In pepper, like in all horticultural crops, quality is an important component of marketable yield. Fruit weight, shape, and uniformity are important quality components in pepper (Aloni et al., 1999; Kissinger et al., 2005; Lim et al., 2007; Navarro et al., 2002). In bell peppers, both shape and size are primarily determined at the preanthesis stage (Munting, 1974). Although genetics play a key role, several other factors also determine fruit shape and size during preanthesis, including temperature and carbohydrate availability (Aloni et al., 1999; Tomer et al., 1998).
Low night temperatures (15 °C or lower) negatively affect bell pepper fruit quality. One of the more striking effects of LNT on pepper flower development is an increase in ovary diameter without a concomitant increase in locule number, which results in “swollen” ovaries and malformed fruit (Aloni et al., 1999; Polowick and Sawhney, 1985; Shaked et al., 2004).
As the duration of LNT increases beyond ≈1 week, both the percentage of ovaries that exhibit swelling (Aloni et al., 1999; Polowick and Sawhney, 1985) and the extent of ovary swelling (Cruz-Huerta et al., 2011) increase. Increased source-sink ratio also increases the proportion of swollen ovaries. Flower fresh weight (FW) on defruited pepper plants was three to four times higher than flower FW on fruiting plants, which resulted in swollen ovaries and malformed fruit (Aloni et al., 1999).
The incidence of swollen ovaries in bell pepper resulting from either LNT or high source-sink ratio has also been correlated with increased ovary carbohydrate concentration (Aloni et al., 1999). Under LNT, pepper plants exhibit slower growth rates, resulting in decreased shoot dry weight (DW) compared with plants grown under higher night temperatures (Mercado et al., 1997). The decreased growth rate may be the result of decreased photosynthesis, because night temperatures below 15 °C may decrease ribulose-1,5-bisphosphate regeneration and Pi availability for recycling (Hendrickson et al., 2004a, 2004b) and/or activities of photosynthetic/carbohydrate metabolizing enzymes (Bertamini et al., 2005; Sundar and Reddy, 2000). Tropical crops, including pepper, are especially sensitive to LNT. Bhatt and Srinivasa-Rao (1993) reported that net CO2 exchange rates in pepper were higher at night temperatures of 22 °C compared with 17 °C. However, night temperature effects on photosynthesis have not been studied in relation to ovary swelling in pepper. In some species such as cotton (Gossypium hirsutum) and bean (Phaseolus vulgaris), however, leaves developed under LNT may acclimate to such conditions, resulting in carbon exchange rates as high as in plants growing under higher temperatures (Singh et al., 2005; Wolfe and Kelly, 1992). It is unknown whether pepper leaves developed under LNT undergo acclimation and regain high photosynthetic rates. If so, then similar photosynthetic rates and slower growth rates under LNT may cause excess carbohydrate accumulation in floral ovaries, resulting in swelling and fruit deformation.
Vegetative growth and photosynthetic rates in pepper are also affected by source-sink ratios. In bell pepper, fruiting decreased vegetative growth rates (Bhatt and Srinivasa-Rao, 1989; Hall and Milthorpe, 1978) and increased leaf photosynthetic rates (Cruz-Huerta et al., 2005) compared with defruited plants. Defruiting, which increases the source-sink ratio, increased starch concentration in stems (Hall and Milthorpe, 1978) and the incidence of flower deformation and swollen ovaries (Aloni et al., 1999). In addition, the percentage of swollen flowers was inversely related to the number of growing fruit on the plant and directly proportional to the concentration of reducing sugars and starch in the flowers developed in those plants (Aloni et al., 1999). This suggests that although defruiting may decrease photosynthetic rates, the decreased carbohydrate production may not be sufficient to maintain an optimum source-sink balance. This may lead to greater assimilate accumulation in flower buds on defruited compared with fruiting plants, resulting in ovary swelling and fruit deformation.
Although ovary swelling is favored by LNT or increased source:sink ratio, the interaction between night temperature and source-sink modification on ovary carbohydrate accumulation and swelling has not been investigated. Thus, the hypothesis tested in the present experiment is that ovary swelling—whether resulting from LNT effects on photosynthesis and vegetative growth and/or resulting from fruiting effects on the source-sink ratio—results from increased non-structural carbohydrate accumulation in ovaries before and at anthesis. The objectives were to determine the interaction between night temperature and fruiting on 1) ovary swelling and vegetative growth; 2) leaf net CER and the photosynthetic acclimation ability of leaves to LNT; and 3) soluble sugar and starch concentrations in ovaries at anthesis.
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