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Susceptibility of sweet cherry (Prunus avium L.) fruit to rain cracking increases toward maturity and is thought to be related to increases in both tissue pressure (Y fruit P ) and cell pressure (Y cell P ). Furthermore, at a given water potential (Y), one might expect the increase in Y fruit P and the Y cell P to balance the decrease in osmotic potential (Y fruit P ). The objectives of our study were to quantify Y fruit P and Y cell P in developing sweet cherry using vapor pressure osmometry (VPO), compression plate (CP), and the cell pressure probe (CPP). In addition, the tissue water potential Y was determined by quantifying the bending of strips of fruit skin and the change in projected area of discs excised from the flesh when incubated in a range of sucrose solutions of varying osmotic potentials (Y P ). Fruit growth followed a sigmoid pattern with time with the Stage II/Stage III transition occurring at ’55 days after full bloom (DAFB). The Y and the Y fruit P were constant up to ’55 DAFB but decreased to –2.8 MPa at maturity. The Y cell P calculated by subtracting the Y fruit P from Y tissue averaged ’350 kPa up to 48 DAFB and then decreased at a decreasing rate to ’21 kPa toward maturity. The Y determined from bending assays using excised skin strips or from water uptake of excised flesh discs was essentially constant up to ’48 DAFB, then decreased until ’75 DAFB and remained constant thereafter. These Y values were in good agreement with those determined by VPO. The Y fruit P as determined by CP passed through a transient peak at ’41 DAFB, then decreased until ’63 DAFB and remained constant and low until maturity. Similarly, Y P by CPP increased from 27 to 48 DAFB, remained constant until ’55 DAFB, and then decreased until maturity. Our data demonstrate a consistent decrease in Y fruit P and Y cell P that coincides with a decrease in Y fruit P of sweet cherry during Stage III. Because Y fruit P and Y cell P are low relative to Y fruit P , the change in Y fruit parallels that in Y fruit P . The reason for the low turgor most likely lies in the accumulation of apoplastic solutes. These prevent a catastrophic increase in pressure that would otherwise lead to the bursting of individual cells and the cracking of entire fruit. Many soft and fleshy fruit crack on exposure of the fruit to surface water. Osmotic water uptake that occurs along a gradient in water potential between the surface water on the fruit and the water inside the fruit is thought to be causal. The water potential of the fruit Y comprises two major components, an Y fruit P caused by the presence of significant concentrations of carbohydrates. The second component is the tissue pressure inside the fruit (Y fruit P ) that results from the strained and stressed fruit skin that holds the flesh under pressure in analogy with the turgor of an individual cell (Y P ) generated by the strained and stressed cell wall. Thus, the Y fruit P stimulates ingress of water, whereas the pressures Y fruit P and Y cell P work in opposition to one another and tend to limit the ingress of water. The Y fruit P of expressed juices of soft and fleshy fruit are very low (highly negative) and for mature (sweet) cherry and grape (Vitis vinifera L.), values of Y fruit P as low as –5 MPa have been reported (Tyerman et al., 2004). For a cherry grown under irrigated conditions and examined just before dawn, one might anticipate near-equilibrium water potential conditions within the fruit and so fruit (Y) and cell (Y) water potentials that are close both to one another and also to zero (i.e., Y Y 0). This assumption is based on the following arguments. First, the fruit surface is covered by a cuticle that represents the dominant rate-limiting barrier to transpiration. Thus, internal resistors to water transfer such as the plasma membranes should not be a significant factor. Their permeability is usually around three orders of magnitude larger than that of the cuticle. Second, transpiration at night is low (air humidity is usually high so driving force is usually low) and therefore, we would expect any gradient in water potential (that is unlikely to be present in the first place) to have disappeared as a result of equilibration. Third, we are not aware of any published data reporting an internal water potential gradient within a fruit. Under the conditions described, the rapid decreases in Y fruit P as maturity is approached (Knoche et al., 2004) might be expected to be balanced by corresponding rapid increases in tissue pressure (Y fruit P ) and cell pressure (Y cell P ). We note that the concept of tissue pressure is analogous to that of the cell pressure (or turgor) of an individual cell but refers to the whole fruit (occasionally Y fruit P is also referred to as fruit turgor). Consistent with these hypothetical increases in Y fruit P and Y P would be an increase in the fruit’s susceptibility to cracking, which fits with the idea that its susceptibility is related to increased tissue pressure caused by water uptake (Measham et al., 2009; Sekse, 1995; Sekse et al., 2005). Based on this understanding, water uptake increases fruit volume and, hence, fruit surface area. The fruit skin, being thus strained, accumulates increasing levels of stress, which generates a tissue Received for publication 2 Mar. 2014. Accepted for publication 10 Apr. 2014. This research was funded in part by a grant from the Deutsche Forschungsgemeinschaft. We thank Friederike Schroeder and Simon Sitzenstock for technical assistance and Drs. Fritz Lenz and Bishnu P. Khanal for helpful comments on an earlier version of this manuscript. Corresponding author. E-mail: moritz.knoche@obst.uni-hannover.de. J. AMER. SOC. HORT. SCI. 139(4):349–355. 2014. 349 pressure Y fruit P . Failure of the skin is expected to occur when its limit of extensibility is exceeded. The hypothetical tissue pressure threshold at which skin failure and cracking occurs is referred to as the critical tissue pressure (turgor) (Considine and Kriedemann, 1972; Measham et al., 2009). According to the critical tissue pressure concept, one would expect Y fruit P in a cherry to increase through the final stages of development [Stage III, ‘‘final swell’’ (Lilleland and Newsome, 1934; Tukey, 1934)] when the cracking susceptibility also increases (Christensen, 1996). Both lines of arguments imply that we should expect to record increases in Y fruit P and Y cell P during Stage III of development. Although this logic would seem to offer a satisfactory conceptual framework within which to understand fruit cracking, recent measurements of Y fruit P and Y cell P in mature cherry indicate: 1) that Y fruit P and Y cell P do not have high values but, instead, have values very close to zero (i.e., atmospheric pressure) (Knoche et al., 2014). Furthermore, 2) laboratory manipulations of fruit water status (e.g., by permitting isolated fruit to take up water by osmosis from a bathing solution or to lose water by transpiration to dry air) have essentially no effect at all on Y fruit P (Knoche et al., 2014). Thus, we infer that this conceptual framework may not be as useful as would first appear. To provide a more useful understanding of fruit water relations in cherry, Y fruit P and Y cell P were quantified in developing fruit using a vapor pressure osmometer, a compression plate, and a cell pressure probe. The value Y fruit was also determined by following changes in the curvatures of strips of fruit skin (with a layer of adhering flesh) and changes in the projected surface areas of excised flesh discs when incubated in sucrose solutions of a range of osmotic potentials. The study focused on Stage III of fruit development because this is the period characterized by particularly rapid changes in cracking susceptibility and it is also when Y fruit P , measured on expressed juice samples, suffers its most marked decrease (increase in negativity). Materials and Methods PLANT MATERIAL. Developing fruit of the ‘Regina’ sweet cherry of uniform size and color were sampled at regular intervals from greenhouse-grown trees grafted on ‘Gisela 5’ rootstocks (Prunus cerasus L. · Prunus canescens Bois) at the Horticultural Research Station of the Leibniz University Hannover at Sarstedt/Ruthe, Germany (lat. 52 14# N, long. 9 49# E). Unless otherwise specified, fruit was processed on the day of sampling. Fruit mass was determined [n = 20 (CPA 225D and BP 211D; Sartorius, Göttingen, Germany)] and fruit color quantified in the CIE 1976 (L*, a*, b*) scale as defined by the Commission Internationale de l’É-clariage (McGuire, 1992) using a chromometer (CR-200; Minolta, Osaka, Japan). To describe the change in color of the developing fruit, the hue angle was calculated as described by McGuire (1992). Three readings were taken per fruit on a total of 20 fruit per sampling date. MEASURING Y, Y fruit P , AND Y cell P USING VPO. Values of Y, Y fruit P , and Y cell P of the flesh of developing ‘Regina’ sweet cherry were measured using a vapor pressure osmometer (VAPRO 5520; Wescor, Logan, UT). Briefly, a skin + flesh disc (8 mm diameter) was excised from the cheek of a visually perfect fruit and the adhering flesh trimmed to 2 mm thickness. The sample was blotted gently using soft tissue paper (7216; Kimberly-Clark, Reigate, U.K.) and, within 60 s of excision, was placed in the sample holder (9.5 mm diameter, 4.5 mm depth) of the osmometer. The value of the tissue water potential (Y) was read at stability, 15 min after closure of the sample chamber. Preliminary studies established that this interval was sufficient to establish an equilibrium reading (Knoche et al., 2014). Next, the sample chamber was opened, the tissue disc was thoroughly crushed in the sample holder using a cylindrical glass rod (matching diameter of 9.5 mm), the chamber again closed, and a second value for Y was recorded after a further 15-min equilibration time. This second reading is assumed to represent an e

Water Potential and Its Components in Developing Sweet Cherry