Biological and Practical Importance of Light Microenvironments in a Tree: Participatory Research to Match Teaching and Learning Styles G
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Biological and Practical Importance of Light Microenvironments in a Tree: Participatory Research to Match Teaching and Learning Styles G. A. Picchioni,* S. A. Weinbaum, D. L. Daniel, and H. Karaca ABSTRACT usual profit-generating portion of the tree, are driven by the photosynthetic activity of nearby leaves and the importation Productivity and crop quality vary strongly among the diverse light microenvironments within a tree’s canopy. An effective of carbohydrates (Marschner, 1995, p. 131). The level of nat- teaching exercise to convey this biologically and agriculturally ural irradiance within tree canopies is spatially linked to the important principle to plant science students is lacking. We ap- allocation of metabolic resources such as reduced carbon (C) plied a simple participatory learning approach to teach the im- and nitrogen (N) (Flore and Lakso, 1989; Weinbaum et al., portance of light microenvironment in the dense canopy of a ma- 1989). Accordingly, the light microclimate conditions the lo- ture pecan tree [Carya illinoinensis (Wangenh.) K. Koch ‘Sch- calized availability of C and N and determines in large mea- ley’]. Leaves and nuts were sampled along a steep light flux gra- sure the productivity and quality of the tree’s crop within that dient through the canopy, and specific leaf weight (leaf weight per portion of the canopy. Intracanopy light distribution is thus unit leaf area), was used as a simple, proven, and integrative mea- both physiologically and economically important. sure to quantify the incident light gradient. Simple regression At a given level of management, a tree’s quantity and qual- analysis revealed the importance of specific leaf weight (light ex- ity of yield involve a complex interaction between light in- posure) as a predisposing factor controlling nut quality and ni- tensity, relative light distribution, time, space, and the translo- trogen (N) allocation. Multiple regression analysis further aided cation of high-energy products from leaves to fruit. In addi- systematic learning in that it exposed students to statistical com- tion, the availability of light and number of fruit on a branch plexities in determining major sources of variation in nut at- during 1 yr can have a pronounced impact on the production tributes (“masking” of one independent variable by another). and quality of that branch during the succeeding year, which This exercise enabled participatory learning in the scientific describes a localized carry-over effect (Klein et al, 1991b; method, including collection and processing of field samples, statistical analysis, data interpretation, and hypothesis testing. Tustin et al., 1992). The self-generated findings induced a sense of ownership among As shown by the literature, the study of tree canopy–light students, resulting in positive learning outcomes and strength- relationships combines practical and scientific learning ele- ening the match between teaching and learning styles beyond the ments along with challenges to the teacher, all of which are traditional classroom lecture approach. The work also provided consistent with the needs of a plant science curriculum. Our an experiential, discovery-based setting that addressed a deficit own observations on teaching at the undergraduate level in- in the scientific pecan literature. The exercise is simple, inex- dicate that (i) it is difficult to adequately convey this type of pensive, fits nicely within a semester time-frame, and broadly ap- subject matter through traditional lectures and readings, and plicable. It encourages active student participation in an in- (ii) undergraduate students generally prefer a hands-on labo- quiry-driven environment. ratory setting to enliven their initial exposure to new and technical material. Despite student preferences for active learning, it has recently been stated that overhead trans- OR NEARLY TWO CENTURIES, sunlight has been recognized parencies and chalk boards remain the primary teaching tools Fas an indispensable natural resource for plant growth and in today’s agricultural classroom (Whittington, 1997a). Thus, survival (Epstein, 1972, p. 8), and interest in its role in the pro- we either lack or underutilize creative, experiential teaching ductivity and quality of tree crops has increased. The contin- techniques. Increased use of hands-on, participatory learning ued relevance of this topic is apparent in trade journals devoted strategies to teach technical subjects would undoubtedly im- to tree crop production (e.g., Green, 1997; Phillips, 1997; prove students’ critical thinking skills, understanding and use McEachern, 1999), as well as in scientific journals (e.g., of the scientific method, spirit of inquiry, ability to interpret Kuden and Son, 1997; Fallahi and Kilby, 1997; Worley and experimental data, practical experience, and employability Mullinix, 1997; Grossman and DeJong, 1998; Guimond et al., upon graduation (Salvador et al., 1995; Stearns, 1995; Riley, 1998; Genard et al., 1998). 1997; Cantliffe and Kostewicz, 1998; Corak and Grabau, The biological and agricultural significance of light (pho- 1988). tosynthetic photon flux) gradients within a tree’s canopy is im- Although technical accounts of the influence of light on tree portant to convey to plant science students. Dry matter accu- productivity and fruit quality are numerous, we could find no mulation (growth) and chemical composition of fruit, the published material that utilized this concept for teaching. Thus, an experientially driven method to demonstrate phe- G.A. Picchioni, Dep. of Agronomy and Horticulture, New Mexico State notypic plasticity and the importance of light and light distri- Univ., Las Cruces, NM 88003; S.A. Weinbaum, Dep. of Pomology, Univ. of California, Davis, CA 95616; D.L. Daniel, Dep. of Economics/International bution on tree productivity and fruit quality appeared war- Business and Univ. Statistics Center, New Mexico State Univ., Las Cruces, ranted. Such a method would ideally be simple, require min- NM 88003; and H. Karaca, Soil Science Dep., Mustafa Kemal Univ., imal expense and equipment, fit within the limited time frame Serinyol-Hatay 31000, Turkey. Received 14 Sept. 1999. Corresponding au- of the syllabus, and require active participation by the students. thor ([email protected]). Specific leaf weight (leaf mass per unit leaf area) varies Published in J. Nat. Resour. Life Sci. Educ. 29:78–87 (2000). with light intensity (Tustin et al., 1992), and represents an in- http://www.JNRLSE.org tegration of incident light upon a given position inside the tree 78 • J. Nat. Resour. Life Sci. Educ., Vol. 29, 2000 canopy (Weinbaum et al., 1989; Klein et al., 1991a). Com- Sprugel et al., 1991; Weinbaum et al., 1994). Thus, we relied pared with other techniques of light assessment, using specific on simple and multiple regression analyses to provide a sys- leaf weight as a bioassay to quantify specific light microen- tematic linkage between observational data and positive in- vironments within tree canopies is easy (Marini and Barden, terpretive outcomes. We anticipated that this approach would 1981). In addition to its strong dependence on natural irradi- help plant science students develop a combined awareness of ance, specific leaf weight has also been shown to be positively the biological and agricultural significance of light distribu- correlated with the allocation of N to leaves, leaf photosyn- tion in the tree canopy. thetic capacity, and the number, growth, and quality of fruit at discrete canopy positions. Such findings have been ob- MATERIALS AND METHODS tained for peach [Prunus persica (L.) Batsch] (DeJong and Suggested Reading Doyle, 1985), prune (Prunus domestica L.) (Weinbaum et al., 1989; Southwick et al., 1990), apple (Malus × domestica Ideally, students should be given preparatory time in the Borkh.) (Barritt et al., 1987 and 1991), and walnut (Juglans form of literature review,orareading primer. The contents regia L.) (Klein et al., 1991a, 1991b). extracted from that preparation could be summarized in writ- Pecan [Carya illinoinensis (Wangenh.) K. Koch] is the ing and evaluated for grading purposes, if so desired by the only U.S. commercial tree fruit and nut crop indigenous to instructor.We focused on important light-dependent tree phys- North America. Production statistics illustrate the global im- iological parameters and included pecan-specific literature portance of U.S. pecan cultivation (National Agricultural Sta- when appropriate. Selections (below) are only suggested and tistics Service, 1998) and the expanding land areas in the could no doubt be expanded with more thorough review of lit- semiarid southwestern U.S., including far west Texas, south- erature. ern New Mexico, much of Arizona, and central California (Peña, 1995). There are virtually no published data on the re- Measurement, significance, and biochemical/structural de- lationship between irradiance, specific leaf weight, and pecan terminants of specific leaf weight (Marini and Barden, 1981; Weinbaum et al., 1989) nut production and quality. Related research on pecan has been The relationship between specific leaf weight, light, and limited to pruning and tree thinning studies to improve light crop quality (Barritt et al., 1987; Southwick et al., 1990; penetration and maintain production and quality (Worley, Corelli-Grapadelli and Coston, 1991; Klein et al., 1985, 1993; Amling et al., 1985; Worley and Mullinix, 1997). 1991b; Tustin et al., 1992; Campbell and Marini, 1992) Those studies were based on entire,