Final fruit size is decided earlier than you think
Citrus fruit grow in three phases. By the time the crop looks ready, most of the decisions about its size have already been made.
Citrus fruit development follows three phases. Phase I is cell division and slow growth, lasting roughly two months from flowering to the June drop. Phase II is rapid growth by cell enlargement and water accumulation, lasting four to six months. In Phase III, ripening, growth largely stops.1
The size is set in Phase II
Australian guidance for navel oranges puts it plainly: once cell division ends and enlargement begins, the fruit has a predetermined final size as early as the start of enlargement, provided the trees are not stressed.2 In lemons, Phase II is likewise the stage that determines final fruit size, and earlier cultivars show a steeper growth slope during it.3
Early prediction works
Because the curve is so regular, it can be modelled. In Valencia Late oranges, logistic growth models fitted to weekly caliper data reached R² of 0.94 to 0.96, and Gompertz models 0.95 to 0.97.4 For Washington Navel, measuring tagged fruit every two weeks during early Phase II predicted final size and size distribution five to six months before harvest, with R² of 0.81 to 0.85 against actual harvest weight over three seasons.5,6
An early warning threshold
The same guide gives a practical trigger: if more than half of the fruit are still smaller than 40 mm at the start of the enlargement period, the crop load is too high or the trees or their management have a problem, and action is needed.2
What this means in the orchard
Measuring only at harvest is measuring the result. Weekly measurements from early Phase II give a growth curve that can be compared with the expected one, while thinning, irrigation and nutrition can still change the outcome.
Sources
- Iglesias D.J. et al. (2007). Physiology of citrus fruiting. Brazilian Journal of Plant Physiology 19(4). PDF
- Bevington K. et al. (2003). Fruit Size Management Guide, Part 1. Australian Citrus Growers. PDF
- NSW Agriculture / Citrus Australia. Growing Lemons in Australia: a production manual, chapter 20.
- Avanza M.M., Bramardi S.J., Mazza S.M. (2008). Statistical models to describe the fruit growth pattern in sweet orange ‘Valencia late’. Spanish Journal of Agricultural Research 6(4). PDF
- Khurshid T., Braysher B. (2009). Early fruit size prediction model using cubic smoothing splines for ‘Washington Navel’ oranges. International Journal of Fruit Science 9. doi
- Neupane C., Pereira M., Koirala A., Walsh K.B. (2023). Fruit sizing in orchard: a review from caliper to machine vision with deep learning. Sensors 23(8):3868. doi