Comparative analysis of two greenhouse microclimates in the sub-humid climate of South Africa.

Abstract

Development of small-scale greenhouse facilities that are suitable for the southern African agro-climatic conditions is one of the options that can be adopted to help alleviate food insecurity and malnutrition in Southern African countries. A comparative study was undertaken to analyse the temperature and relative humidity (RH) of two types of greenhouse tunnels, namely, the fan-pad evaporatively-cooled (FPVT) and the open-ended naturally-ventilated tunnels (NVT). The effect of the microclimates on crop growth, yield and quality, using tomatoes during the summer months and lettuce in winter, were also investigated. The effects of the integrated agro-technologies involving greenhouse microclimate and storage conditions on the postharvest quality of four tomato cultivars was also studied. The daytime temperature was lower in the FPVT than in the NVT during both summer and winter periods. Night-time temperatures were not significantly (P>0.05) different. The RH was higher in the FPVT than in the NVT, both during the day and at night. There was also a marked vertical and horizontal variation of temperature and relative humidity. Tomato vegetative growth was significantly (P<0.05) higher in the FPVT, but there were no significant (P>0.05) differences in the total yield and the total marketable yield of tomatoes grown in the FPVT and the NVT. However, the total yield was 24% higher in the NVT, but the total marketable yield was 8.3% higher in the FPVT. Highly significant (P<0.01) cultivar differences were observed, with Bona performing the best in terms of total yield and total marketable yield under both NVT and FPVT microclimates. During winter, there were no significant (P>0.05) differences in the growth and yield of lettuce crops resulting from the effect of the microclimate, although there was a higher incidence (by 26.9%) of leaf tip-burn in the FPVT. Between the two lettuce cultivars, growth was faster and the yield was 11.5% higher for Ballerina than for Nadine. However, Ballerina also had 24 to 29.6% more leaf tip-burnt leaves than Nadine. The integrated agro-technology study involving pre-harvest and postharvest treatments, which included the effect of greenhouse microclimate and postharvest storage environmental conditions on the texture, colour, TSS and pH of the four tomato cultivars, had significant (P>0.05) effects. NVT-grown tomatoes had and retained a firmer texture (by 7.4%) for a longer period, had a higher TSS value (by 10.3%), but a more rapid colour change, from mature-green to red-ripe, than FPVT tomatoes. Among the four tomato cultivars, Bona was the least firm and changed colour from green to red more rapidly, but it had the highest TSS and acidity than of the other three cultivars. With cold storage conditions at 13°C and 85% RH, the firmness of Bona was improved by 24.1% and the colour changes were reduced by 18.6%, compared to those under ambient air conditions (23± 2°C and 52± 4% RH). The cultivars Star 9037 and Star 9009 had the highest firmness and changed colour the least, but had the lowest TSS and highest pH values. This study has provided information about the microclimate of two types of greenhouses and their performance, in terms of the effects on growth, yield and quality of four tomato cultivars in summer and two lettuce cultivars in winter. The microclimate in the FPVT encouraged vegetative growth and improved the quality of the tomatoes. In the NVT, although the total yields were higher, the higher temperatures and insect/pests reduced the quality of the tomatoes. In winter, although the daytime temperatures were higher in the NVT, lettuce growth was not negatively affected, whereas in the FPVT, high humidity resulted in lower quality lettuce. The study also provided information on the integrated agro-technologies, involving greenhouse microclimate and postharvest storage environmental conditions on the postharvest quality attributes of tomatoes available on the South African market. NVT-grown tomatoes need cold postharvest storage, to reduce the ripening, in terms of colour, whereas FPVT grown tomatoes need cold postharvest storage conditions, in order to maintain a firmer texture for a longer period. It has also shown that the selection of the most suitable cultivar is important for profitable cultivation of vegetable crops in greenhouses. The study has further shown that small-scale naturally-ventilated greenhouses can be used for commercially-oriented agriculture to produce crops, with the aim to reduce poverty, create employment and address malnutrition. In addition, the study has shown that there is a need for further research aimed at reducing the daytime heat load of the greenhouses during the summer months, and the night-time heat loss during winter and a cost-benefit analysis, to establish the cost of production associated with the two tunnels. Furthermore, greenhouse microclimate research can be extended to the other South African climatic regions, to provide extensive information about the performance of these facilities and their effects on other greenhouse crops such as peppers and cucumbers.