Mature Sweet Cherries Have Low Turgor

The pressure inside a mature sweet cherry (Prunus avium L.) fruit is thought to be an important factor in rain cracking. However, to our knowledge, this pressure has never been quantified directly. The objectives of this study are to quantify: 1) the cell turgor (Y cell P ) in fruit using a cell pressure probe (CPP) and a vapor pressure osmometer (VPO); and 2) the tissue pressure in a fruit (Y fruit P ) using both a fruit pressure probe (FPP) and a compression-plate technique (CP). The value of Y cell P in mesocarp cells of mature sweet cherry fruit averaged 28.1 kPa in ‘Samba’ and 17.5 kPa in ‘Sam’ at depths below the fruit surface between 200 and 400 mm. A Y cell P (range 38 to 64 kPa for different cultivars) calculated from the tissue water potential (Y tissue = –2968 to –4035 kPa) and the osmotic potential (Y fruit P ) (Y fruit P = –3020 to –4116 kPa) of excised mesocarp discs as determined by VPO was of the same order of magnitude as that by CPP. Similar low Y fruit P values were obtained by FPP (range 8.0 to 11.8 kPa across cultivars). The Y fruit P were consistently lower than the Y cell P values measured by CPP or by VPO. The Y fruit P value in the mesocarp increased slightly with increasing depth below the surface. However, Y fruit P was always negligible (e.g., ‘Samba’ Y fruit P = 10 kPa) compared with either Y fruit P (‘Samba’ Y fruit P = –2395 kPa) or calculated water potential (Y ) (‘Samba’ Y fruit = –2385 kPa). When subjecting intact fruit to CP, linear relationships were obtained between the forces applied and the resulting aplanation areas. The Y fruit P values obtained by CP (range in sweet cherry 18.4 to 36.1 kPa) were somewhat larger than the Y fruit P values obtained by FPP (range in sweet cherry 8.0 to 11.8 kPa). Incubating fruit for up to 7.5 h in deionized water or for up to 96 h in air enclosed above dry silica gel had no measurable effects on Y fruit P . The low Y cell P and the low Y fruit P values are not unique to sweet cherry. Values of the same order of magnitude were obtained also in mature sour cherry (Prunus cerasus L.), european plum (Prunus domestica L.), grape (Vitis vinifera L.), gooseberry (Ribes uva-crispa L.), red currant (Ribes rubrum L.), black currant (Ribes nigrum L.), blueberry (Vaccinium corymbosum L.), and tomato (Solanum lycopersicum L.). Possible explanations for the very low values of Y cell P and Y fruit P are discussed. Rain-induced cracking severely limits production of many soft-textured, drupe, and berry fruits. Sweet cherry is a prominent example of the former (Christensen, 1996). Cracking is commonly assumed to result from increased fruit turgor, caused by osmotic water uptake through the wetted fruit skin or the pedicel during and after precipitation or heavy dew (Considine and Kriedemann, 1972; Measham et al., 2009; Sekse, 1995; Sekse et al., 2005). As a consequence of water uptake, fruit volume and, hence, skin surface area must increase. It is believed that as the skin surface area increases, the fruit will crack because the skin area increase exceeds the natural limit of its extensibility. The associated pressure immediately before cracking is referred to as the critical turgor pressure (Considine and Kriedemann, 1972; Measham et al., 2009). In discussions of plant water relations, cell turgor pressure is usually understood in terms of the sensible pressure in the semifluid protoplast when it is constrained within a taut cell wall. Cell turgor is usually (but not always) defined relative to atmospheric pressure. In this explanation of fruit cracking, the term ‘‘turgor’’ is used analogously to this to refer to the tissue pressure in a soft fruit’s semifluid parenchymatous mesocarp when it is constrained within a taut fruit skin. That is, the entire fruit is considered to behave in a way analogous to a single plant cell. Although the critical turgor pressure model seems to offer a logical conceptual framework for explaining fruit cracking, it has not, to our knowledge, been critically examined and the evidence supporting it is mostly circumstantial or indirect. The objectives of our present study therefore were 1) to quantify Y fruit P and Y cell P in mature sweet cherry fruit using pressure probes, water vapor pressure osmometry, and compression plates; and 2) to establish the effects of water uptake and water loss on Y fruit P . Materials and Methods PLANT MATERIAL. Fruits of uniform size and color were harvested at commercial maturity from greenhouse-grown sweet cherries (‘Kordia’, ‘Korvic’, ‘Sam’, ‘Samba’, ‘Sweetheart’, ‘Staccato’) and sour cherries (‘Ungarische Traubige’) both grafted on ‘Gisela 5’ rootstocks (P. cerasus · Prunus canescens Bois) or from field-grown european plum (‘Doppelte Hauspflaume’ grafted on Prunus insitia L. ‘St. Julien A’), gooseberry (‘Invicta’), red currant (‘Rovada’), black currant (‘Titania’), and blueberry (‘Elliott’) at the Horticultural Research Station in Sarstedt (lat. 52 14# N, long. 9 49# E) or the Herrenhausen campus garden of the Leibniz University Hannover (lat. 52 23# N, long. 9 42# E). Grapes were obtained from a commercial vineyard at Gleidingen [‘Fanny’ (lat. 52 16# N, long. 9 42# E)] or an experimental vineyard at Neustadt/ Weinstraße [‘Riesling’ (lat. 49 22# N, long. 8 11# E)]. Tomato (‘Micro-Tom’) was cultivated in a greenhouse on the Received for publication 12 Aug. 2013. Accepted for publication 15 Oct. 2013. This research was funded in part by a grant from the Deutsche Forschungsge-