Pak. J. Bot., 40(3): 1013-1023, 2008. SPRING APPLICATION OF GROWTH REGULATORS AFFECTS FRUIT QUALITY OF ‘BLOOD RED’ SWEET ORANGE BASHARAT ALI SALEEM 1 , AMAN ULLAH MALIK 1 , M. ASLAM PERVEZ 1 , AHMAD SATTAR KHAN 1 AND M. NAWAZ KHAN 2 1 Institute of Horticultural Sciences, University of Agriculture, Faisalabad, Pakistan 2 Orange Research Institute, Sargodha, Pakistan Corresponding author’s E-mail: basharatuaf@yahoo.com Abstract Spring applications of two plant growth regulators (GA 3 and 2, 4-D) alone and in combination, were tested on ‘Blood Red’ sweet orange trees at full bloom. Ultimate effects of these growth regulators were studied on external and internal fruit quality. Fruit weight, diameter, peel thickness and peel quantity were significantly decreased by the growth regulator treatments compared with control while juice contents (%), pulp (%), reducing sugars, non-reducing sugars and total sugars, seeds quantity and quality were significantly improved by GA 3 treatments compared with control. TSS (%), Vitamin C contents were increased by growth regulators treatments compared with non treated ones. In organoleptic tasting, taste, peel colour, pulp colour and appearance were also improved by growth regulator treatments compared with control. In conclusion mixture treatments performed best with regards to biochemical parameters compared with control. Introduction Fruit quality reflects numerous external and internal attributes, on the basis of which, minimum standards of palatability and commercial acceptability have been established over the years (Davies & Albrigo, 1994). In citrus, external features like fruit colour, size, and peel texture are the important parameters to estimate the quality of the fruit, while internal characters contributing to fruit quality include amount and quality of juice, seediness, vitamin C contents, total soluble solids (TSS), titratable acidity (TA) and TSS: TA ratio (Ahmed, 2006). The composition of citrus fruit varies with cultivar, climate, rootstock and cultural practices (Davies & Albrigo 1994; Sattar, 1999; Ahmed, 2006). Today, Pakistan stands at 13 th and 10 th position among top citrus producing and exporting countries of the world respectively (Anon., 2005). The average yield of citrus in Pakistan is about 10 t ha -1 which is far less than world average citurs yield 30 t ha -1 (Anon., 1993). Annually, 1.7 MMT citrus is produced from a total area of 185 thousand hectares in Pakistan (Anon., 2005). The citrus industry in Pakistan has turned into monoculture comprising of ‘Kinnow’ mandarin although four decades ago among other species sweet orange was the major component of citrus plantation in the country (Malik et al., 1993). Low productivity and inferior fruit quality are the major reasons of depletion of sweet orange from our citrus industry. Oversized fruit with low juice contents having poor fruit quality discouraged the citrus growers and thus ‘Kinnow’ mandarin has replaced most of the sweet orange area in the Punjab province. The application of plant growth regulator (PGR) can provide significant economic advantages to citrus growers when used in appropriate situations as these have proven effective in stimulating a number of desired responses such as increase in fruit size and delay in fruit maturity (Coggins Jr & Hield, 1968). Fruit development is thought to be triggered by hormones as it is evident from the report by Talon et al., (1990) that the endogenous gibberellin status of the developing citrus ovaries is the limiting factor for the initiation of fruit development. Application of Gibberellic acid (GA 3 ) before or at full