
differences amongst the four cultivars. Fruit cracking was found to be directly correlated with
fruit size, as the large-sized cultivars ‘Malory’ and ‘FA593’ were more susceptible than the
other two cultivars. Plants grown under shadenet were prone to fruit cracking and raincheck
as well as early blight. Higher yields were obtained when plants were produced in the open
bag system under temperature controlled conditions and in the closed system under shadenet.
Growing tomatoes in the non-temperature controlled tunnel resulted in high incidences of
fruit cracking, poor yield and pre-mature fruit ripening probably due to high and fluctuating
temperatures under such conditions. The average marketable yield was 88% and 59% of the
total yield in the temperature controlled and non-temperature controlled tunnels, respectively.
A further experiment was carried out to improve yield and quality of tunnel tomatoes
using beneficial micro-organisms, i.e., arbuscular mycorrhiza fungi (AMF) at different nutrient concentrations. Tomato seedlings were treated with Mycoroot™ containing four
mycorrhiza species (Glomus etunicatum, Paraglomus occultum, Glomus clarum and Glomus
mossea) at transplanting and subsequently transferred to either a temperature controlled or a
non-temperature controlled tunnel under the recommended (100%) or reduced (75 and 50%)
nutrient concentrations. Sawdust was used as a growing medium in this experiment.
Application of AMF neither enhanced plant growth, yield, nor fruit mineral nutrient
concentrations; although fruit Mn and Zn concentrations in the temperature controlled tunnel
increased significantly following AMF application. Plants grown in the non-temperature
controlled tunnel had significantly poorer plant growth, and lower yield and lower fruit
mineral concentrations, compared with fruit from plants in the temperature controlled tunnel.
Tomato plants in the non-temperature controlled tunnel had higher levels of micro-elements
in leaf tissue, compared with those in the temperature controlled tunnel. The highest yields
were obtained from plants fertigated with 75% of the recommended nutrient concentration, as
compared with the 100 and 50% nutrient concentrations.
When coir was subsequently used as the growing medium, Mycoroot™ applied at
seeding and transplanting did not enhance mycorrhizal colonization or fruit quality. Growing
tomatoes under reduced nutrient supply reduced the total soluble solids in the juice of the
fruit, but improved total and marketable yield, as well as the number of marketable fruit. This
effect was more substantial in the temperature controlled than in the non-temperature
controlled tunnel. Fruit firmness and leaf chlorophyll concentrations were significantly
higher in plants grown in the temperature controlled tunnel. Growing tomatoes in sawdust
improved the leaf Mn and Ca concentration over that of tomato plants grown in coir.
Mycorrhizal colonisation did not have a beneficial effect on tomato yield and quality.
The study indicated that cultivar selection was important in obtaining the highest
yield and quality of tomato using the closed hydroponic system under shadenet and the open
bag hydroponic system in the temperature controlled tunnel. Temperature controlled tunnels
with a pad–and-fan cooling system are still an effective way of cooling the tunnel
environment which resulted in high yield and high quality of tomatoes with a higher fruit
mineral content than that obtained under non-temperature controlled conditions where only
natural ventilation is relied on. Results also demonstrated that mycorrhizal colonization in soilless condition has limited beneficial effects in allowing for better nutrient uptake and
thereby for improved yield and quality of tomatoes. Further studies, including different
media, nutrient composition and concentrations, need to be carried out to investigate the
possible causes of AMF failure to improve yield, despite good AMF root colonization. === Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
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