Tung tree is a member of the Euphorbiaceae family, a native tree species that has been cultivated for more than 1000 years in China. Along with the oil-tea tree (Camellia oleifera), walnut (Juglans regia), and tallow tree (Sapium sebiferum), tung tree is one of the four major woody oil trees in China. Tung trees have been cultivated in central and southern areas of China (lat. 22°15′–34°30′N; long. 99°41′–122°07′E) and are usually cultivated in mountainous and hilly areas at less than 1000 m above sea level. Mature tung trees can reach ≈5 m, with a crown circumference of ≈4.5 m (Tan et al., 2011). Tung oil, which is extracted from tung seeds, exhibits traits that are highly valued in many industries (Park et al., 2008; Pfister et al., 2008), including rapid drying, chemical resistance, adhesiveness, and sleekness. These properties make tung oil a valuable drying ingredient in paints, varnishes, and other coatings and finishes (Cao and Shockey, 2012; Li et al., 2017b). With recent human population increases, tung trees have become a valuable biofuel species, with the potential to help resolve energy shortage problems (Tan et al., 2011). Furthermore, tung trees grow quickly, yielding fruit within 3 years as a result of their high photosynthetic efficiency (Li et al., 2017a).
The primary problem in tung tree production is the weak or dead lower branches of mature tung trees caused by heavy shade of the upper branches, resulting in a lower yield. For example, tung trees in a shady, sloped planting site had a 58.4% yield reduction with smaller fruit than those grown in a sunny planting site (Li and Zhu, 2014). In addition, most of the lower branches of mature tung trees appear to be dead, which seriously affects the healthy development of the tung tree industry in China (Fig. 1). To date, there have not been any studies on the effects of growth and photosynthesis of tung trees under different light intensities. Currently, the lowest DLI for suitable for tung tree growth is not known, especially in southern China, which has frequent cloudy and rainy days. By studying the growth and physiological responses of tung tree seedlings to different light intensities, the minimum DLI requirement can be determined for the growth of tung trees, which provides a theoretic basis and technical support for the determination of a suitable planting density of tung trees.
The productivity of plants depends on soil, LA index and efficiency of light conversion, and CO2 absorption (Lone and Khan, 2007). Light is a major environmental factor that affects leaf photosynthesis, traits, and plant growth, and determines the geographic distribution of plants (Kim et al., 2011). Plants experiencing shade stress often exhibit serious dysfunctions in terms of appearance and physiology, including reduced photosynthetic potential, stomatal density, and damage to various cellular structures (Holland and Richardson, 2009; Kim et al., 2011; Nobel et al., 1993; Tsukaya, 2005). Reduced photosynthesis may be the result of either stomatal closure restricting the availability of CO2 for carboxylation or nonstomatal inhibition caused by abiotic stress on the photosynthetic apparatus (Chen et al., 2009; Colla et al., 2012b; Rivelli et al., 2002). However, plants adopt different strategies to adapt to shade stress. For example, to absorb sufficient light energy, LA and plant height increase under shade conditions, increasing light-harvesting capabilities in a light-limited environment (Huang et al., 2016; Johnston and Onwueme, 1998; Khan et al., 2000). Similarly, superoxide dismutase and peroxidase in leaves increase in response to shade stress (Ou et al., 2015). Many authors (Morandi et al., 2011; Zibordi et al., 2009) have reported that shade causes a decrease in the net C exchange rates in young apple canopies. The decrease in fruit growth rate in young apple trees is mainly the result of a reduction in import through the phloem rather than a direct effect of shading on fruit sink strength. Iqbal et al. (2012) hypothesized that the photosynthetic potential of leaves that were lower on the plant axis was less than that of the upper leaves in the plant canopy. Previous studies have shown that light-use efficiency and decreased photosynthetic activity from the apex of the plant to the lower axis (Khan and Lone, 2005; Lone et al., 2008). Lugassi-Ben-Hamo et al. (2010) reported that shade treatments may have an adverse effect on plant growth or flower yield and quality in Lisianthus. Several studies have shown that shade tolerance is associated with a wide range of traits, including pigment biosynthesis, photosynthesis, and morphological and physiological traits (Huang et al., 2016; Khan et al., 2000; Kim et al., 2011).
In our study, we investigated the changes in growth, chlorophyll content, relative water content, photosynthesis, and enzyme activity under different light intensities. The aim of our study was to understand the acclimation mechanism under lower light conditions and to determine the optimal DLI for tung tree seedlings. From this DLI, we can determine the proper planting density and cultivation technique, such as pruning, to prevent mutual shading and ensure sufficient light for tung tree growth and high yield.
Blakey, R.J. & Bower, J.P. 2009 Bertling I.: Influence of water and ABA supply on the ripening pattern of avocado (Persea americana Mill.) fruit and the prediction of water content using near infrared spectroscopy Postharvest Biol. Technol. 53 72 76
Bower, J.P. 1985 Some aspects of water relations on avocado (Persea americana Mill.) tree and fruit physiology. Department of Horticultural Science, University of Natal, Pietermaritzburg, PhD Diss.
Bower, J.P. & Cutting, J.G.M. 1988 Avocado fruit development and ripening physiology, p. 229–271. In: J. Janick (ed.). Horticultural reviews, 10. Timber Press, Portland, OR
Brodribb, T. 1996 Dynamics of changing intercellular CO2 concentration (Ci) during drought and determination of minimum functional Ci Plant Physiol. 111 179 185
Cao, H.P. & Shockey, J.M. 2012 Comparison of TaqMan and SYBR green qPCR methods for quantitative gene expression in tung tree tissues J. Agr. Food Chem. 60 12296 12303
Chen, S.P., Bai, Y.F., Zhang, L.X. & Han, X.G. 2005 Comparing physiological responses of two dominant grass species to nitrogen addition in Xilin River Basin of China Environ. Exp. Bot. 53 65 75
Chen, L.H., Gong, P., Yang, W.Q., Zhang, J. & Hu, T.X. 2012 Effects of allelopathy from the early decomposition of Eucalyptus leaf litter on the photosynthetic characteristics of Brassica chinensis L J. Sichuan Agr. Univ. 30 174 180
Chen, B.L., Yang, H.K., Ma, Y.N., Liu, J.R., Lv, F.J. & Chen, J. 2016 Effect of shading on yield, fiber quality and physiological characteristics of cotton subtending leaves on different fruiting positions Photosynthetica 1 11
Chen, W., Zou, D., Guo, W., Xu, H., Shi, D. & Yang, C. 2009 Effects of salt stress on growth, photosynthesis and solute accumulation in three poplar cultivars Photosynthetica 47 415 421
Colla, G., Rouphael, Y. & Cardarelli, M. 2012a Vegetable crops: Improvement of tolerance to adverse chemical soil conditions by grafting. In: Improving Crop Resistance to Abiotic Stress, Vol. 1 and Vol. 2.
Colla, G., Rouphael, Y., Leonardi, C., Bie, Z.L. & Colla, G. 2010 Role of grafting in vegetable crops grown under saline conditions Scientia Hort. 127 147 155
Colla, G., Rouphael, Y., Rea, E. & Cardarelli, M. 2012b Grafting cucumber plants enhance tolerance to sodium chloride and sulfate salinization Scientia Hort. 135 177 185
Farquhar, G.D., Von, C.S. & Berry, J.A. 1980 A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species Planta 149 1 1361 1369
Holland, N. & Richardson, A.D. 2009 Stomatal length correlates with elevation of growth in four temperate species J. Sustain. For. 28 63 73
Hu, L., Yu, J., Liao, W., Zhang, G., Xie, J., Lv, J., Xiao, X., Yang, B., Zhou, R. & Bu, R. 2015 Moderate ammonium:nitrate alleviates low light intensity stress in mini Chinese cabbage seedling by regulating root architecture and photosynthesis Scientia Hort. 186 143 153
Huang, W., Hu, H., Hu, T., Chen, H., Wang, Q. & Chen, G. 2015 Impact of aqueous extracts of Cinnamomum septentrionale leaf litter on the growth and photosynthetic characteristics of Eucalyptus grandis seedlings New For. 46 561 576
Huang, C.J., Wei, G., Jie, Y.C., Xu, J.J., Anjum, S.A. & Tanveer, M. 2016 Effect of shade on plant traits, gas exchange and chlorophyll content in four ramie cultivars Photosynthetica 54 390 395
Iqbal, N., Masood, A. & Khan, N.A. 2012 Analyzing the significance of defoliation in growth, photosynthetic compensation and source-sink relations Photosynthetica 50 161 170
Johnston, M. & Onwueme, I.C. 1998 Effects of shade on photosynthetic pigments in the tropical root crops: Yam, taro, tannia, cassava and sweet potato Exp. Agr. 34 301 312
Khan, N.A. & Lone, P.M. 2005 Effects of early and late season defoliation on photosynthesis, growth and yield of mustard (Brassica juncea L.) Braz. J. Plant Physiol. 17 181 186
Khan, S.R., Rose, R., Haase, D.L. & Sabin, T.E. 2000 Effects of shade on morphology, chlorophyll concentration, and chlorophyll fluorescence of four Pacific Northwest conifer species New For. 19 171 186
Kim, S.J., Yu, D.J., Kim, T.C. & Lee, H.J. 2011 Growth and photosynthetic characteristics of blueberry (Vaccinium corymbosum, cv. Bluecrop) under various shade levels Scientia Hort. 129 486 492
Li, Z., Long, H.X., Zhang, L., Liu, Z.M., Cao, H.P., Shi, M.W. & Tan, X.F. 2017a The complete chloroplast genome sequence of tung tree (Vernicia fordii): Organization and phylogenetic relationships with other angiosperms Sci. Rep. 7 1869 doi: 10.1038/s41598-017-02076-6.
Li, Z., Tan, X.F., Lu, K., Liu, Z.M. & Wu, L.L. 2017b The effect of CaCl2, on calcium content, photosynthesis, and chlorophyll fluorescence of tung tree seedlings under drought conditions Photosynthetica 55 553 560
Li, Q.M. & Zhu, Z.P. 2014 Ecological factors affecting the growth and fruiting of Vernicia fordii in Shangnan County Shanxi For. Sci. Technol. 3 82 84
Lichtenthaler, H.K., Babani, F. & Langsdorf, G. 2007 Chlorophyll fluorescence imaging of photosynthetic activity in sun and shade leaves of trees Photosynth. Res. 93 235 244
Lone, P.M. & Khan, N.A. 2007 The effects of rate and timing of N fertilizer on growth, photosynthesis, N accumulation and yield of mustard (Brassica juncea) subjected to defoliation Environ. Exp. Bot. 60 318 323
Lone, P.M., Nazar, R., Singh, S. & Khan, N.A. 2008 Effects of timing of defoliation on nitrogen assimilation and associated changes in ethylene biosynthesis in mustard (Brassica juncea) Biologia 63 207 210
Lugassi-Ben-Hamo, M., Kitron, M., Bustan, A. & Zaccai, M. 2010 Effect of shade regime on flower development, yield and quality in Lisianthus Scientia Hort. 124 248 253
Morandi, B., Zibordi, M., Losciale, P., Manfrini, L., Pierpaoli, E. & Grappadelli, L.C. 2011 Shading decreases the growth rate of young apple fruit by reducing their phloem import Scientia Hort. 127 347 352
Nobel, P.S., Forseth, I.N. & Long, S.P. 1993 Canopy structure and light interception, p. 79–90. In: D.O. Hall, J.M.O. Scurlock, H.R. Bolhar-Nordenkampf, R.C. Leegood, and S.P. Long (eds.). Photosynthesis and production in a changing environment. Chapman and Hall, London, UK
Ou, L.J., Wei, G., Zhang, Z.Q., Dai, X.Z. & Zou, X.X. 2015 Effects of low temperature and low irradiance on the physiological characteristics and related gene expression of different pepper species Photosynthetica 53 85 94
Park, J.Y., Kim, D.K., Wang, Z.M., Lu, P., Park, S.C. & Lee, J.S. 2008 Production and characterization of biodiesel from tung oil Appl. Biochem. Biotechnol. 148 109 117
Pfister, D.P., Baker, J.R., Henna, P.H., Lu, Y. & Larock, R.C. 2008 Preparation and properties of tung oil-based composites using spent germ as a natural filler J. Appl. Polym. Sci. 108 3618 3625
Rivelli, A.R., Lovelli, S. & Perinola, M. 2002 Effects of salinity on gas exchange, water relations and growth of sunflower (Helianthus annuus) Funct. Plant Biol. 29 1405 1415
Sarijeva, G., Knapp, M. & Lichtenthaler, H.K. 2007 Differences in photosynthetic activity, chlorophyll and carotenoid levels, and in chlorophyll fluorescence parameters in green sun and shade leaves of Gingko and Fagus J. Plant Physiol. 164 950 955
Schwarz, D., Iersel, M.W.V., Ingram, K.T., Kläring, H.P., Horst, W.J. & Schenk, M.K. 2001 Nutrient solution concentration effects on growth and photosynthesis of tomato grown hydroponically: Plant nutrition. Springer, Netherlands
Shi, J.N., Wu, M.X. & Cha, J.J. 1979 Studies on plant phosphoenolpyruvate carboxylase I: Separation and properties of PEP carboxylase isoenzymes J. Plant Physiol. 5 225 236
Sui, X.L., Mao, S.L., Wang, L.H., Zhang, B.X. & Zhang, Z.X. 2012 Effect of low light on the characteristics of photosynthesis and chlorophyll a fluorescence during leaf development of sweet pepper J. Integr. Agr. 11 1633 1643
Talts, P., Pärnik, T., Gardeström, P. & Keerberg, O. 2004 Respiratory acclimation in Arabidopsis thaliana leaves at low temperature J. Plant Physiol. 161 573 579
Tan, X.F., Jiang, G.X. & Tan, F.Y. 2011 Research report on industrialization development strategy Vernicia fordii in China Nonwood Forest Res. 29 1 5
Yan, N., Wang, X.Q., Xu, X.F., Guo, D.P., Wang, Z.D., Zhang, J.Z., Hyde, K.D. & Liu, H.L. 2013 Plant growth and photosynthetic performance of Zizania latifolia are altered by endophytic Ustilago esculenta infection Physiol. Mol. Plant Pathol. 83 75 83
Ye, Z.P. 2007 A new model for relationship between irradiance and the rate of photosynthesis in Oryza sativa Photosynthetica 45 4 1361 1369
Ye, Z.P. & Yu, Q. 2008 Comparison of new and several classical models of photo-synthesis in response to irradiance J. Plant Ecol. 32 6 1361 1369
Zibordi, M., Domingos, S. & Grappadelli, L.C. 2009 Thinning apples via shading: An appraisal under field conditions J. Pomol. Hort. Sci. 84 6 1361 1369