86 Table 3.2 Plant growth and reproduction under two CO2 regimes (~430 ppm (aCO2) and
~500 ppm (eCO2) of sweetcorn cultivar STAR7719.
STAR7719
Parameters aCO2 eCO2
Significant
Difference Mean LSD Leaf area (mm2) 174.3 179.8 0.786ns 177.1 44.64 Plant height (cm) 123.4 180.9 0.001*** 152.2 11.13 Leaf number/plant 8.33 9.56 0.001*** 8.94 0.512 Days to tasselling 67.33 56.89 0.001*** 62.11 1.680 Days to anthesis 74.22 64.22 0.001*** 69.22 2.400 Days to silking 75.33 67.67 0.001*** 71.50 2.400
*, **, *** indicate that the parameter is significant at P<0.05, P<0.01 or P<0.001, respectively, under ambient (~430 ppm, aCO2) versus elevated (CO2~500 ppm, eCO2).
The influence of eCO2 on above ground fresh and dry biomass (Table 2.3) was highly significant, resulting in an above ground fresh mass of 522g under eCO2, while under aCO2 a fresh mass of 334g was recorded. Above ground dry mass was also higher under eCO2 (139.4g) compared with aCO2 (76.4g). Elevated CO2 further significantly enhanced root fresh and dry mass (Table 2.3). No significant differences were found in fresh cob mass; nevertheless, significant differences were found in individual ear mass. Ears were highly significantly (P<0.01) longer (19.8 cm) under eCO2 than under aCO2 (13.2 cm).
Table 3.3 Above ground biomass, fresh cob mass and length parameters under ~430 ppm (aCO2) and ~500 ppm (eCO2) of sweetcorn cultivar STAR7719.
STAR7719 Plant parameter aCO2 eCO2
Significant
Difference Mean LSD
Ear mass (g) 163 219 0.056* 191.0 58.5
Fresh ear length (cm) 13.2 19.8 0.013** 16.5 4.78 Fresh cob mass (g) 104.2 134.1 0.140ns 119.1 42.09 AG fresh mass (g) 334 522 0.005*** 428 111.9 AG dry mass (g) 76.4 139.4 0.009*** 107.9 42.25 Fresh root mass (g) 61.9 88.8 0.018** 75.4 20.93 Dry root mass (g) 8.77 13.93 0.002*** 11.35 4.131
*, **, *** indicates that the term is significant at P<0.05, P<0.01 and P<0.001 respectively, where ns = non-significant at aCO2 (~430 ppm) and eCO2 (~500 ppm).
87 A delayed seedling emergence in the supersweet ‘Assegai’ hybrid cultivar, agrees with findings by Emmalea (2018), who described supersweet (sh2) hybrids to have a relatively low field emergence, due to seeds having low food reserves in the form of starch. Pedro et al.
(2021) also outlined that hinderances such as susceptibility to cold stress and wet soils, contribute immensely to reducing sweetcorn emergence and seedling vigour. The environmental variables in both glasshouses such as the temperature (ranged from 25-35oC) and relative humidity (RH) ranged from (60-80 %) were measured with agrometeorology tools during South African winter season. The solar radiance for both glasshouses was natural and ranged between 200-425 W/m2 depending on a day’s photoperiodic light. Prior to planting, the physicochemical properties of soil from three soil samples were analysed (Tables 2.3, 2.4, and 2.5, as described in descriptive chapter 2) as well as soil moisture percentages for the two sets of four consecutive weeks (Appendices 8 and 9) were recorded.
Due to cooler temperature, a delay of over one month in DTE and slow growth was found in
‘Assegai’ plants. Lodging in control ‘Assegai’ plants subsequently resulted in death of these plants. Emmalea (2018) stipulated that the supersweet hybrids contain the sh2 gene and are characterised by shrunken endosperms (fig. 2.3). This feature is aligned with the modified carbohydrate synthesis in the high sucrose containing maize endosperm, endosperm starch levels at maturity.
During the time of sowing, germination and emergence could have also affected seed performance, resulting in slowed development due to the cooler winter season with low temperatures in the glasshouse with temperature between 19 and 24oC during the day and 19oC at night, seemingly causing seeds to remain dormant or emerge slowly. Wrinkled cultivars (supersweet hybrids cultivars, with enhanced sugar concentrations in the fruity) were reported to have a smaller amount of food than standard cultivars and, consequently, grow slower and are vulnerable to stand problems (Emmalea, 2018). Additionally, two plants of the ‘Assegai’
eCO2 treatment lodged and died, hence, data were only recorded from the remaining, healthy plants. Under eCO2 conditions ‘STAR7719’ emerged between 7 and 8 days after sowing, meanwhile, in control the emergence was visible between 8 and 10 days after sowing.
Emergence is governed by environmental conditions driving early embryo growth. Cell elongation was likely limited due to relatively low temperatures in the glasshouse during winter season (Wolfe, 1991); this could have affected, plant height (180.9 under eCO2 and 123.4 under aCO2), visible as relatively short maize plants in both eCO2 as well as aCO2 glasshouse
88 but growth improved under eCO2. Significant differences were found between the ‘Assegai’
eCO2 treatment and the control, resulting in over a month of delay in seed emergence of the control compared with seed emergence of the treatment (figure 3.1). This led to lodging and, subsequently, no data being could be recorded for control plants. Significant differences were found in plant height and leaf number per plant for treatment compared with control (Table 3.2). As a C4 crop, Driscoll et al. (2006) reported maize to respond relatively well to CO2
enrichment. These authors also reported that the epidermal cells of leaves were larger after six weeks of transferring plants to eCO2 of 700 ppm compared with plants maintained under 350 ppm CO2.
The two concentrations of CO2 used in this experiment (~400 ppm for aCO2 and Mycelium CO2 powered generator bags with a concentration of ~500 ppm for eCO2) resulted in significant differences in various vegetative and yield parameters (Table 3.2 and 3.3). Vanaja et al. (2006) elaborated that growing plants in controlled environment chambers can result in root growth restriction in pot-grown plants. Similarly, in the present study, the days to achieving certain phenological milestone (days to tasselling, anthesis and silking) were reduced. This allowed maize plants under eCO2 to overcome adverse environmental conditions, while control plants succumbed to lodging (Table 3.3). Elevated CO2 had significant positive effects on sweetcorn, increasing fresh as well as dry root mass;
additionally, eCO2 enhanced above ground fresh and dry biomass (Table 3.3). The advancement in growth and development of maize plant under eCO2 lead to a shortened growing season due to earlier reproductive development (56-67 days to tasselling) (Table 3.3), indicating the potential to grow even two maize crops in one season.
Elevating CO2 could, therefore, shorten the period to produce certain horticultural crops, allowing two sweetcorn crops to be produced in one season. At ~500ppm, individual cob mass and leaf area of individual plants were not significantly affected, however, the IPCC (2007) mentioned the possibility of a rise in CO2 to 550 ppm, possibly even escalating to 700 ppm by the end of 21st century. Such an increase is likely to strongly affect growth parameters measured in this experiment, fast-forwarding growth and development of sweetcorn, as well as resulting in faster maturation of the crop.
Elevated CO2 not only fast-forward ontogenetic development, but also produced tall plants (Table 3.2). The higher leaf number combined with unchanged chlorophyll concentration and above ground mass are possibly responsible for a higher photosynthetic rate (Lara and Andreo,
89 2011). The total biomass from data recorded in the present study was greatly enhanced except the for-fruit yield (Table 3.3) this was possibly due to plants being susceptible to a colder growing season with lower temperatures (Waqas et al., 2021). Coupled with this are reports on declining maize yields declining under eCO2, attributed to a strongly reduced metabolite transport coupled with a reduced photosynthetic activity (Foyer et al., 2002). The effect of CO2
enrichment in the environment plants are grown in has been debated in C4 plants, as these already possess a mechanism to concentrate CO2 in the bundle sheath cells, such that C4 plants could be unaffected by eCO2 concentration in the atmosphere (Lara and Andreo, 2011). This thinking, however, reflects ignorance and has resulted in little interest in investigation on C4 plants and their response to eCO2 conditions, increasing the CO2 to possibly 550 ppm needs to be investigated for its effects on sweetcorn production.
Ear mass was positively impacted by eCO2, similar to ear length; however, cob mass was not significant enhanced (Table 3.3). This might have been due to the experiment being conducted in winter, as this limited Rubisco activity has been reported at cooler temperatures (Kubien et al., 2003). Vanaja et al. (2015) reported maize yield being improved under eCO2, as elevated CO2 triggered the partitioning of biomass towards cobs or grains. Reddy et al. (2010) conducted a fifteen year-literature survey on the influence of eCO2 among numerous C3, C4 and CAM plants by providing data on forty C3, two C4 as well as three CAM species. Several authors further found positive, significant responses of C3 plants to eCO2 conditions in terms of photosynthetic acclimation, i.e., a change in photosynthetic efficiency of leaves due to long exposure to eCO2, while in C4 crops, such as Sorghum and Panicum, certain negative responses to eCO2 have been reported. For CAM plants, such as pineapple, agave and kalanchoe, positive responses in plant growth and development were reported (Reddy et al., 2012).
The outcome of the survey conducted by Reddy et al. (2010), portrayed little evidence of investigations on the response of C4 species to eCO2. Vanaja et al. (2015), however, found a response of the C4 crop maize to eCO2, which was solely attributed to the efficient partitioning of reproductive biomass towards the reproductive rather than towards vegetative parts.
Furthermore, inconsistent reports on the response of C4 plants to eCO2 and differences in photosynthetic responses to eCO2 are attributed to plant species, differences in experimental techniques, the period of treatment, the age of the plant and the growing season (Sage, 2002).