Effects of water-deficit stress and gibberellic acid on floral gene expression and floral determinacy in 'Washington' navel orange

Authors: Tang, Lisa; LOVATT, CAROL - University Of California

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2022
Publication Date: 7/1/2022
Citation: Tang, L., Lovatt, C. 2022. Effects of water-deficit stress and gibberellic acid on floral gene expression and floral determinacy in 'Washington' navel orange. Journal of the American Society for Horticultural Science. 147(4):183-195. https://doi.org/10.21273/JASHS05213-22.
DOI: https://doi.org/10.21273/JASHS05213-22

Interpretive Summary: Flowering in citrus can be triggered by drought stress, but how the process is regulated is poorly understood. To better understand this phenomenon, water deficit and flower-inhibiting gibberellic acid foliar applications were employed as tools to increase and reduce flowering of navel orange trees, respectively, in order to compare gene expression in well-irrigated trees that did not produce flowers. We identified the key genes in flower induction in response to water deficit. With the same results obtained in our previous work using low temperature as a floral stimulus, we further inferred that both stresses promote citrus flowering via overlapping pathways. The results of this study provide a scientific base for the use of alternative irrigation approaches, such deficit irrigation, to sustain flowering and yield in citrus producing areas where winter low temperatures have become insufficient for flower induction due to climate change.

Technical Abstract: Effects of water-deficit stress and foliar-applied gibberellic acid (GA3) on ‘Washington’ navel orange (Citrus sinensis) floral gene expression and inflorescence number were quantified. Trees subjected to eight weeks of water-deficit stress [average stem water potential (SWP) -2.86 MPa] followed by three weeks of re-irrigation (SWP recovered to > -1.00 MPa) produced more inflorescences in week 11 than trees well-irrigated (SWP > -1.00 MPa) for the full 11 weeks (P < 0.001). After eight weeks of water-deficit stress, bud expression of flowering locus t (FT), suppressor of overexpression of constans 1 (SOC1), leafy (LFY), apetala1 (AP1), apetala2 (AP2), sepallata1 (SEP1), pistillata (PI), and agamous (AG) increased during the re-irrigation period (weeks nine and 10), but only AP1, AP2, SEP1, PI, and AG expression increased to levels significantly greater than that of well-irrigated trees. Foliar-applied GA3 (50 mg·L-1) in weeks two through eight of the water-deficit stress treatment did not reduce bud FT, SOC1 or LFY expression, but prevented the upregulation AP1, AP2, SEP1, PI, and AG expression that occurred during re-irrigation in water-deficit stressed trees not treated with GA3. Applications of GA3 to water-deficit stressed trees reduced inflorescence number 95% compared to stressed trees without GA3. Thus, GA3 inhibited citrus floral development in response to water-deficit stress through downregulating AP1 and AP2 expression, which likely led to the failed activation of the downstream floral organ identity genes. The results reported herein suggest that bud determinacy and subsequent floral development in response to water-deficit stress in ‘Washington’ navel orange are controlled by AP1 and AP2 transcript levels, which regulate downstream floral organ identity gene activity and the effect of GA3 on citrus flower formation. The water-deficit stress floral-induction pathway provides an alternative to low-temperature induction that increases the potential for successful flowering in citrus trees grown in areas experiencing warmer, drier winters due to global climate change.