Water, heat and fruit size: timing is everything
Deficit irrigation can save a fifth of the water without hurting yield, or cost a sixth of the crop. The difference is when it happens.
Spanish research on regulated deficit irrigation in citrus shows that the same water saving can be harmless or expensive depending on the growth phase in which it falls.
Sensitive and tolerant windows
Flowering and fruit set are highly sensitive: water stress then increases fruit drop and reduces yield.1 In Navelina oranges, restricting water to 40 to 60 percent during early summer saved 16 to 23 percent of water without affecting yield or fruit quality.2 In Salustiana, a 20 percent seasonal saving in spring cost 8 percent of yield, while a restriction late in fruit growth cost only 4 percent, and those losses came from smaller fruit rather than fewer fruit.1
Late Phase II is the danger zone. In a commercial clementine orchard, a 28 percent seasonal restriction cut yield by 15 percent, mainly through smaller fruit.1 In late Fortune mandarins, late Phase II was the period most sensitive to water stress, and the mean equatorial diameter fell 10 percent below the control under stress.3
Thresholds you can measure
Studies report midday stem water potential thresholds of about −1.3 to −1.5 MPa if fruit size reductions are to be avoided.3 Fruit growth itself is a signal: in satsuma mandarin, a daily diameter increase below 0.25 mm indicated a predawn leaf water potential of −0.8 MPa or lower.4
Heat changes the peel
In satsuma, temperatures 2 °C or more above average during early fruit development increased peel puffing and reduced quality. Higher fruit weight under early heat came partly from a higher peel share.5 Colour needs cold: satsuma peel began to degreen only once daily minimum temperatures fell to 16 °C or below.6
What this means in the orchard
A weekly growth curve per block is an early stress sensor. When diameter gains slow down in Phase II, check irrigation first. Saving water is safest outside flowering and late Phase II.
Sources
- Ruiz-Sánchez M.C., Domingo R., Castel J.R. (2010). Review. Deficit irrigation in fruit trees and vines in Spain. Spanish Journal of Agricultural Research 8(S2). doi
- Gasque M., Granero B., Turegano J.V., González-Altozano P. (2010). Regulated deficit irrigation effects on yield, fruit quality and vegetative growth of ‘Navelina’ citrus trees. Spanish Journal of Agricultural Research 8(S2). doi
- Conesa M.R. et al. (2018). Deficit irrigation in commercial mandarin trees: water relations, yield and quality responses. Spanish Journal of Agricultural Research 16(3). doi
- Komiya et al. (2025). Development of a fruit size estimation method using Mask RCNN for water stress estimation in Satsuma mandarin. PLoS ONE 20(7). doi
- Sato K., Ikoma Y. (2020). Effects of elevated temperatures during flowering to physiological fruit drop and at maturation on fruit quality of satsuma mandarin. Journal of Agricultural Meteorology 76(1). PDF
- Mitalo O.W. et al. (2022). Examining the role of low temperature in satsuma mandarin fruit peel degreening. Frontiers in Plant Science 13. doi