Vol.:(0123456789) 1 3 Journal of Plant Growth Regulation https://doi.org/10.1007/s00344-020-10180-7 Photooxidative Stress Modulation of Endogenous Phytohormone and Antioxidant Accumulations and Fruit Maturity in Date Palm (Phoenix dactylifera L.) Hussein J. Shareef 1  · Jameel M. Al‑Khayri 2 Received: 10 December 2019 / Accepted: 15 June 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract A common modern cultivation practice is bagging the fruit bunch if date palm (Phoenix dactylifera L.) which may infu- ence fruit maturity and nutraceutical quality. Exposure of fruits to photooxidative stress induces changes in the endogenous concentrations of plant hormones and other metabolites, which may cause accelerated fruit maturity. This study was con- ducted to examine the efect of exposure to direct and indirect sunlight on date palm fruit development. The indirect sunlight treatment was simulated by fruit bunch bagging, a common practice in modern date production. The exposure of date palm fruits to direct sunlight-induced photooxidative stress causing an increased concentration of ascorbic acid and decreased content of chlorophyll, anthocyanins, carotenoids, and phenols compared to the fruit bagging treatment. Direct sunlight also reduced the concentration of phytohormones, including indoleacetic acid, gibberellin, and zeatin, but increased abscisic acid accumulation. The directly-exposed fruits reached a partially-mature stage (Rutab) in August, whereas the bagged fruits remained at the immature stage (Khalal). This study is the frst to describe the biochemical basis of the observed improve- ment of date palm fruit development in response to reduced light intensity. Besides, it provides insights into controlling date palm fruit maturity and subsequently prolonging the shelf life dates on the tree; thus, extending the marketing period for the beneft of the farmers. Keywords Abscisic acid · Anthocyanin · Ascorbic acid · Carotene · Chlorophyll · Direct sunlight · Fruit bagging · Phenol Introduction Sunlight contributes to the ripening of many fruits, including date palm. Date palm (Phoenix dactylifera L.) is the lead- ing fruit crop in Iraq and surrounding countries (Khieral- lah et al. 2015). It is an evergreen tree bearing fruit during the summer and continuing into autumn in some varieties. The fruits have high nutritional value and are used in many food industries and have a high market value (El Hadrami and Al-Khayri 2012). Among the palms, dates occupy third place in terms of economic importance in the world after oil palm and coconut. The desired qualities of the consumer are cleanliness, bright color and size of the fruit. Field practices that help increase fruit size involve covering the clusters to block the sunlight from reaching the fruits (Shareef and Alapresm 2012). Sunlight reaching the fruit can cause damage at the cel- lular level, including the breakdown of photosynthesis and peroxidation of the lipid and photooxidative of phytohor- mones such as indoleacetic acid (IAA), which is consid- ered most afected by high radiation (Šebela et al. 2017). Light accelerates the process of fruit maturation through the process of photooxidation as the light-emitting plant hor- mones such as gibberellin (GA 3 ) and zeatin (ZT) break down (Shareef 2010). Some plant hormones, such as abscisic acid (ABA), are known as stress hormones that play a critical role in plant growth, regulation of growth, stomata closure, and fruit abscission (Yang et al. 2011). IAA is a multifunctional plant hormone that is necessary not only for plant growth * Jameel M. Al-Khayri jkhayri@kfu.edu.sa Hussein J. Shareef hussein.shareef@uobasrah.edu.iq 1 Date Palm Research Center, University of Basrah, Basrah, Iraq 2 Department of Agricultural Biotechnology, College of Agriculture and Food Sciences, King Faisal University, Al-Ahsa 31982, Saudi Arabia