Effects of water stress on fruit growth and water relations between fruits and leaves in a hedgerow olive orchard

Abstract

Yield, the final goal in agricultural systems, is highly determined by fruit growth. However, most works on deficit irrigation did not consider the physiology and dynamics of fruit growth. In this study, we explore the effect of water relations between leaves and fruits on fruit growth in well-watered (WW) and water-stressed (WS) trees in a super-high-density olive orchard (cv. Arbequina) over a full irrigation season (June–October of 2017) in southern Spain. Both leaf and fruit water potential were measured, along with pressure-volume curves. Concomitantly, fruit growth and diurnal changes in six fruits of each irrigation treatment were continuously monitored with fruit gauges, which recorded changes in the equatorial diameter of the fruit. Fruit water status and growth were greatly affected by water stress, the latter recovering markedly, following our regulated deficit irrigation strategy. The gradient between leaf and fruit water potential was positive most of the time for both irrigation treatments, and greater for WS than WW trees, which suggests water flow into the fruit. However, we also observed negative fruit growth in WS which, together with the positive gradient, suggests that water flow from the parent plant to the fruit was insufficient to balance fruit transpiration under conditions of water deficit. Although a strong correlation was found between leaf water potential and the amplitude of fruit contraction in WW trees, this relationship did not hold for WS trees, suggesting a progressive decoupling of fruit water status from leaves as water stress progressed. Accordingly, an osmotic adjustment as evidenced by the analyses of pressure-volume curves occurred in WS fruits towards the end of the season. These results indicate that leaf-fruit water potential measurements may not be sufficient to study fruit-leaf water relationships in the canopy; however, fruit gauges may serve as highly useful instruments to understand the water status and daily dynamics of fruit growth.