Increasing economic and environmental pressures on the horticultural industry have prompted interest in sustainability over the past several years (Hall et al., 2009; Krug et al., 2008; Mattson et al., 2009). Sustainable horticultural practices have been described as methods that reduce environmental impact, conserve resources, support a stable community and high quality of life, and maintain economic viability (U.S. Government, 2007). Such practices include the proper use of supplemental lighting and efficient irrigation in greenhouses to optimize energy and water use. Understanding the precise light and water requirements of crops is the key to using supplemental lighting and water efficiently.
Insufficient light limits growth for many crops. Reduced light results in lower shoot biomass, reduced leaf and flower production, and a lower total leaf area in many species (Faust et al., 2005; Oh et al., 2009; Pennisi et al., 2005). Growers provide supplemental lighting to increase the total photosynthetically active radiation [PAR (400–700 nm)] a plant receives throughout a day, the sum of which is termed DLI. Increasing DLI can increase the growth rate of sun-adapted species, thus reducing the time it takes to produce a marketable crop (Faust, 2003; Fisher and Both, 2004). However, increasing DLI may not result in a more marketable, rapidly growing crop for shade-adapted species. Differences in light requirements exist among species and sometimes cultivars. For example, increasing DLI beyond 19 mol·m−2·d−1 resulted in no further increases in dry weight of Begonia semperflorens-cultorum (wax begonia) and Impatiens walleriana (bedding impatiens) (Faust et al., 2005). Increasing the DLI beyond 11.5 mol·m−2·d−1 yielded no additional increase in leaf count or number of flowers in the shade-adapted plant Cyclamen persicum (cyclamen) (Oh et al., 2009). Conversely, shoot dry weights of sun-adapted Zinnia elegans (bedding zinnia) and Tagetes erecta (african marigold) continued to increase as DLIs reached 43 mol·m−2·d−1 (Faust et al., 2005).
Plant genotype determines the plasticity of the response to increased DLI (Lambers et al., 2008). When exposed to high light, the sun-adapted plant Phaseolus vulgaris (common bean) has a greater capacity to adjust allocation of nitrogen resources toward ribulose-1,5-bisphosphate carboxylase (rubisco) synthesis (a factor that can limit photosynthetic capacity at high light levels) compared with the shade-adapted plant Alocasia macrorrhizos (giant taro). Meanwhile, when both species are grown in shade, giant taro allocates more nitrogen proportionally toward chlorophyll development to increase light interception (thus increasing photosynthetic activity at low light levels) compared with the common bean (Seeman et al., 1987). Differences can even exist between cultivars. Miller et al. (2001) reported that two cultivars of Angelonia angustifolia (angelonia) had different quantum yields (micromoles of CO2 fixed per incident micromole photon) and light compensation points (LCP), which correlated with a faster growth rate in the more photosynthetically efficient cultivar.
To understand if it is beneficial to supply crops with additional light, it is necessary to grow plants under various DLIs while observing both physiological and morphological characteristics. If an irrigation system is not responsive to changes in substrate water status as a result of increased evaporative demand, water stress can occur in plants exposed to high light or, conversely, plants growing in low light may experience periods of excessive water supply (Burnett and van Iersel, 2012). Capacitance sensor automated irrigation systems (Nemali and van Iersel, 2006), which trigger an irrigation valve to open if θ is too low, maintain a stable θ despite differences in plant water use under various DLIs (van Iersel et al., 2010), thus eliminating the question of whether water stress plays a role in studies assessing plant response to light.
There are many other benefits to such innovative irrigation technology, including the minimization of water consumption and elimination of effluent that could contain environmentally damaging nitrates and phosphates (Burnett and van Iersel, 2008; Nemali and van Iersel, 2006; Pierzynski et al., 2005). A study comparing the efficiency of this irrigation method with cyclic irrigation regulated by timers revealed that, over the course of a month, cyclic irrigation used 30 times more water than the system based on θ (Ristvey and Lea-Cox, 2008).
If capacitance sensor automated irrigation is to be used on a broad scale, it is important to know how plants respond to θ maintained at a steady, low level. Plant response to reduced water availability varies depending on the frequency, duration, and intensity of the stress imposed (Kramer and Boyer, 1995; Pospíšilová and Dodd, 2005). However, most previous studies have exposed plants to brief, severe drought. For example, Eakes et al. (1991) allowed plants to dry down to visible wilt before rehydration, and Zollinger et al. (2006) watered plants once every 2 or 4 weeks to simulate drought. Investigators in other experiments refrained from watering until containers reached a certain target weight (Niu et al., 2006; Sánchez-Blanco et al., 2009; Starman and Lombardini, 2006). Although these investigations provide key insights into how certain crops respond to these specific types of water stress, their methods likely resulted in much greater fluctuations in θ compared with plants irrigated with more refined methods such as imposing θ treatments using a capacitance sensor automated irrigation system. Capacitance sensor automation can provide well-defined irrigation treatments that can be compared from one study to the next, which is an essential component to building a better understanding of the influence of water deficit on plant functions (Jones, 2007).
WUEc is a direct and valid means of assessing the efficiency in which a plant produces biomass relative to the amount of water applied and can be accurately measured using a capacitance sensor automated irrigation system that can monitor how much water is applied. Regulating irrigation in this manner may improve WUEc but can also reduce plant growth (Burnett and van Iersel, 2008). Therefore, it is important to understand both the physiological and morphological implications of producing plants under various θ. Plants grown at low θ may have reduced leaf areas resulting from reduced meristem activity and/or insufficient turgor pressure for cell expansion (Lambers et al., 2008; van Volkenburgh, 1999). Although this may lower whole plant water loss by limiting evaporative surface area, it may also reduce whole plant photosynthesis and marketability of the crop (Franco et al., 2006). In some cases, stomatal conductance (gS) may not fully recover after periods of sustained or periodic water stress or previously stressed plants may acclimate through increased sensitivity of stomata to abscisic acid, the primary hormone responsible for stomatal closure during water stress (Pospíšilová and Dodd, 2005). Even non-stomatal limitations to photosynthesis such as the maximum carboxylation rate of rubisco (Vcmax) and the light saturated electron transport rate driving the regeneration of ribulose-1,5-bisphosphate (Jmax) can be influenced by reduced water availability (Flexas and Medrano, 2002). Instantaneous measures of WUEl are commonly used to assess WUE because it is easily determined using leaf gas exchange measurements (Long and Bernacchi. 2003; Singh and Reddy, 2011; Starman and Lombardini, 2006; Zollinger et al., 2006). Because these measurements are generally taken on a part of a single leaf, WUEl may not accurately reflect whole plant physiological responses.
Heuchera americana is an evergreen herbaceous perennial native to rocky, dry, deciduous understory habitats ranging from Ontario to Louisiana and Nebraska to Connecticut (Skillman and Osmond, 1998; U.S. Department of Agriculture, 2009). Although daylength becomes shorter in the winter, understory plants are often exposed to DLIs three to 10 times higher than those observed in the summer as a result of an open canopy. Despite freezing temperatures, photosynthetic capacity increases during winter high light periods without any sign of photoinhibition (Skillman et al., 1996). To our knowledge, net photosynthetic rate and estimates of photosynthetic capacity (other than chlorophyll fluorescence) have not been determined for american alumroot growing under DLIs higher than those observed in its native habitat. These estimations would be helpful indicators of appropriate DLIs for growing this crop in a greenhouse for commercial sales. Also, there is little more than anecdotal information regarding the specific water requirements of american alumroot (Cullina, 2000; Heims and Ware, 2005). Therefore, the objectives of this investigation were to: 1) determine the optimal θ set point for american alumroot production; 2) determine whether capacitance sensor regulated deficit irrigation has an impact on stomatal and non-stomatal limitations to photosynthesis; 3) explore whether american alumroot has the physiological capacity to acclimate to DLIs higher than those experienced in its native habitat; and 4) quantify and compare the long-term, whole-plant and instantaneous, leaf-level WUE of american alumroot under differing water and light availability.
Andersen, P.C., Knox, G.W. & Norcini, J.G. 1991 Light-intensity influences growth and leaf physiology of Aucuba japonica variegata HortScience 26 1485 1488
Bernacchi, C.J., Portis, A.R., Nakano, H., von Caemmerer, S. & Long, S.P. 2002 Temperature response of mesophyll conductance. Implications for the determination of rubisco enzyme kinetics and for limitations to photosynthesis in vivo Plant Physiol. 130 1992 1998
Burnett, S.E., Pennisi, S.V., Thomas, P.A. & van Iersel, M.W. 2005 Controlled drought affects morphology and anatomy of Salvia splendens J. Amer. Soc. Hort. Sci. 130 775 781
Burnett, S.E. & van Iersel, M.W. 2008 Morphology and irrigation efficiency of Gaura lindheimeri grown with capacitance-sensor controlled irrigation HortScience 43 1555 1560
Burnett, S.E. & van Iersel, M.W. 2012 Watering, irrigation systems, and control Williams K. Water and nutrient management for floriculture crops University of California Press Berkley, CA (In Press).
Eakes, D.J., Wright, R.D. & Seiler, J.R. 1991 Water relations of Salvia splendens ‘Bonfire’ as influenced by potassium nutrition and moisture stress conditioning J. Amer. Soc. Hort. Sci. 116 712 715
Faust, J.E., Holcombe, V., Rajapakse, N.C. & Layne, D.R. 2005 The effect of daily light integral on bedding plant growth and flowering HortScience 40 645 649
Fisher, P.R. & Both, A. 2004 Supplemental lighting and energy costs 43 46 Fisher P.R. & Runkle E. Lighting up profits: Understanding greenhouse lighting Meister Media Worldwide Willoughby, OH
Flexas, J. & Medrano, H. 2002 Drought-inhibition of photosynthesis in C3 plants: Stomatal and non-stomatal limitations revisited Ann. Bot. (Lond.) 89 183 189
Flexas, J., Ribas-Carbo, M., Diaz-Espejo, A., Galmes, J. & Medrano, H. 2008 Mesophyll conductance to CO2: Current knowledge and future prospects Plant Cell Environ. 31 602 621
Franco, J.A., Martinez-Sanchez, J.J., Fernandez, J.A. & Banon, S. 2006 Selection and production of ornamental plants for landscaping and xeroscaping in semi-arid environments J. Hort. Sci. Biotechnol. 81 3 17
Gent, M.P.N. 2008 Density and duration of shade affect water and nutrient use in greenhouse tomato J. Amer. Soc. Hort. Sci. 133 333 340
Hall, T.J., Dennis, J.H., Lopez, R.G. & Marshall, M.I. 2009 Factors affecting growers’ willingness to adopt sustainable floriculture practices HortScience 44 1346 1351
Jones, H.G. 2007 Monitoring plant and soil water status: Established and novel methods revisited and their relevance to studies of drought tolerance J. Expt. Bot. 58 119 130
Kim, J. & van Iersel, M.W. 2009 Daily water use of abutilon and lantana at various substrate water contents Proc. Southern Nursery Assn. Res. Conf 54 12 16
Kim, J., van Iersel, M.W. & Burnett, S.E. 2011 Estimating daily water use of two petunia cultivars based on plant and environmental factors HortScience 46 1287 1293
Long, S.P. & Bernacchi, C.J. 2003 Gas exchange measurements, what can they tell us about the underlying limitations to photosynthesis? Procedures and sources of error J. Expt. Bot. 54 2393 2401
Lorenzo, P., Garcia, M.L., Sanchez-Guerrero, M.C., Medrano, E., Caparros, I. & Giminez, M. 2006 Influence of mobile shading on yield, crop transpiration, and water use efficiency Acta Hort. 719 471 478
Miller, A.M., van Iersel, M.W. & Armitage, A.M. 2001 Whole-plant carbon dioxide exchange responses of Angelonia angustifolia to temperature and irradiance J. Amer. Soc. Hort. Sci. 125 606 610
Naasz, R., Michel, J.C. & Charpentier, S. 2005 Measuring hysteretic hydraulic properties of peat and pine bark using a transient method Soil Sci. Soc. Amer. J. 69 13 22
Nemali, K.S. & van Iersel, M.W. 2004 Acclimation of wax begonia to light intensity: Changes in photosynthesis, respiration, and chlorophyll concentration J. Amer. Soc. Hort. Sci. 129 745 751
Nemali, K.S. & van Iersel, M.W. 2006 An automated system for controlling drought stress and irrigation in potted plants Sci. Hort. 110 292 297
Nemali, K.S. & van Iersel, M.W. 2008 Physiological responses to different substrate water contents: Screening for high water-use efficiency in bedding plants J. Amer. Soc. Hort. Sci. 133 333 340
Niu, G.H., Rodriguez, D.S. & Wang, Y.T. 2006 Impact of drought and temperature on growth and leaf gas exchange of six bedding plant species under greenhouse conditions HortScience 41 1408 1411
Oh, W., Cheon, I.H., Kim, K.S. & Runkle, E. 2009 Photosynthetic daily light integral influences flowering time and crop characteristics of Cyclamen persicum HortScience 44 341 344
Osmond, C.B. 1994 What is photoinhibition? Some insights from comparison of shade and sun plants 1 24 Baker N.R. & Bowyer J.R. Photoinhibition of photosynthesis: From molecular mechanisms to the field Bios Scientific Oxford, UK
Pearcy, R.W. & Yang, W. 1998 The functional morphology of light capture and carbon gain in the redwood forest understorey plant Adenocaulon bicolor Hook Funct. Ecol. 12 543 552
Pennisi, S., van Iersel, M.W. & Burnett, S.E. 2005 Photosynthetic irradiance and nutrition effects on growth of English ivy in subirrigation systems HortScience 40 1740 1745
Pospíšilová, J. & Dodd, I.C. 2005 Role of plant growth regulators in stomatal limitation to photosynthesis during water stress 811 825 Pessarakli M. Handbook of photosynthesis 2nd Ed Taylor and Francis Group Boca Raton, FL
Prevete, K.J., Fernandez, T. & Miller, W.B. 2000 Drought response of three ornamental herbaceous perennials J. Amer. Soc. Hort. Sci. 125 310 317
Prioul, J.L. & Chartier, P. 1977 Partitioning of transfer and carboxylation components of intracellular resistance to photosynthetic CO2 fixation—Critical analysis of methods used Ann. Bot. (Lond.) 41 789 800
Ristvey, A.G. & Lea-Cox, J.D. 2008 Wireless moisture sensing for real-time management of irrigation in container nursery operations 8 Nov. 2011. <http://www.reeis.usda.gov/web/crisprojectpages/210639.html>.
Rothstein, D.E. & Zak, D.R. 2001 Photosynthetic adaptation and acclimation to exploit seasonal periods of direct irradiance in three temperate, deciduous-forest herbs Funct. Ecol. 15 722 731
Sánchez-Blanco, M.J., Álvarez, S., Navarro, A. & Bañón, S. 2009 Changes in leaf water relations, gas exchange, growth and flowering quality in potted geranium plants irrigated with different water regimes J. Plant Physiol. 166 467 476
Seeman, J.R., Sharkey, T.D., Wang, J.L. & Osmond, C.B. 1987 Environmental effects on photosynthesis, nitrogen-use efficiency, and metabolite pools in leaves of sun and shade plants Plant Physiol. 84 796 802
Singh, S.K. & Reddy, K.R. 2011 Regulation of photosynthesis, fluorescence, stomatal conductance and water-use efficiency of cowpea [Vigna unguiculata (L.) Walp.] under drought J. Photochem. Photobiol. 105 40 50
Skillman, J.B. & Osmond, C.B. 1998 Influence of nitrogen supply and growth irradiance on photoinhibition and recovery in Heuchera americana (Saxifragaceae) Physiol. Plant. 103 567 573
Skillman, J.B., Strain, B.R. & Osmond, C.B. 1996 Contrasting patterns of photosynthetic acclimation and photoinhibition in two evergreen herbs from a winter deciduous forest Oecologia 107 446 455
Stamm, P. & Kumar, P.P. 2010 The phytohormone signal network regulating elongation growth during shade avoidance J. Expt. Bot. 61 2889 2903
Starman, T. & Lombardini, L. 2006 Growth, gas exchange, and chlorophyll fluorescence of four ornamental herbaceous perennials during water deficit conditions J. Amer. Soc. Hort. Sci. 131 469 475
U.S. Government 2007 U.S. code, title 7, chapter 64: Agricultural research, extension, teaching, subchapter 1: Findings, purposes, and definitions 27 Aug. 2010. <http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=browse_usc&docid=Cite:+7USC3103>.
van Iersel, M.W., Kang, J.G. & Burnett, S.E. 2010 Growth and water use of petunia as affected by substrate water content and daily light integral HortScience 45 277 282
Wallach, R. 2008 Physical characteristics of soilless media 41 116 Raviv M. & Lieth J.H. Soilless culture: Theory and practice Elsevier, Amsterdam The Netherlands
Yeh, D.M. & Wang, H.M. 2000 Effects of irradiance on growth, net photosynthesis and indoor performance of the shade-adapted plant, maidenhair fern J. Hort. Sci. Biotechnol. 75 293 298
Zollinger, N., Kjelgren, R., Cerny-Koenig, T., Kopp, K. & Koenig, R. 2006 Drought response of six ornamental herbaceous perennials Sci. Hort. 109 267 274