Unraveling water relations in growing fruit: insights from the epidermal growth regulation hypothesis

Abstract

This review focuses on the intricate water relationships between internal and external tissues in growing fruits within the framework of the epidermal growth control hypothesis. It considers the components of water potential, including turgor pressure and osmotic potential of both internal and external tissues, taking into account factors such as fruit growth rate, sugar accumulation, cell wall metabolism, and climacteric. It also examines the effects of environmental conditions, genetic factors, and physiological influences in modifying water relations. By emphasizing the significance of skin tissue water potential components as indicators of growth stress, the review underlines their importance for a comprehensive understanding of water relations and associated physiological disorders in growing fruit.

Keywords: environmental factors; fruit cracking; fruit growth; osmotic potential; physiological disorders; purple spot; skin tissue; turgor pressure.

Figure 1

Time course of water potential (A), osmotic potential (B), and cell turgor pressure (C) in the flesh and skin tissue of loquat through fruit growth in non-fruit-thinned plants. Arrows indicate the time of color break. Adapted from Gariglio et al. (2008a).

Figure 2

Time course of water potential (A), osmotic potential (B), and cell turgor pressure (C) in the flesh and skin tissue of loquat through fruit growth in plants thinned to one fruit per panicle. Arrows indicate the time of color break. Adapted from Gariglio et al. (2008a).

Figure 3

Xylem backflow in grape: phloem photoassimilates in sink tissues are unloaded preferentially symplasmically (A). However, during ripening, this process shifts to apoplastic pathways (B). The unloading of sucrose symplastically results in the accumulation of this substance within flesh cells, which, in turn, causes osmotic water uptake and an increase in turgor pressure. Conversely, when sucrose is partially unloaded via apoplastic pathways, it accumulates in the intercellular spaces of the flesh tissue. Consequently, when there is an excess influx of phloem water and sugars into the fruit, they are recirculated through the xylem (xylem backflow) as turgor pressure slightly exceeds the resistance offered by the external tissue. Light blue arrow: cellular osmotic water uptake; dashed arrow: turgor pressure; curved arrow: excess photoassimilates and water phloem recirculated through the xylem (xylem backflow). Created based on the results and discussion of Zhang and Keller (2016).

Figure 4

Water potential (Ψ_a), osmotic potential (π), and cell turgor pressure (P) in flesh and skin tissue of loquat fruit at color break with low (A) and high (B) risk of purple spot. Ψ_a = π + P. Dashed arrows indicate water flow; solid arrows indicate the origin of turgor. In the flesh, turgor is a consequence of osmotic water uptake (active turgor); in the skin, turgor is a consequence of flesh expansion reducing skin cell volume (passive turgor).