The cultivated strawberry was originally derived from the accidental hybridization of two wild, octoploid species, Fragaria chiloensis and Fragaria virginiana (Darrow, 1966). The resulting octoploid hybrid, Fragaria ×ananassa, is the basis for the modern strawberry cultivar. The discovery of the narrow genetic base of the cultivated strawberry (Sjulin and Dale, 1987) has led to a renewed interest in the use of wild Fragaria species in breeding. High photosynthetic rate is one of the many interesting characteristics observed in some of the wild species and may be exploited to improve photosynthetic rates of the cultivated strawberry (Hancock et al., 1989a). Reported net carbon exchange rate for the cultivated strawberry ranges from 15 to 25 μmol·m−2·s−1 (Hancock et al., 1989b; Schaffer et al., 1986); light saturation for strawberry cultivars is between 800 and 1000 μmol·m−2·s−1 (Cameron and Hartley, 1990; Ferree and Stang, 1988). Source-sink ratio can cause photosynthetic rates to fluctuate with sink demand (Iglesias et al., 2002). Strawberry fruit is a large sink and can account for 40% to 50% of total plant dry matter accumulation (Forney and Breen, 1985) and the presence of fruit increases NCER when compared with non-fruiting plants (Forney and Breen, 1985; Schaffer et al., 1986; Strik, 1987). In the strawberry cultivar Brighton, the photosynthetic capacity of the plant was not sufficient to meet sink demands during fruiting so translocation of reserve carbohydrates took place to meet demands of the developing fruit (Forney and Breen, 1985).
Two avenues to improve photosynthetic capacity are increasing the maximum photosynthetic rates under full light conditions and improving the stability of photosynthetic rates under a variable range of conditions. There has been limited success in yield improvement through selection for high photosynthetic rates (Hancock et al., 1989a) despite evidence that up to 90% of plant dry matter is accumulated through photosynthesis (Zelitch, 1982). The increases in yield over the last century have been largely the result of increases in harvest index and light interception; however, the role that photosynthesis has played is not completely understood (Richards, 2000). In several grain crops, maximum harvest indices are being approached and the need to increase the photosynthetic capacity may become necessary to realize further yield improvements (Richards, 2000). If genes for desirable photosynthetic characteristics can be incorporated into strawberry cultivars, there may be an opportunity to increase yield potential through increased dry matter accumulation and partitioning to fruit and storage organs. With adequate nitrogen (N) supply and light, increasing the amount of carbon assimilated per unit of N in the leaf could lead to increased biomass (Lawlor, 2002).
The use of wild Fragaria species to increase NCER has been investigated using the octoploid species, F. chiloensis, which has been shown to have 20% to 70% higher NCER than the strawberry cultivars (Cameron and Hartley, 1990). Hancock et al. (1989a) conducted a study to determine if these higher NCERs would be observed in F. chiloensis × cultivar hybrids and found that mean NCERs of hybrids were positively correlated with percent F. chiloensis germplasm incorporated into the hybrid; i.e., as the proportion of F. chiloensis increased, NCER also increased. These findings suggested that the high photosynthetic capacity of F. chiloensis was heritable and appeared to be quantitatively controlled because the relationship between NCER and gene composition was linear.
A second avenue to improve plant productivity is by increasing photosynthetic adaptability over a range of light conditions. Hancock et al. (1989b) suggested that it is the stability of NCER over the growing season and during critical periods of development that are associated with yield and found a positive correlation between the seasonal stability of NCER and yield in strawberry. Strik (1987) measured NCER for strawberry cultivars throughout the season and found that it was positively related to the harvest index but not to yield. Crown dry weight, leaf dry weight, and leaf area during flower bud initiation (FBI) were correlated with yield the next season indicating the importance of vegetative growth during FBI in determining yield potential (Strik, 1987; Strik and Proctor, 1986). In perennial crops such as strawberry in which competing sinks (e.g., fruiting and vegetative growth) are constantly present, high photosynthetic rates at critical times will help to ensure that demands for carbohydrates can be met. Single leaf photosynthetic measurements can be a useful measure of photosynthetic potential (Hancock et al., 1989b) for this type of measurement.
Most of the interest in using wild species germplasm to improve the cultivated strawberry has been focused on the progenitor species, F. chiloensis and F. virginiana. NCERs for strawberry cultivars are intermediate to its progenitor species, F. virginana and F. chiloensis (Hancock et al., 1989a). Previous research has reported NCER for F. virginiana of ≈7 μmol·m−2·s−1 (Jurik, 1983; Jurik et al., 1979). In comparison, Hancock et al. (1989a) found mean maximum rates for five F. chiloensis clones to be 21.5 and 15.4 μmol·m−2·s−1 in the field and greenhouse, respectively. In a study that compared NCER of F. chiloensis with the cultivar Totem, F. chiloensis had rates that were 20% to 70% higher (Cameron and Hartley, 1990). In addition to the octoploid species, several lower-ploidy Fragaria species (2×, 4×, and 6×) have desirable characteristics such as extreme vigor (Harbut et al., 2009), unique flavors, disease resistance (Xue et al., 2005), and temperature adaptability (Harbut et al., 2010) that hold great potential as breeding germplasm (Harbut and Sullivan, 2004; Xue et al., 2005). The lower-ploidy species are found in a wide range of habitats including temperate, Mediterranean, grassland, and subtropical environments (Darrow, 1966; Hancock, 1990, 1999), which may influence photosynthetic capacity. Some of the species such as F. orientalis (4×) are found primarily in alpine forests and cold, dry areas in Asia, whereas F. moschata (6×) is found in heavily shaded forest habitats (Darrow, 1966). F. vesca (2×) is the most widely adapted species in the genus and inhabits regions of North America, South America, Europe, Asia, and Hawaii (Hancock, 1990, 1999). This species is found in a range of habitats including cool, high-light alpine conditions, shaded forests, and humid, temperate coastal habitats and is considered both heat- and cold-tolerant (Darrow, 1966; Staudt, 1989). If these Fragaria species have undergone ecotypic differentiation, adaptive traits such as photosynthetic rates, shade, and heat tolerance will be genetically based and therefore can be exploited through breeding to improve the adaptive strategies of the cultivated strawberry.
Attempts to carry out interspecific hybridizations have been made over several decades and were met with limited success resulting in the lower-ploidy species being largely overlooked for breeding purposes (Sangiacomo and Sullivan, 1994). Efforts to develop a method to incorporate lower-ploidy Fragaria species were started at the University of Guelph by Evans (1982) and were continued by Bors (2000) and Bors and Sullivan (1998). This method led to the creation of Fragaria species hybrids or synthetic octoploids. Each SO is a complex interspecific hybrid composed of at least two Fragaria species and can be easily crossed with the cultivated strawberry. The yield and vegetative growth of three lower-ploidy Fragaria species, SOs, F1 hybrids with the cultivated strawberry, were evaluated in field trials in Ontario, Canada (Harbut et al., 2009). SOs had high vegetative vigor and the F1 hybrids were also vigorous with a high flower number. The vegetative vigor and reproductive potential suggested high photosynthetic rates. The effect of temperature on NCER and dry matter partitioning has been addressed (Harbut et al., 2010). In the present study the photosynthetic characteristics of the lower-ploidy species, the SOs with breeding potential and the early generation progeny of SOs, and the cultivated strawberry are evaluated. The effect of ecological background on photosynthetic characteristics is also evaluated.
BorsR.H.2000A streamlined synthetic octoploid system that emphasizes Fragaria vesca as a bridge species. PhD diss. Univ. of Guelph Guelph Ontario Canada
DarrowG.M.1966The strawberry. History breeding and physiology. Holt Rinehart and Winston New York NY
GiauffretC.BonhommeR.DerieuxM.1997Heterosis in maize for biomass production, leaf area establishment, and radiation use efficiency under cool spring conditionsMaydica421319
HancockJ.F.1999Strawberries. CABI Publishing Wallingford UK
HancockJ.F.BringhurstR.S.1978Interpopulational differentiation and adaptation in the perennial, diploid species Fragaria vesca LAmer. J. Bot.65795803
HarbutR.M.SullivanJ.A.ProctorJ.T.A.SwartzH.J.2009Early generation performance of Fragaria species hybrids in crosses with cultivated strawberryCan. J. Plant Sci.8911171126
HarbutR.M.SullivanJ.A.ProctorJ.T.A.SwartzH.J.2010Temperature affects dry matter production and net carbon exchange rate of lower-ploidy Fragaria species and species hybridsCan. J. Plant Sci.90885892
IglesiasD.J.LlisoI.TadeoF.R.TalonM.2002Regulation of photosynthesis through souce:sink imbalance in citrus is mediated by carbohydrate content in leavesPhysiol. Plant.116563572
JiaoD.LiX.2001Cultivar differences in photosynthetic tolerance to photooxidation and shading in rice (Oryza sativa L.)Photosynthetica39167175
JurikT.W.ChabotJ.F.ChabotB.F.1979Ontogeny of photosynthetic performance in Fragaria virginiana under changing light regimesPlant Physiol.63542547
LarsonK.D.1994Stawberry p. 271–298. In: Schaffer B. and P.C. Andersen (eds.). Handbook of environmental physiology of fruit crops. Vol. I. CRC Press Boca Raton FL
LawlorD.W.2002Carbon and nitrogen assimilation in relation to yield; mechanisms are the key to understanding production systemsJ. Expt. Bot.53773787
MelchingerA.E.1999Genetic diversity and heterosis p. 99–118. In: Coors J.G. and S. Pandey (eds.). The genetics and exploitation of heterosis in crops. Amer. Soc. Agron. Madison WI
PotterD.LubyJ.J.HarrisonR.E.2000Phylogenetic relationships among species of Fragaria (Rosaceae) inferred from non-coding nuclear and chloroplast DNA sequencesSyst. Bot.25337348
SangiacomoM.A.SullivanJ.A.1994Introgression of wild species into the cultivated strawberry using synthetic octoploidsTheor. Appl. Genet.88349354
SarkerM.A.Z.MurayamaS.IshimineY.TsuzukiE.2001Physio-morphological characters of F1 hybrids of rice (Oryza sativa L.) in japonica-indica crosses: I. Heterosis for photosynthesisPlant Prod. Sci.4196201
SchafferB.BardenJ.A.WilliamsJ.M.1986Whole plant photosynthesis and dry matter partitioning in fruiting and deblossomed day-neutral strawberry plantsJ. Amer. Soc. Hort. Sci.111430433
SteelR.G.D.TorrieJ.H.DickeyD.A.1997Principles and procedures of statistics: A biometrical approach. 3rd Ed. McGraw-Hill New York NY
StrikB.C.1987Photosynthesis yield component analysis and growth analysis of strawberry. PhD diss. Univ. of Guelph Guelph Ontario Canada
StrikB.C.ProctorJ.T.A.1988The importance of growth during flower bud differentiation to maximizing yield in strawberry genotypesFruit Var. J.424548
XueS.BorsR.H.StrelkovS.E.2005Resistance sources to Xanthomonas fragariae in non-octoploid strawberry speciesJ. Amer. Soc. Hort. Sci.4016531656