Citrus fruits are of significant global importance with an estimated annual production of 166 million MT, of which the sweet oranges (Citrus sinensis (L.) Osbeck) are the most abundant, constituting more than 45% of global citrus production [1] . Other renown and commercially preferred Citrus species include: mandarin (Citrus reticulata Blanco), pomelo (Citrus maxima (Burm.) Merr.), lemon (Citrus limon (L.) Osbeck), lime (Citrus aurantiifolia (Christm.) Swingle) and citron (Citrus medica L.), grapefruit (Citrus paradisi Macfad.) [2] . Citrus is especially cultivated in tropical and subtropical regions as well as the Mediterranean Basin and regions with a Mediterranean climate however, it is productive in a wide range of climates also encompassing; equatorial, cool, hot, humid and semi-arid [3] [4] . Citrus fruits are widely consumed and have nutritional and health benefits as they are an important source of Vitamin C, folate, potassium and carotenoids and other phytonutrients of physiological importance such as polyphenols, flavonoids, coumarins and terpenes [3] .

Citrus production in Uganda is dominated by oranges and is mostly concentrated in the Kyoga plains agroecological zone in Eastern Uganda mostly spanning Teso subregion [5] . Orange production in Teso region has been on the increase, rising from 200,000 MT valued at US$ 30 million in the 90’s to 360,000 MT valued at US$ 47 million in 2009 to 769,177 MT valued at about US$ 154 million in 2015 [6] - [8] . Oranges in Teso sub region are harvested throughout the year with a peak season in September to December. The main varieties grown in the region are: Washington Navel, sweet Valencia, Hamlin and the local orange [5] . Navel oranges usually have large, seedless fruits and therefore good eating quality while Valencia is known for having high juice and sugar content, good rind firmness, adaptability to different agroecologies and long shelf life [3] .

In Uganda, oranges are largely marketed in the fresh form of which the desirable quality attributes by traders are colour, maturity level, size, taste and texture [5] . The pre-harvest quality of oranges is variable depending on many factors such as climate, maturity stage, soil conditions, scion/rootstock combination and agronomic practices among others [3] [9] [10] . More so, a given variety will exhibit significantly different quality in regions of dissimilar climate [11] . Maturity of oranges is complex entailing physiological processes that cause internal and external changes in the fruit flesh and peel respectively [12] . The maturity of oranges is defined by quality indicators namely: juice content, Total soluble solids (TSS), acidity, sugar to acid ratio and colour [13] [14] . Fruit size and weight are also important quality parameters especially for fruit trade [12] . Although external colour is a key indicator of orange fruit maturity, oranges in the tropics tend to retain their green colour at maturity due to partial degradation of chlorophyll associated with high average temperatures [9] . Therefore, it is important to have a holistic consideration of maturity.

As reported by [5] , most fruit juice processing firms in Uganda did not use oranges from Teso subregion as raw material as they did not meet consumers’ desirable quality in terms of colour and taste of the end-products. Information on the quality of oranges grown in Teso remains limited or unreported. The current study sought to bridge this gap. Therefore, the objective of this study was to investigate the seasonal profile of orange fruit quality in Teso subregion in terms of physical and chemical characteristics during wet and dry seasons in the period 2019, 2022 and 2023.

Citrus fruit weight and size are important towards determining their commercial destination. Large and medium sized fruits with equatorial diameter > 70 mm and average weight 152.5 g are preferred for the fresh fruit market while the smaller ones are channelled to industrial juice processing [20] [21] . The bigger wet season Washington navel oranges span size codes 3, 4 and 5 (73 - 92 mm) while the intermediate dry season Washington and wet season Valencia fall within codes 5, 6 and 7 (67 - 84 mm). The dry season Valencia as well as Hamlin for both seasons were relatively smaller and lie in the category 7 and 8 (64 - 76 mm) [14] . Washington navel is the most widely grown of the navel oranges globally and is characterized by large fruit size and being seedless [22] . The bigger and heavier fruits in the wet season are attributed to excess water available to the trees [9] . Based on size criteria, Washington navel from both seasons as well as wet season Valencia are fit for the fresh market while the Hamlin for both seasons and dry season Valencia are ideal for processing.

Peel thickness is dependent on variety, maturity stage and climate conditions. The peel thickness for Valencia was similar to that reported by [23] . Thin peels are associated with higher juice yield and are thus preferred for processing while the thicker peel goes into the fresh market as they are generally regarded as easier to peel and are more shelf stable [24] [25] . According to [23] , smaller peel thickness was considered at ≤3 mm. The peels for all the varieties in our study were at least 4mm regardless of season, therefore they could ostensibly be described as having intermediate thickness, the exception being dry season Washington navel whose peel was significantly thicker. Fruits grown in dry and hot conditions tend to have thicker and harder peels [3] . Thicker peels like those of the dry season Washington navel provide opportunities for oil and pectin extraction.

TSS is a good indicator of the sugar content and thus sweetness of oranges as 80% of it is contributed by sucrose, fructose and glucose [26] . The recommended minimum value of TSS of oranges is 8 - 10 ˚Brix [27] [28] . All the dry season oranges, regardless of variety as well as wet season Washington navel, met this requirement. In addition, the wet season Hamlin and Valencia were borderline. Lower TSS in the wet season can be explained by dilution of sugars by excess water available in the soil [9] . The relatively low TSS content in the fruits generally can be explained by warm temperatures of lowland tropics which accelerate respiration rates and fruit maturity resulting in insufficient time to accumulate maximum TSS while acidity also declines fast thus giving a sharp increase in TSS to acid ratio [3] [9] .

Citric acid is the main organic acid in citrus fruits varying from 0.5% to 1.5% in oranges [22] . Variation in acidity depends on the variety, rootstock, level of harvest maturity, geographical location and season [9] [22] [23] . [29] evaluated fruits from orange tree tops on different rootstocks, reporting TA in range 0.36% to 1.47% c.a. In Tanzania, [30] reported TA of 0.60% to 1.65% c.a. from Valencia and Navel oranges at different harvest stages and storage time. The TA for wet and dry season oranges in this study was comparable with the aforementioned studies. However, overall TA was low across the three varieties especially for the wet season fruits. During the wet season, acid levels decrease due to the dilution effect of excess water [9] . The generally low acidity is associated with high temperatures in tropical climates that accelerate physiological processes such as respiration resulting in rapid breakdown of organic acids [3] [9] . A minimum TA of 0.5 and 0.75% is recommended for industrial processing and fresh consumption respectively [23] [24] [31] . The dry season oranges particularly Valencia met this criterion.

Citrus fruits are one of the main sources of vitamin C for human nutrition of which oranges and lemons are richer in vitamin C than other citrus fruit [3] . Vitamin C content of fresh oranges (C. sinensis) varies between 23 and 83 mg per 100 g. This is influenced by several factors such as maturity stage, variety, climate, handling and storage conditions; tending to decrease after harvest [22] [32] . The vitamin C content was within this reported range for all varieties across seasons. In addition, it was similar to that reported for Valencia and Navel oranges (31.3 - 50.4 mg/100g) in Tanzania [30] . Seasonal variation of vitamin C mirrored that of TA and can be explained similarly.

The pH of the oranges (3.1 - 3.6) is similar to that reported for sweet oranges [33] [34] . The organic acids in oranges especially citric acid together with malic and succinic acid are responsible for the acidic pH [33] .

Juice content is an important citrus fruit quality parameter. The required minimum juice content for navel and other oranges (includes Valencia and Hamlin) is 33% and 35% respectively [14] . Only wet season Valencia met this minimum threshold. More so, a juice content of at least 50% is recommended for orange fruits to be considered for industrial juice production [28] , of which none of the cultivars matched this. Oranges with thick peels have relatively low juice content and extraction levels [35] . Lower juice content in the dry season could be linked to reduced water available to the trees. [36] reported a decrease in juice content of limes due to soil water deficit.

Sugar to acid ratio is a good indicator of citrus fruit maturity. According to East African Standards, the minimum ratio should be 6.5 [14] . Furthermore, for juice processing a ratio of 14 - 16 is recommended [28] . All the oranges had high sugar to acid ratio with a minimum of 17 in the dry season Valencia. The high sugar to acid ratio is mostly due to the low acidity associated with fruits grown in a hot climate [3] . However, a very high sugar: acid ratio results in an insipid taste [37] .

As reported by [10] the area of origin and inherent unique characteristics of some citrus cultivars contribute to their suitability to certain climates. Valencia is a late cultivar which develops and matures over a longer period compared to Washington and Hamlin (mid-season cultivars) [3] [10] . This could explain its better overall characteristics. Moreover, Valencia is the most widely grown cultivar globally and is associated with a high juice and sugar content with good rind firmness, giving it a longer shelf life [3] .

The study showed that fruit quality of oranges in Teso was dependent on the prevailing season at harvest and the variety with significant differences. Based on fruit size, Washington navel is suited for the fresh market and Hamlin for processing. Valencia was intermediate and thus serves a dual purpose. The dry season fruits across cultivars met the minimum TSS content requirements and optimum levels of acidity for both industrial juice processing and the fresh market. Both wet and especially dry season Valencia provide sugar: acid ratio within the optimum range for industrial juice processing. Therefore, we recommend that Valencia be prioritized for scaled production due to its versatility in meeting the requirements of both the fresh market and industrial juice processing. Breeding, propagation and cultural practices should be geared towards increasing juice content while maintaining optimal sugar: acid ratio of the available cultivars regardless of season. The current study did not consider consumer preferences of the different orange varieties across seasons and therefore this aspect is recommended for subsequent research.

This work was funded by the Korea Partnership for Innovation of Agriculture (KOPIA). We appreciate the National Agricultural Research Organisation for the financial and institutional support. We are grateful to the District Local Governments of Soroti, Kalaki, Kumi and Ngora as well as Teso Tropical Fruits Cooperative Union Ltd. (TEFCU) and the citrus farmers for their dedicated support and cooperation.