Strawberry fruit skins are far more permeable to osmotic water uptake than to transpirational water loss

Abstract

Water movements through the fruit skin play critical roles in many disorders of strawberry (Fragaria × ananassa Duch.) such as water soaking, cracking and shriveling. The objective was to identify the mechanisms of fruit water loss (dry skin, transpiration) and water uptake (wet skin, osmosis). Fruits were held above dried silica gel or incubated in deionized water. Water movements were quantified gravimetrically. Transpiration and osmotic uptake increased linearly with time. Abrading the thin cuticle (0.62 g m-2) increased rates of transpiration 2.6–fold, the rates of osmotic uptake 7.9-fold. The osmotic potential of the expressed juice was nearly the same for green and for white fruit but decreased in red fruit stages. Fruit turgor was low throughout development, except for green fruit. There was no relationship between the rates of water movement and fruit osmotic potential. The skin permeance for transpiration and for osmotic uptake were both high (relative to other fruit species) but were two orders of magnitude greater for osmotic uptake than for transpiration. Incubating fruit in isotonic solutions of osmolytes of different sizes resulted in increases in fruit mass that depended on the osmolyte. The rate of osmotic uptake decreased asymptotically as molecular size of the osmolyte increased. When transpiration and osmotic uptake experiments were conducted sequentially on the same fruit, the rates of transpiration were higher for fruit previously incubated in water. Fluorescence microscopy revealed considerable microcracking in a fruit previously incubated in water. Our findings indicate that the high permeance for osmotic uptake is accounted for by an extremely thin cuticle and by viscous water flow through microcracks and along polar pathways.

Water Soaking Disorder in Strawberries: Triggers, Factors, and Mechanisms

“…(1) Surface wetness induced microcracking in the cuticle of the strawberry fruit when incubated in water in this and our earlier study (Hurtado et al, 2021 ) or in isotonic solutions (PEG 6000) ( Figure 9 ). This is consistent with the literature reports of other fleshy fruit crops, including sweet cherry (Knoche and Peschel, 2006 ), apples (Knoche and Grimm, 2008 ; Khanal et al, 2021 ), Ribes berries (Khanal et al, 2011 ), and grapes (Becker and Knoche, 2012a ).…”

“…Microcracks impair the barrier properties of the cuticle. They represent a pathway for a rapid and localized water uptake by viscous flow through the strawberry fruit surface (Hurtado et al, 2021 ). Microcracking is an essential, but not the only requirement, in water soaking.…”

“…It is important to note that microcracking of the cuticle due to moisture exposure also accounts for the localized nature of water soaking. Microcracks impair the cuticle's barrier function and allow a rapid, localized water uptake (Hurtado et al, 2021 ). This also explains why partial incubation of fruit resulted in water soaking only in the exposed regions.…”

“…First, the barrier function of the cuticle is impaired by the formation of microcracks in response to surface wetness ( Figure 2 ). Because the cuticle of a strawberry is very thin (Hurtado et al, 2021 ), microcracks allow a rapid, localized water uptake into the subtending tissues. This uptake occurs by viscous flow and is more rapid than that by diffusion through an intact cuticle (Hurtado et al, 2021 ).…”

“…Because the cuticle of a strawberry is very thin (Hurtado et al, 2021 ), microcracks allow a rapid, localized water uptake into the subtending tissues. This uptake occurs by viscous flow and is more rapid than that by diffusion through an intact cuticle (Hurtado et al, 2021 ). Unlike in apple, there is no evidence of any repair processes in strawberry: wax deposition in a microcrack (Curry, 2009 ) or the formation of a periderm (Evert, 2006 ; Macnee et al, 2020 ).…”