2. LITERATURE REVIEW
3.4 Discussion
3.4.4 Effects on tomato plant growth, fruit yield and quality
69
(2004), who had stated that relative humidity tends to be lower close to the vent openings than in the interior sections of the greenhouse.
During the night, there was less variation in the air moisture content than during the day (Figure 3.8). The relative humidity variation has been found to be driven by solar radiation (Kittas et al., 2003), therefore, at night, when solar radiation effect was zero, there was less variation. The relative humidity was higher at 2.3 m than at 1.15 m. At night, the cover has been found to be the coolest surface in the greenhouse (Piscia et al., 2012). The higher relative humidity closer to the polyethylene cover, combined with the cooler cover, would have been the cause of the condensation on the inside of the tunnel, as mentioned in Section 3.6.2 and shown in Figure 3.13. Inside the NVT, as during the daytime, the relative humidity was higher at 1.15 m within the crop canopy, than at 2.3 m.
70
al., 2003). These results differ from those reported by Willits and Li (2005), Mutwiwa et al.
(2007) and Max et al. (2009), who reported faster growth rates in tomatoes grown in a naturally-ventilated greenhouse than those in fan-pad ventilated greenhouses in a tropical climate. The differences could have been due to the different climatic regions under which the studies were conducted.
The differences in the growth rate among the cultivars could have been due to genotypic differences. The indeterminate cultivars were consistently taller and generated more leaves than the semi-determinate cultivars. The increase in the height of Bona, Star 9037 and Star 9009 was faster in the FPVT, when compared to their growths in the NVT. The consistently lower temperatures and higher air moisture content (Figures 3.3 and 3.5) in the FPVT enhanced the vegetative growth of these three cultivars (Figures 3.9-3.12). Zeal plant height and leaf generation were not influenced by environmental conditions in the two tunnels. Abdelmageed and Gruda (2009) reported that heat tolerant cultivars would perform better under high temperature conditions than non-heat tolerant cultivars. Zeal was more heat tolerant than Bona, Star 9037 and Star 9009. These results are in agreement with Max et al.
(2009), who also observed no significant differences (P>0.05) in tomato leaf area and vegetative biomass in a fan-pad ventilated and a naturally-ventilated greenhouse.
Although the total and marketable yields did not differ significantly (P>0.05) between the two tunnels (Table 3.2), the total yield was 24% higher in the NVT than in the FPVT. Conversely, total marketable yield percentage was higher in the FPVT, with 84% of the total yield, compared to 70% in the NVT. These results agree with those reported by Willits and Li (2005), Mutwiwa et al. (2007), Teitel et al. (2007) and Max et al. (2009), who reported significantly higher total yields from naturally-ventilated greenhouses in tomato and rose plants. On the contrary, Maboko et al. (2010) reported higher total yield from a FPVT than a NVT. The differences could possibly be attributed to the different cultivars used and their responses to climatic conditions (Carli et al., 2011).The total yield for the cultivar Bona was higher than that of Zeal. The higher rate of vegetative growth observed in Bona may have resulted in the higher total yields, compared to those of Zeal.
The high unmarketable yield in the NVT was mainly due to the large quantity of deformed, undersized and insect-damaged fruit yields (Table 3.2) that were observed in this tunnel.
71
These results agree with those of Maboko et al. (2010), who reported 41% and 12%
unmarketable tomato yield from a NVT and a FPVT, respectively. The mean mass of undersized fruits was found to be significantly higher (P<0.001) in the NVT than in the FPVT. These results are in agreement with those of Willits and Li (2005), Max et al. (2009) and Maboko et al. (2010), who all reported significantly higher (P<0.05) undersized yields from the NVT than the FPVT. The development of smaller sized fruits has been associated with higher temperatures, as reported by Adams et al. (2001), who observed a lower mean fruit size at higher temperatures. In addition, temperatures above 30°C impair the development, release and viability of pollen (Abdul-Baki and Stommel, 1995; Peet et al., 2003), leading to poor fertilization and the production of misshapen fruits (Waquant, 1995;
cited by Dorais et al., 2001). Temperatures in NVT were above 30°C for up to 4 hours during the daytime in December and January. This would have had an effect on the development of pollen and fruit-set in the NVT, resulting in the high percentage of undersized and deformed fruit in this tunnel. The high difference in undersized fruit mass between Bona and Zeal could be attributed to genotypic differences. According to Hanson et al. (2002), the possession of the heat tolerant gene, would allow fruit set under high temperature environments. Cultivar Bona could be lacking the heat tolerant gene, thus the higher undersized fruit mass, compared to Zeal.
The microclimate in the FPVT did not have a significant effect on the development of blossom end-rot and fruit cracking, physiological disorders that are often associated with high relative humidity in fan-pad evaporative cooled greenhouses (Dorais et al., 2004). High relative humidity, particularly at night, has been shown to be positively correlated to tomato fruit cracking (Estergaard et al., 2001). Under conditions of high relative humidity, leaf transpiration rates are reduced and the water supply to the cells and tissue of other organs, such as fruit, are increased (Peet and Willits, 1995; Dorais et al., 2004). The resulting increase in turgor pressure and stress on the skin of the tomatoes has the potential to cause fruit and skin cracks in tomatoes (Dorais et al., 2004). The higher relative humidity, especially in the early morning, late afternoon and at night observed in both tunnels, could have led to approximately the same mass of total cracked fruits. The results are contrary to those reported by Max et al. (2009), who found a higher number of cracked fruits in a FPVT than in a NVT.
Zeal was the most susceptible to cracking, with 8.3% of its total yield rendered unmarketable due to cracking. Bona was the most tolerant to cracking, with <1% of its total yield rendered
72
unmarketable as a result of fruit cracking. These differences were most probably due to genotypic differences.
The incidence of blossom end-rot has been associated with low relative humidity and high vapour pressure deficits (Max et al., 2009). Insufficient translocation of calcium to the fruits under dry air conditions has been identified as the main cause of blossom end-rot (Blanc, 1986). At low relative humidity and high vapour pressure deficit, leaves tend to out-compete fruits in the competition for water supply (Adams and Holder, 1992; Bertin et al., 2000), thus limiting the supply of calcium to the fruit. In spite of the fact that the relative humidity was low in the NVT, especially in the middle of the day (Figure 3.5), this did not increase the incidence of BER in this tunnel, when compared to the FPVT. These results agree with those reported by Leonardi et al. (2000) and Max et al. (2009), who reported no significant differences at two different levels of vapour pressure deficit. Zeal grown in the NVT was the most affected by blossom end-rot, which would suggest that it is vulnerable to low relative humidity at high temperatures in the NVT.
There was a higher percentage of insect-damaged fruit in the NVT than in the FPVT.
Although this has little to do with greenhouse microclimate, it highlights some of the additional problems that may arise in controlled environment crop production. Naturally- ventilated greenhouses are often fitted with porous plastic insect screens to eliminate insect pests from the greenhouse and reduce the use the chemicals (Bartzanas et al., 2002; Klose and Tantau, 2004; Valera et al., 2006). However, insect screens will only keep out those insects which are larger than the openings between the threads of the screen mesh (Klose and Tantau, 2004; Katsoulas et al., 2006; Teitel, et al., 2009). For this experiment, Knittex ® 40% netting, with irregularly-shaped openings was used to keep insect pests out of the NVT. While this screen was effective against most tomato insect pests, it was ineffective against red spider mite. Up to 13.5% of tomatoes harvested in the NVT, compared to 2.75% in FPVT were rendered unmarketable, due to red spider mite attack. Red spider mite has been found to thrive under hot and dry conditions (Gotoh et al., 2010; Suzuki et al., 2012), the conditions which were more prevalent in the NVT than in the FPVT. Consequently, the tomato crop in the NVT was sprayed three times, in an attempt to control the red spider mite. This suggests that there might be a need for more pest control measures in the NVT than the FPVT, which might raise the costs for growing crops in the NVT.
73