Pecan is the most valuable nut tree native to North America (Hall, 2000). Its native range extends from northern Illinois and southeastern Iowa to the Gulf Coast of the United States where it grows abundantly along the Mississippi River, the rivers of central and eastern Oklahoma, and the Edwards Plateau in Texas (Thompson and Grauke, 1991). Isolated populations are also found in Mexico, where it grows as far south as the state of Oaxaca (Thompson and Grauke, 1991). Today, pecan is cultivated outside its native range, and commercial production has been expanded to many other regions of the United States and Mexico.
The tree canopy is a complex solar collection system possessing leaf and shoot subsystems consisting of numerous parts. Orchard crops, such as pecan, typically intercept 65% to 70% of the available sunlight (Wood, 1996) with up to 95% light interception in overcrowded, unpruned orchards (Lombardini, 2006). Orchard profitability depends on efficient absorption and use of light because sunlight is the source of energy that drives the biological production of dry mass (Garriz et al., 1998). Therefore, many training and pruning techniques are designed to maximize total light interception as well as to ensure good light penetration into the canopy (Li and Lakso, 2004). Low irradiance affects net photosynthesis (PN) directly by reducing the utilization of photon energy, but this effect differs between plants and is dependent on their saturation irradiance (Gregoriou et al., 2007). Thus, the relationship between PN and PPF density provides information that is useful as a physiological model of leaf, plant, or canopy growth (Hanson et al., 1987).
Morphological, physiological, and biochemical modifications are required for acclimation of photosynthesis to high and low light levels (Björkman, 1981; Taiz and Zeiger, 2006). Leaves on the outside of the tree canopy are typically adapted to high light (sun leaves), while leaves growing in the more shaded, inner canopy generally exhibit adaptations to low light conditions (shade leaves) (Lambers and Poorter, 1992). Sun and shade leaves have additional contrasting characteristics. Sun leaves are able to increase light-saturated photosynthetic capacity by increasing soluble protein, rubisco activity, and components of the electron-transport chain (Olsen et al., 2002). Shade leaves have inherently low photosynthetic rates, contain more total chlorophyll per reaction center, have a smaller Chl a/b ratio, and are generally thinner than sun leaves (Björkman, 1981; Taiz and Zeiger, 2006).
Carbon assimilation is not the only component of plant gas exchange affected by solar radiation intensity. Species limited in natural distribution to moist sunny locations have dark respiration rate (Rd) values in the range of 0.67 to 1.33 μmol·m−2·s−1 of CO2, whereas Rd in species from the shaded floor of dense forests range from 0.10 to 0.27 μmol·m−2·s−1 of CO2 (Björkman, 1968). Light compensation point (LCP) is the level of illumination at which photosynthetic fixation of carbon dioxide matches respiratory loss. Reported LCP values for various fruit trees species range between 1 and 25 μmol·m−2·s−1 for shade leaves and between 28 and 67 μmol·m−2·s−1 for sun leaves (Vanden Heuvel et al., 2004). In sun leaves of nut trees, reported LCP values are 29 μmol·m−2·s−1 (Rosati et al., 2006) and 56 μmol·m−2·s−1 (Higgins et al., 1992) for almond [Prunus dulcis (Mill.) D.A. Webb] and 52 μmol·m−2·s−1 for hazelnut (Corylus avellana L.) (Hampson et al., 1996).
Several authors have studied the effect of light availability in orchard crops. Shading reduced total leaf area in walnut (Juglans regia L.) (Atanasova et al., 2003; Ryugo et al., 1980), altered leaf chlorophyll concentration and Chl a/b ratio in grapefruit (Citrus paradisi Macf.) and sweet orange (Citrus sinensis L.) (Syvertsen and Smith, 1984), hazelnut (Hampson et al., 1996), and peach [Prunus persica (L.) Batsch.] (Kappel and Flore, 1983), decreased the stomatal density in hazelnut (Hampson et al., 1996) and olive (Olea europaea L.) (Gregoriou et al., 2007), and reduced Rd, maximum net CO2 assimilation (Amax), lowered LCP in grapevine (Vitis vinifera L.) (Vanden Heuvel et al., 2004), and affected light-saturated photosynthesis in apple (Malus ×domestica Borkh.) (Li and Lakso, 2004). When pecan trees were shaded for 14 d with shadecloth (30% full sunlight), PN and transpiration rate were reduced compared with leaves in full sunlight by 60% to 70% and 50%, respectively (Andersen and Brodbeck, 1995). The same study also showed that short-term shading induced physiological adaptations such as reduced Rd, LCP and light saturation point.
Pruning is a common practice in orchard management for the improvement of light penetration and utilization and for increasing flowering and fruiting. Hedge-pruning increased the amount of light penetration between pecan trees, but not within individual tree canopies, and was not associated with any change in production and nut quality (Malstrom et al., 1982). A more recent study showed that some pruning techniques induced a proportional increase in the fraction of total solar radiation intercepted by the tree canopies, and that crowding reduced the amount of light intercepted per unit of leaf area (Lombardini, 2006).
While the short-term effect of artificially shaded leaves of young pecan trees has been investigated early in the growing season (Andersen and Brodbeck, 1995), there is no information about the photosynthetic performance of sun and shade leaves of mature pecan trees. In particular, little is known about the change in photosynthesis activity of pecan leaves throughout the growing season. The objective of the present work was to quantify the effects of differences in light intensity on the morphological characteristics and seasonal physiological performance of sun and shade leaves of field-grown pecan trees. The two cultivars chosen (Pawnee and Stuart) differ in their history and utilization and are probably the two most important ones used commercially (Grauke and Thompson, 1995; Thompson and Grauke, 1991; Thompson and Grauke, 2000; Thompson and Hunter, 1985). ‘Stuart’ has been the most popular cultivar for almost a century (Grauke and Thompson, 1995), while ‘Pawnee’ has recently become the most widely planted pecan cultivar (Thompson and Grauke, 2000). Both cultivars have a moderate tendency to biennial bearing, but there are few published results on their seasonal carbon assimilation and the effect of additional light received at the leaf level.
Andersen, P.C. & Brodbeck, B.V. 1995 Light preconditions and fluctuating irradiance levels influence gas exchange of pecan leaves 148 159 Smith M.W. , Reid W. & Wood B.W. Sustaining pecan productivity into the 21st century: 2nd Natl. Pecan Wkshp. Proc. U.S. Dept. Agr., Agr. Res. Serv. 1995–3
- Search Google Scholar
- Export Citation
Andersen, P.C. Brodbeck, B.V. 1995 Light preconditions and fluctuating irradiance levels influence gas exchange of pecan leaves 148 159 Smith M.W. Reid W. Wood B.W. Sustaining pecan productivity into the 21st century: 2nd Natl. Pecan Wkshp. Proc. U.S. Dept. Agr., Agr. Res. Serv. 1995–3
Aschan, G. & Pfanz, H. 2003 Non-foliar photosynthesis: A strategy of additional carbon acquisition Flora 198 81 97
Atanasova, L. , Stefanov, D. , Yordanov, I. , Kornova, K. & Kavardzikov, L. 2003 Comparative characteristics of growth and photosynthesis of sun and shade leaves from normal and pendulum walnut (Juglans regia L.) trees Photosynthetica 41 289 292
Björkman, O. 1968 Carboxydismutase activity in shade-adapted and sun-adapted species of higher plants Physiol. Plant. 21 1 10
Björkman, O. 1981 Responses to different quantum flux densities 57 107 Lange O.L. , Nobel P.S. , Osmond C.B. & Zeigler H. Encyclopedia of plant physiology, New Series Springer Berlin
Crews, C.E. , Worley, R.E. , Syvertsen, J.P. & Bausher, M.G. 1980 Carboxylase activity and seasonal changes in CO2 assimilation rates in three cultivars of pecan J. Amer. Soc. Hort. Sci. 105 798 801
Garriz, P.I. , Colavita, G.M. & Alvarez, H.L. 1998 Fruit and spur leaf growth and quality as influenced by low irradiance levels in pear Scientia Hort. 77 195 205
Grauke, L.J. & Thompson, T.E. 1995 Pecan cultivars 12 June 2009 <http://extension-horticulture.tamu.edu/carya/pecans/cvintro.htm>.
Grauke, L.J. , Storey, J.B. & Emino, E.R. 1987 Influence of leaf age on the upper and lower leaf surface features of juvenile and adult pecan leaves J. Amer. Soc. Hort. Sci. 112 835 841
Greer, D.H. 1995 Effect of canopy position on the susceptibility of kiwifruit (Actinidia deliciosa) leaves on vines in an orchard environment to photoinhibition throughout the growing season Aust. J. Plant Physiol. 22 299 309
Gregoriou, K. , Pontikis, K. & Vemmos, S. 2007 Effects of reduced irradiance on leaf morphology, photosynthetic capacity, and fruit yield in olive Photosynthetica 45 172 181
Hampson, C.R. , Azarenko, A.N. & Potter, J.R. 1996 Photosynthetic rate, flowering, and yield component alteration in hazelnut in response to different light environments J. Amer. Soc. Hort. Sci. 121 1103 1111
Hanson, P.J. , McRoberts, R.E. , Isebrands, J.G. & Dixon, R.K. 1987 An optimal sampling strategy for determining CO2 exchange rate as a function of photosynthetic photon flux density Photosynthetica 21 98 101
Higgins, S.S. , Larsen, F.E. , Bendel, R.B. , Radamaker, G.K. , Bassman, J.H. , Bidlake, W.R. & Wir, A.A. 1992 Comparative gas exchange characteristics of potted, glasshouse-grown almond, apple, fig, grape, olive, peach and asian pear Scientia Hort. 52 313 329
- Search Google Scholar
- Export Citation
Higgins, S.S. Larsen, F.E. Bendel, R.B. Radamaker, G.K. Bassman, J.H. Bidlake, W.R. Wir, A.A. 1992 Comparative gas exchange characteristics of potted, glasshouse-grown almond, apple, fig, grape, olive, peach and asian pearScientia Hort. 52 313 329 10.1016/0304-4238(92)90032-8
Jarvis, P.G. & McNaughton, K.G. 1986 Stomatal control of transpiration: Scaling up from leaf to region Adv. Ecol. Res 15 1 49
Kappel, F. & Flore, J.A. 1983 Effect of shade on photosynthesis, specific leaf weight, leaf chlorophyll content, and morphology of young peach trees J. Amer. Soc. Hort. Sci. 108 541 544
Kirk, J.T.O. 1968 Studies on the dependence of chlorophyll synthesis on protein synthesis in Euglena gracilis, together with a nomogram for determination of chlorophyll concentration Planta 78 200 207
Lambers, H. & Poorter, H. 1992 Inherent variation in growth rate between higher plants: A search for physiological causes and ecological consequences Adv. Ecol. Res 23 187 261
Li, K.T. & Lakso, A.N. 2004 Photosynthetic characteristics of apple spur leaves after summer pruning to improve exposure to light HortScience 39 969 972
Lombardini, L. 2006 One-time pruning of pecan trees induced limited and short-term benefits in canopy light penetration, yield and nut quality HortScience 41 1469 1473
Malstrom, H.L. , Riley, T.D. & Jones, J.R. 1982 Continuous hedge pruning affects light penetration, and nut production of ‘Western’ pecan trees Pecan Qrtly. 16 4 15
Markwell, J. , Osterman, J. & Mitchell, J. 1995 Calibration of the Minolta SPAD-502 leaf chlorophyll meter Photosynth. Res. 46 467 472
Olsen, R.T. , Ruter, J.M. & Rieger, M.W. 2002 Photosynthetic responses of container-grown Illicium L. taxa to sun and shade J. Amer. Soc. Hort. Sci. 127 919 924
Rosati, A. , Metcalf, S.G. , Buchner, R.P. , Fulton, A.E. & Lampinen, B.D. 2006 Physiological effects of kaolin applications in well-irrigated and water-stressed walnut and almond trees Ann. Bot. (Lond.) 98 267 275
Ryugo, K. , Marangoni, B. & Ramos, D.E. 1980 Light intensity and fruiting effects on carbohydrate contents, spur development, and return bloom of ‘Hartley’ walnut J. Amer. Soc. Hort. Sci. 105 223 227
Sagaram, M. , Lombardini, L. & Grauke, L.J. 2007 Variation in leaf anatomy of pecan cultivars from three ecogeographic locations J. Amer. Soc. Hort. Sci. 132 592 596
Sparks, D. & Brack, C.E. 1972 Return bloom and fruit set of pecan from leaf and fruit removal HortScience 7 131 132
Syvertsen, J.P. & Smith, M.L. 1984 Light acclimation in citrus leaves. I. Changes in physical characteristics, chlorophyll, and nitrogen content J. Amer. Soc. Hort. Sci. 109 807 812
Vanden Heuvel, J.E. , Proctor, J.T.A. , Fisher, K.H. & Sullivan, J.A. 2004 Shading affects morphology, dry-matter partitioning, and photosynthetic response of greenhouse-grown ‘Chardonnay’ grapevines HortScience 39 65 70
Wood, B.W. 1988 Fruiting affects photosynthesis and senescence of pecan leaves J. Amer. Soc. Hort. Sci. 113 432 436