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Differential sensitivity of C3 and C4 turfgrass species to increasing atmospheric vapor pressure deficit
Environmental and Experimental Botany 67 (2009) 372–376
Contents lists available at ScienceDirect
Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot
Differential sensitivity of C3 and C4 turfgrass species to increasing atmospheric vapor pressure deficit Benjamin G. Wherley, Thomas R. Sinclair ∗ Agronomy Physiology Laboratory, University of Florida, P.O. Box 110965, Gainesville, FL 32611-0965, United States
a r t i c l e
i n f o
Article history: Received 25 February 2009 Received in revised form 7 July 2009 Accepted 8 July 2009 Keywords: Turfgrass C3 C4 Transpiration Vapor pressure deficit VPD
a b s t r a c t Temperature and vapor pressure deficit (VPD) effects on turfgrass growth are almost always confounded in experiments because VPD commonly is substantially increased in elevated-temperature treatments. The objective of this study as to examine specifically the influence of VPD on transpiration response of four ‘warm-season’ (C4 ) and four ‘cool-season’ (C3 ) turfgrasses to increasing VPD at a stable temperature (29.3 ± 1.5 ◦ C). Although transpiration rates were noticeably lower in C4 grasses, transpiration rates increased linearly in response to increasing VPD across the range of 0.8–3.0 kPa. In contrast, transpiration rates of C3 increased sharply with increasing VPD across the range of low VPDs, but became constrained at higher VPDs (>1.35 kPa). Restricted transpiration rate at elevated VPD was most evident in Agrostis palustris and Lolium perenne. Assuming restricted transpiration rates reflect a limitation on leaf CO2 uptake, these results indicate that the commonly observed decline in growth of C3 (and success of C4 ) grasses at elevated temperature may include a sensitivity to elevated VPD. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Warm-season C4 turfgrasses are able to photosynthesize at high rates at elevated temperatures, which explains their relative abundance in hot, dry climates (Taiz and Zeiger, 1998). Unlike the C3 turfgrass species which have a common European origin, the C4 species have centers of origin ranging from Africa, South America, and Asia (Hartley and Williams, 1956; Beard, 1973). As such, it is not surprising that the apparent temperature optimum for growth of C3 grasses is much lower (15–24 ◦ C) than that of C4 grasses (27–35 ◦ C) (Beard, 1973). High temperature stress has long been considered the primary factor limiting the use of cool-season (C3 ) grasses in warm climates (Beard and Daniel, 1965; Carrow, 1996; Beard, 1997; Huang et al., 1998). A wide variety of C3 turfgrasses have been shown to respond to super-optimal temperatures through decreased photosynthetic rates, chlorophyll contents, carbohydrate accumulation, and root and shoot growth (Wehner and Watschke, 1981; White et al., 1988; Howard and Watschke, 1991; Huang et al., 1998). One confounding factor that is often overlooked in temperature studies, however, is the influence of the vapor pressure deficit (VPD) of the atmosphere surrounding the plants. VPD is the difference between atmospheric saturation vapor pressure at a given temperature minus the actual water vapor pressure. Since saturation vapor pressure of the atmosphere increases exponentially with temperature, VPD also commonly increases greatly in experiments with
∗ Corresponding author. Tel.: +1 352 392 6180; fax: +1 352 392 6139. E-mail address: trsinc@ufl.edu (T.R. Sinclair). 0098-8472/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2009.07.003
elevated temperature treatments. Therefore, VPD provides a very direct indication of the atmospheric moisture conditions independent of temperature (Anderson, 1936). Temperature and VPD are almost always confounded in experiments because usually little or no attempt is made to control VPD when differing temperatures are imposed. Since potential evapotranspiration rate is directly related to VPD, the water flux demand can be a substantial hidden variable in nearly all temperature experiments (Sinclair and Bennett, 1998). The confounding influence of VPD may be particularly important in plant species where stomatal conductance begins decreasing with increasing VPD. For example, in a number of plant species, stomata conductance has been observed to decrease between VPDs of 1.0 and 2.5 kPa (Bunce, 1981). A limited maximum transpiration rate has been observed when atmospheric VPD exceeds ∼2 kPa. The common observation of this phenomenon is afternoon depression of leaf photosynthesis rates, restriction of evaporative cooling of leaves, and increased leaf temperatures in many plant species (Hirasawa and Hsiao, 1999; Isoda and Wang, 2002). Based on evapotranspiration studies in the field, it appears that turfgrass species may also differ in their response to changing humidity (Aronson et al., 1987; Sheffer, 1979). In one of the few studies examining the interactive effects of temperature and VPD on turfgrass growth, Sinclair et al. (2007) discovered that growth of Festuca arundinacea, a cool-season (C3 ) species, while expected to decline with increasing temperatures over the range of 18–27 ◦ C, actually increased markedly with increasing temperature when VPD was held constant at a low value. Conversely, growth declined in their study when the grass was exposed to increasing VPD and
B.G. Wherley, T.R. Sinclair / Environmental and Experimental Botany 67 (2009) 372–376
temperature was held constant at an optimal 21 ◦ C. The authors concluded that the growth decline of this C3 grass species at elevated temperature may involve plant sensitivity to VPD. It is not known to what extent this occurs in other cool-season or warm-season grass species. The objective of the current study was to compare the transpiration rate response of C3 and C4 turfgrasses to increasing VPD at a stable temperature. If a difference in response to VPD is observed between C3 and C4 turfgrasses, this could be an important contributing factor in defining climate zones of adaptability. That is, sensitivity to increasing VPD, which is commonly associated with elevated temperature, might contribute to the lack of success of a species in warm-temperature climate zone. 2. Materials and methods Three sets of experiments (Table 1) were carried out between February and May 2008 in a glasshouse at the University of Florida campus (29◦ 38 N, 82◦ 22 W) in Gainesville, FL. The study included four C4 species: ‘Argentine’ bahiagrass (Paspalum notatum Flugg), ‘Tifway 419’ bermudagrass (Cynodon transvaalensis Burtt-Davy × C. dactylon (L.) Pers.), ‘Floratam’ St. Augustine grass (Stenotaphrum secundatum (Walt.) Kuntze), ‘Empire’ zoysiagrass (Zoysia japonica Steud.). Four C3 species were also studied: ‘Penncross’ creeping bentgrass (Agrostis palustris Huds.), ‘TQ Elite’ perennial ryegrass (Lolium perenne L.), ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.), and annual bluegrass (Poa annua var. annua). All grasses were grown in pots constructed from 10 cm diameter, 20 cm tall PVC pipes fitted with a flat end cap. A small hole was drilled into the center of the end cap for drainage. The top, open end of the pipes was fitted with a toilet flange for attaching a chamber to the pots when VPD measurements were made. Sod pieces (2.5 cm depth) of the C4 species were removed from established research plots at the University of Florida G.C. Horn Memorial Turfgrass Research Field Laboratory, Citra, FL, using a 10-cm diameter golf cup cutter. The sod was washed free of soil and established atop medium-coarse textured sand in PVC pots. C3 species were removed in a similar fashion from 10 cm deep trays of sod which had been established from seed and grown for 6 weeks in the greenhouse prior to beginning the experiment. Greenhouse temperatures during the establishment of turfgrass in PVC pots were controlled at 29/22 ◦ C (day/night). A complete slow-release fertilizer (24–5–11, Turfgro Professional, Sandford, FL) was applied at the beginning of each establishment period at 5 g N m−2 and plants were kept well watered with two or three irrigations per week. Grasses were given 4–6 weeks to fully root in the soil prior to VPD experiments. Visual observations confirmed that grasses had fully developed root systems which reached the container bottoms at the beginning of each experiment. All grasses were clipped weekly to the approximate recommended heights of cut for the given species. This corresponded to a height of 6.4 cm for all species except A. palustris, which was clipped to 8 mm. At the time of VPD measurement, all grasses had developed full canopies shading the soil surface so there was very little soil evaporation during the VPD tests. Measurements of transpiration rate of the plants were made simultaneously on 3 replicate pots for each of the C3 or C4 species, Table 1 Establishment and VPD test dates for the turfgrasses used in the experiments. Set 1 Establishment date VPD test dates
C4 C3
Set 2
Set 3
13 January
15 February
20 February
14,19 February 3, 5 March
21 March 26 March
31 March, 2 April 21, 22 April
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i.e., 12 plants per test. The plants in Sets 1 and 3 were measured on 2 days and in Set 2 on only 1 day (Table 1). Therefore, for each species there was the possibility of up to 45 observations (3 replicates × 5 days × 3 VPD levels). However, any measurements obtained when container temperature exceeded 32 ◦ C were discarded. Also, plants of Poa annus grew very poorly in Set 3 so this species was not measured at these later times resulting in a decreased number of potential observations. Consequently, the actual number of observations was less than the maximum and varied among species. All VPD tests were carried out during cloudless periods. The evening prior to each experiment, plants were fully watered and allowed to drain overnight. VPD treatments were imposed on grasses by attaching a chamber system to the PVC pots (Fletcher et al., 2007). This involved attaching a 26-cm diameter food container lid (with a 10-cm diameter circle cut out of the center) to the toilet flange at the top of the pot. A 6-L transparent plastic food container (Rubbermaid Commercial Products LLC, Winchester, VA) was then inverted and placed over the turf and sealed into the previously installed lid. The chambers extinguished approximately 25% of the solar radiation. The whole assembly was then placed in a greenhouse covered with density-neutral shade fabric. Light levels in the greenhouse were approximately 70% of irradiance incident to the greenhouse. Photosynthetic photon flux within VPD chambers was measured prior to each experiment using a quantum meter (Apogee Instruments, Logan UT). For all experiments, light levels in VPD chambers exceeded 650 mol m−2 s−1 . Each VPD container was equipped with a 12 V, 7.6 cm diameter computer box fan (Northern Tool and Equipment, Burnsville, MN) to mix air inside the chamber. The circulation provided by the fan was found through previous experiments to provide near equality between leaf and air temperatures. A pocket relative humidity/temperature pen (Extech Instruments, League City, TX) was mounted through the side wall of each container to measure the chamber environment. To achieve a range of VPD treatments, various humidity levels in the chambers were established by varying air flow rate into the chambers and by pretreatment of the air. On each day when a set of plants were tested, conditions in the VPD chambers were adjusted to expose the plants to three VPD levels. Measurements were initiated at the lowest VPD followed by the mid and high VPD settings to avoid any influence of exposure to high VPD on subsequent VPD tests. A low VPD treatment was achieved in the chambers by flowing air at approximately 0.4 L min−1 into each chamber using two pumps (DOA0P704, Gast, Benton Harbor, MI) with their intake tubes placed inside an atomizing humidifier (Herrmidifier, Sanford, NC). A medium VPD was achieved by flowing ambient greenhouse air into the containers at 2–3 L min−1 from two air compressors (Kobalt model L-215902, Lowe’s Companies Inc., North Wilksboro, NC). The highest VPD was achieved by first flowing air from the two compressors through PVC tubes of 76 mm diameter filled with six-mesh indicating Drierite (W.A. Hammond Drierite Co. Ltd., Xenia, OH) into the containers at 2–3 L min−1 . All flow rates were monitored using flow meters (Model FL-2043, Omega, Stamford, CT), one for each chamber. Measurement of transpiration rate at each VPD level was done after allowing the plants and air in the container to equilibrate for 30 min. VPD was calculated for each chamber based on the measured temperature and relative humidity. During each humidity treatment, air temperature and relative humidity were used to calculate an average VPD based on three measurements taken every 20 min during the 60 min measurement periods. At each setting, the humidity levels in the chambers proved to be stable over the 60 min period within individual chambers, although the VPD varied somewhat among chambers.
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For each temperature and humidity setting the transpiration rate per plant was calculated based on the change in mass between the start and end of each VPD measurement period. Mass was determined by weighing pots on a balance with a resolution of 0.1 g (Model SI-8001, Denver Instrument, Denver, CO). For all treatments, transpiration rates were measured over 60 min periods. Data were analyzed by regressing transpiration rate against VPD using observations from individual plants within a species. For each species and experiment, the data were analyzed using both linear and quadratic terms (PROC REG, SAS). Slopes and intercepts for individual experiments were compared to one another and no differences were found (˛ = 0.05), indicating that the relationship between VPD and transpiration did not differ over experiments within a species. Data were then pooled for a species for further analysis using GraphPad Prism 2.01 (Graph pad Software Inc., San Diego, CA, 1996). Pooled data were analyzed by plotting all individual transpiration rate (Tr ) data against VPD for each species. Initially, a two-segment linear regression using GraphPad Prism software was applied to the data: If VPD < X0 ,
Tr = S1 (VPD) + C1
(1)
If VPD = X0 ,
Tr = S2 (VPD) + C2
(2)
where X0 is the breakpoint between the two line segments, S1 and S2 the slopes of the first and second line segments, respectively. In the regression, the second line segment is constrained to intersect with the first line segment at X0 . The software uses all the data obtained for a species and solves for the regression coefficients of both linear segments including the breakpoint. The slopes of the two linear regressions (S1 and S2 ) were statistically compared to determine if they differed significantly (P < 0.05). If the slopes differed, the two-segment linear regression results were retained. When the slopes were not significantly different,
a simple linear regression was applied to all the data for that species. 3. Results Air temperatures inside the humidity containers for the data retained for this analysis were normally distributed with a mean temperature of 29.3 ± 1.5 ◦ C (S.D.). Hence, the main basis for variation in VPD in the chambers resulted from changes in humidity as a result of the air flow rate and the air source introduced into the chambers. Relative humidity in these tests ranged between 31 and 79%. Calculated VPDs were between 0.79 and 2.99 kPa. For all C4 species, a single linear regression provided the best fit (R2 = 0.57–0.79, all cases P < 0.0001) (Fig. 1). Only slight differences in water loss rates were detected among the C4 species. Slopes ranged from 0.27 mg H2 O pot−1 s−1 in C. traansvalensis and S. secundatum to 0.35 mg H2 O pot−1 s−1 in P. notatum and Z. japonica for each 1 kPa increase in VPD (Fig. 1). The two-segment linear regression provided the best fit for all C3 species (Fig. 2). (These data were also analyzed using a quadratic model but the R2 for these regressions were not superior to the two-segment linear regression.) As VPD increased within the low VPD range, transpiration of the C3 grasses increased sharply at rates ranging from 0.66 to 1.41 mg H2 O pot−1 s−1 (A. palustris and L. perenne, respectively) for each 1 kPa increase in atmospheric VPD. An apparent limitation on transpiration rate became evident at VPD in the range of 1.3 and 1.8 kPa. A. palustris had a higher breakpoint than the other three C3 grasses (Fig. 2). Noticeably lower rates of water loss rates were obtained for C3 species when VPD exceeded their VPD breakpoint. Limitation on transpiration rate occurring in response to increasing VPD for a given species could be seen by comparing slopes at low vs. high VPD (slope 1 vs. slope 2). As such, the most substantial constraints on transpiration rate among the C3 species occurred in A. palustris and L. perenne, while F. arundinacea showed the least sensitivity to increasing VPD.
Fig. 1. Transpiration rates of four C4 grasses in response to VPD. Each observation is shown as a single datum.
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Fig. 2. Transpiration rates of four C3 grasses in response to VPD. Each observation is shown as a single datum. Double-linear regressions provided a better fit for all C3 data than single-linear regression.
4. Discussion We are unaware of any previous attempts to evaluate the responses of C3 and C4 turfgrass species to VPD. These results demonstrated that as a group the C4 and C3 turfgrass species differed substantially in their response and sensitivity to VPD. While the C4 species appeared to lack sensitivity to VPD within the range studied, C3 species showed limitations on transpiration rate as VPD increased beyond about 1.3 kPa. Of course, this study was limited to a single cultivar per species so it is necessary to remember that variation in VPD sensitivity is certainly possible within a species (Ebdon et al., 1998). Nonetheless, differences between C4 and C3 turfgrass species in our study were striking. These observed difference in limitations on transpiration rate at high VPD, are likely to extend to decreased CO2 assimilation and growth with increasing VPD in C3 species due to decreased stomata conductance (deWit, 1958; Sinclair et al., 1984) at high VPD. These observed differences between the C3 and C4 grass species is consistent with observations in crops species responses to VPD. Pettigrew et al. (1990) found that grain sorghum (C4 ) gas exchange rates were less sensitive than well-watered soybean (C3 ) to increasing VPD. These results are also in agreement with to those of Kawamitsu et al. (1993), who found that low humidity (i.e., high VPD) caused lower leaf conductance, transpiration, and CO2 assimilation in the C3 Oryza sativa, but had little effect on stomatal responses and CO2 assimilation in the C4 P. maximum. Decreased humidity resulted in lower rates of photosynthesis in C3 Chenopodium album, but had no effect on photosynthesis of two C4 species Amaranthus hybridus and Portulaca oleracea (Bunce, 1983). Rawson et al. (1977) showed that photosynthesis and transpiration rate of whole barley (Horendum vulgare) plants (C3 ) decreased
across a range of VPD from 0.7 to 1.8 kPa (at a constant 26 ◦ C), indicating stomatal sensitivity to VPD. Unfortunately, it is difficult to compare our results to the few other VPD studies on turfgrasses, all of which involve C3 species, due to the limited number of species included in each study. Ebdon et al. (1998) studied evapotranspiration of only P. pratensis (C3 ), although they compared 61 cultivars. Exposure of plants to VPD treatments as high as 2.3 kPa, indicated substantial variability among cultivars in their response to high VPD. Gaussoin et al. (2005) evaluated carbon dioxide exchange rates of P. annua and A. palustris in response to VPD, but only two treatments at low VPD (0.5 and 1.0 kPa) were used. Our observations on F. arundinacea (C3 ) are in agreement with those of Sinclair et al. (2007) who found that shoot growth of this species declined at VPD exceeding ∼1.3 kPa with temperature held constant at 21 ◦ C. While the apparent threshold for regulation of transpiration we observed at 1.35 kPa for F. arundinacea was not as dramatic as with the other C3 species, the observed inhibition of gas exchange at high VPD is likely to contribute to the decreased growth previously reported at high VPD. Our results reveal unique differences in overall transpiration rates as well as sensitivity to VPD between the C4 and C3 turfgrass species. These findings are in agreement with previous research, which has shown that C4 grass species possess considerably lower evapotranspiration rates than C3 species (Beard, 1973; Huang and Fry, 1999). This has been attributed to the fact that C4 species contain the precursor organic acid pathway for capturing CO2 . Photosynthesis rates by C4 species similar to C3 species can be achieved at lower CO2 concentration within the leaves. Therefore, C4 grass species can have decreased stomata apertures and water loss rates while fixing CO2 at rates equal to or greater than those of their C3 counterparts (Taiz and Zeiger, 1998). Conversely, due to the inability
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to concentrate CO2 , C3 species require high stomata conductance. Therefore, C3 species may be more susceptible to encountering limitations on water transport in the plant to guard cells at high VPDs resulting in decreased stomata conductance. If VPDs were to be increased beyond the range studied here, a transpiration limitation might well be observed also in the C4 species. The breakpoint for the C3 species in terms of transpiration occurred in the range of 0.63–0.85 mg s−1 pot−1 (Fig. 2), which is the range of the maximum transpiration rates measured in the C4 species. Therefore, both groups of species may have the same limitation on water transport in the plant but in these experiments the transpiration rate, i.e., VPD conditions, were not sufficiently high to detect a possible breakpoint in C4 species at high VPD. Studies at higher transpiration rates are needed to fully resolve this possibility. However, a breakpoint in C4 species may be of less relevance than in C3 species for most environments, since VPD in many turfgrass would not greatly exceed the range of VPD tested. Acknowledgment This research was made possible in part through funding from the Southwest Florida Water Management District. References Anderson, D.B., 1936. Relative humidity or vapor pressure deficit. Ecology 17, 277–282. Aronson, L.J., Gold, A.J., Hull, R.J., Cisar, J.L., 1987. Evapotranspiration of cool-season turfgrass in the humid northeast. Agron. J. 79, 901–904. Beard, J.B., 1973. Turfgrass: Science and Culture. Prentice Hall, New York. Beard, J., 1997. Dealing with heat stress on golf course turf. Golf Course Manage. 7, 54–59. Beard, J.B., Daniel, W.H., 1965. Effect of temperature and cutting on the growth of creeping bentgrass (Agrostis palustris Huds.) roots. Agron. J. 5, 249–250. Bunce, J.A., 1981. Comparative responses of leaf conductance to humidity in single attached leaves. J. Exp. Bot. 32, 629–634. Bunce, J.A., 1983. Differential sensitivity to humidity of daily photosynthesis in the field in C3 and C4 species. Oecologia 57, 262–265. Carrow, R.N., 1996. Summer decline of bentgrass greens. Golf Course Manage. 6, 51–56.
deWit, C.T., 1958. Transpiration and Crop Yields. Institute of Biological and Chemical Research on Field Crops and Herbage. Wageningen, The Netherlands. No. 64.6. Ebdon, J.S., Petrovic, A.M., Zobel, R.W., 1998. Stability of evapotranspiration rates in Kentucky bluegrass cultivars across low and high evaporative environments. Crop Sci. 38, 135–142. Fletcher, A.L., Sinclair, T.R., Allen Jr., L.H., 2007. Transpiration responses to vapor pressure deficit in well watered ‘slow-wilting’ and commercial soybean. Environ. Exp. Bot. 61, 145–151. Gaussoin, R.E., Branham, B.E., Flore, J.A., 2005. The influence of environmental variables on CO2 exchange rates of three cool-season turfgrasses. Int. Turf. Soc. Res. J. 10, 850–856. Hartley, W., Williams, R.J., 1956. Centres of distribution of cultivated pasture grasses and their significance for plant introduction. In: Proc. 7th Int. Grassld Congr., Palmerston North, New Zealand, p. 190. Hirasawa, T., Hsiao, T.C., 1999. Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field. Field Crop Res. 62, 53–62. Howard, H., Watschke, T.L., 1991. Variable high-temperature tolerance among Kentucky bluegrass cultivars. Agron. J. 83, 689–693. Huang, B., Fry, J.D., 1999. Turfgrass evapotranspiration. In: Kirkham, M.B. (Ed.), Water Use in Crop Production. Food Products Press, Binghamton, NY. Huang, B., Liu, X., Fry, J.D., 1998. Shoot physiological responses of two bentgrass cultivars to high temperature and poor soil aeration. Crop Sci. 38, 1219–1224. Isoda, A., Wang, P., 2002. Leaf temperature and transpiration of field grown cotton and soybean under arid and humid conditions. Plant Prod. Sci. 5, 224–228. Kawamitsu, Y., Yoda, S., Agata, W., 1993. Humidity pretreatment affects the responses of stomata and CO2 assimilation to vapor pressure difference in C3 and C4 plants. Plant Cell Phys. 34, 113–119. Pettigrew, W.T., Hesketh, J.D., Peters, D.B., Woolley, J.T., 1990. A vapor pressure deficit effect on crop canopy photosynthesis. Photosynt. Res. 24, 27–34. Rawson, H.M., Begg, J.E., Woodward, R.G., 1977. The effect of atmospheric humidity on photosynthesis, transpiration, and water use efficiency of leaves of several plant species. Planta 134, 5–10. Sheffer, K.M., 1979. Response of three cool-season turfgrass species to heat and moisture stress. Ph.D. Diss. Univ. of Missouri, Columbia (Diss. Abstr. 80-07193). Sinclair, T.R., Tanner, C.B., Bennett, J.M., 1984. Water-use efficiency in crop production. BioScience 34, 36–40. Sinclair, T.R., Bennett, J.M., 1998. Water. In: Sinclair, T.R., Gardner, F.P. (Eds.), Principles of Ecology in Plant Production. CAB International, pp. 103–120. Sinclair, T., Fiscus, E., Wherley, B., Durham, M., Rufty, T., 2007. Atmospheric vapor pressure deficit is critical in predicting growth response of ‘cool-season’ grass Festuca arundinacea to temperature change. Planta 227, 273–276. Taiz, L., Zeiger, E., 1998. Plant Physiology. Sinauer Associates, Sunderland, MA. Wehner, D.J., Watschke, T.L., 1981. Heat tolerance of Kentucky bluegrasses, perennial ryegrasses, and annual bluegrasses. Agron. J. 73, 79–84. White, R.H., Stefany, P., Comeau, M., 1988. Pre- and poststress temperature influence perennial ryegrass in vitro heat tolerance. Hortscience 23, 1047–1051.
Contents lists available at ScienceDirect
Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot
Differential sensitivity of C3 and C4 turfgrass species to increasing atmospheric vapor pressure deficit Benjamin G. Wherley, Thomas R. Sinclair ∗ Agronomy Physiology Laboratory, University of Florida, P.O. Box 110965, Gainesville, FL 32611-0965, United States
a r t i c l e
i n f o
Article history: Received 25 February 2009 Received in revised form 7 July 2009 Accepted 8 July 2009 Keywords: Turfgrass C3 C4 Transpiration Vapor pressure deficit VPD
a b s t r a c t Temperature and vapor pressure deficit (VPD) effects on turfgrass growth are almost always confounded in experiments because VPD commonly is substantially increased in elevated-temperature treatments. The objective of this study as to examine specifically the influence of VPD on transpiration response of four ‘warm-season’ (C4 ) and four ‘cool-season’ (C3 ) turfgrasses to increasing VPD at a stable temperature (29.3 ± 1.5 ◦ C). Although transpiration rates were noticeably lower in C4 grasses, transpiration rates increased linearly in response to increasing VPD across the range of 0.8–3.0 kPa. In contrast, transpiration rates of C3 increased sharply with increasing VPD across the range of low VPDs, but became constrained at higher VPDs (>1.35 kPa). Restricted transpiration rate at elevated VPD was most evident in Agrostis palustris and Lolium perenne. Assuming restricted transpiration rates reflect a limitation on leaf CO2 uptake, these results indicate that the commonly observed decline in growth of C3 (and success of C4 ) grasses at elevated temperature may include a sensitivity to elevated VPD. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Warm-season C4 turfgrasses are able to photosynthesize at high rates at elevated temperatures, which explains their relative abundance in hot, dry climates (Taiz and Zeiger, 1998). Unlike the C3 turfgrass species which have a common European origin, the C4 species have centers of origin ranging from Africa, South America, and Asia (Hartley and Williams, 1956; Beard, 1973). As such, it is not surprising that the apparent temperature optimum for growth of C3 grasses is much lower (15–24 ◦ C) than that of C4 grasses (27–35 ◦ C) (Beard, 1973). High temperature stress has long been considered the primary factor limiting the use of cool-season (C3 ) grasses in warm climates (Beard and Daniel, 1965; Carrow, 1996; Beard, 1997; Huang et al., 1998). A wide variety of C3 turfgrasses have been shown to respond to super-optimal temperatures through decreased photosynthetic rates, chlorophyll contents, carbohydrate accumulation, and root and shoot growth (Wehner and Watschke, 1981; White et al., 1988; Howard and Watschke, 1991; Huang et al., 1998). One confounding factor that is often overlooked in temperature studies, however, is the influence of the vapor pressure deficit (VPD) of the atmosphere surrounding the plants. VPD is the difference between atmospheric saturation vapor pressure at a given temperature minus the actual water vapor pressure. Since saturation vapor pressure of the atmosphere increases exponentially with temperature, VPD also commonly increases greatly in experiments with
∗ Corresponding author. Tel.: +1 352 392 6180; fax: +1 352 392 6139. E-mail address: trsinc@ufl.edu (T.R. Sinclair). 0098-8472/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2009.07.003
elevated temperature treatments. Therefore, VPD provides a very direct indication of the atmospheric moisture conditions independent of temperature (Anderson, 1936). Temperature and VPD are almost always confounded in experiments because usually little or no attempt is made to control VPD when differing temperatures are imposed. Since potential evapotranspiration rate is directly related to VPD, the water flux demand can be a substantial hidden variable in nearly all temperature experiments (Sinclair and Bennett, 1998). The confounding influence of VPD may be particularly important in plant species where stomatal conductance begins decreasing with increasing VPD. For example, in a number of plant species, stomata conductance has been observed to decrease between VPDs of 1.0 and 2.5 kPa (Bunce, 1981). A limited maximum transpiration rate has been observed when atmospheric VPD exceeds ∼2 kPa. The common observation of this phenomenon is afternoon depression of leaf photosynthesis rates, restriction of evaporative cooling of leaves, and increased leaf temperatures in many plant species (Hirasawa and Hsiao, 1999; Isoda and Wang, 2002). Based on evapotranspiration studies in the field, it appears that turfgrass species may also differ in their response to changing humidity (Aronson et al., 1987; Sheffer, 1979). In one of the few studies examining the interactive effects of temperature and VPD on turfgrass growth, Sinclair et al. (2007) discovered that growth of Festuca arundinacea, a cool-season (C3 ) species, while expected to decline with increasing temperatures over the range of 18–27 ◦ C, actually increased markedly with increasing temperature when VPD was held constant at a low value. Conversely, growth declined in their study when the grass was exposed to increasing VPD and
B.G. Wherley, T.R. Sinclair / Environmental and Experimental Botany 67 (2009) 372–376
temperature was held constant at an optimal 21 ◦ C. The authors concluded that the growth decline of this C3 grass species at elevated temperature may involve plant sensitivity to VPD. It is not known to what extent this occurs in other cool-season or warm-season grass species. The objective of the current study was to compare the transpiration rate response of C3 and C4 turfgrasses to increasing VPD at a stable temperature. If a difference in response to VPD is observed between C3 and C4 turfgrasses, this could be an important contributing factor in defining climate zones of adaptability. That is, sensitivity to increasing VPD, which is commonly associated with elevated temperature, might contribute to the lack of success of a species in warm-temperature climate zone. 2. Materials and methods Three sets of experiments (Table 1) were carried out between February and May 2008 in a glasshouse at the University of Florida campus (29◦ 38 N, 82◦ 22 W) in Gainesville, FL. The study included four C4 species: ‘Argentine’ bahiagrass (Paspalum notatum Flugg), ‘Tifway 419’ bermudagrass (Cynodon transvaalensis Burtt-Davy × C. dactylon (L.) Pers.), ‘Floratam’ St. Augustine grass (Stenotaphrum secundatum (Walt.) Kuntze), ‘Empire’ zoysiagrass (Zoysia japonica Steud.). Four C3 species were also studied: ‘Penncross’ creeping bentgrass (Agrostis palustris Huds.), ‘TQ Elite’ perennial ryegrass (Lolium perenne L.), ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.), and annual bluegrass (Poa annua var. annua). All grasses were grown in pots constructed from 10 cm diameter, 20 cm tall PVC pipes fitted with a flat end cap. A small hole was drilled into the center of the end cap for drainage. The top, open end of the pipes was fitted with a toilet flange for attaching a chamber to the pots when VPD measurements were made. Sod pieces (2.5 cm depth) of the C4 species were removed from established research plots at the University of Florida G.C. Horn Memorial Turfgrass Research Field Laboratory, Citra, FL, using a 10-cm diameter golf cup cutter. The sod was washed free of soil and established atop medium-coarse textured sand in PVC pots. C3 species were removed in a similar fashion from 10 cm deep trays of sod which had been established from seed and grown for 6 weeks in the greenhouse prior to beginning the experiment. Greenhouse temperatures during the establishment of turfgrass in PVC pots were controlled at 29/22 ◦ C (day/night). A complete slow-release fertilizer (24–5–11, Turfgro Professional, Sandford, FL) was applied at the beginning of each establishment period at 5 g N m−2 and plants were kept well watered with two or three irrigations per week. Grasses were given 4–6 weeks to fully root in the soil prior to VPD experiments. Visual observations confirmed that grasses had fully developed root systems which reached the container bottoms at the beginning of each experiment. All grasses were clipped weekly to the approximate recommended heights of cut for the given species. This corresponded to a height of 6.4 cm for all species except A. palustris, which was clipped to 8 mm. At the time of VPD measurement, all grasses had developed full canopies shading the soil surface so there was very little soil evaporation during the VPD tests. Measurements of transpiration rate of the plants were made simultaneously on 3 replicate pots for each of the C3 or C4 species, Table 1 Establishment and VPD test dates for the turfgrasses used in the experiments. Set 1 Establishment date VPD test dates
C4 C3
Set 2
Set 3
13 January
15 February
20 February
14,19 February 3, 5 March
21 March 26 March
31 March, 2 April 21, 22 April
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i.e., 12 plants per test. The plants in Sets 1 and 3 were measured on 2 days and in Set 2 on only 1 day (Table 1). Therefore, for each species there was the possibility of up to 45 observations (3 replicates × 5 days × 3 VPD levels). However, any measurements obtained when container temperature exceeded 32 ◦ C were discarded. Also, plants of Poa annus grew very poorly in Set 3 so this species was not measured at these later times resulting in a decreased number of potential observations. Consequently, the actual number of observations was less than the maximum and varied among species. All VPD tests were carried out during cloudless periods. The evening prior to each experiment, plants were fully watered and allowed to drain overnight. VPD treatments were imposed on grasses by attaching a chamber system to the PVC pots (Fletcher et al., 2007). This involved attaching a 26-cm diameter food container lid (with a 10-cm diameter circle cut out of the center) to the toilet flange at the top of the pot. A 6-L transparent plastic food container (Rubbermaid Commercial Products LLC, Winchester, VA) was then inverted and placed over the turf and sealed into the previously installed lid. The chambers extinguished approximately 25% of the solar radiation. The whole assembly was then placed in a greenhouse covered with density-neutral shade fabric. Light levels in the greenhouse were approximately 70% of irradiance incident to the greenhouse. Photosynthetic photon flux within VPD chambers was measured prior to each experiment using a quantum meter (Apogee Instruments, Logan UT). For all experiments, light levels in VPD chambers exceeded 650 mol m−2 s−1 . Each VPD container was equipped with a 12 V, 7.6 cm diameter computer box fan (Northern Tool and Equipment, Burnsville, MN) to mix air inside the chamber. The circulation provided by the fan was found through previous experiments to provide near equality between leaf and air temperatures. A pocket relative humidity/temperature pen (Extech Instruments, League City, TX) was mounted through the side wall of each container to measure the chamber environment. To achieve a range of VPD treatments, various humidity levels in the chambers were established by varying air flow rate into the chambers and by pretreatment of the air. On each day when a set of plants were tested, conditions in the VPD chambers were adjusted to expose the plants to three VPD levels. Measurements were initiated at the lowest VPD followed by the mid and high VPD settings to avoid any influence of exposure to high VPD on subsequent VPD tests. A low VPD treatment was achieved in the chambers by flowing air at approximately 0.4 L min−1 into each chamber using two pumps (DOA0P704, Gast, Benton Harbor, MI) with their intake tubes placed inside an atomizing humidifier (Herrmidifier, Sanford, NC). A medium VPD was achieved by flowing ambient greenhouse air into the containers at 2–3 L min−1 from two air compressors (Kobalt model L-215902, Lowe’s Companies Inc., North Wilksboro, NC). The highest VPD was achieved by first flowing air from the two compressors through PVC tubes of 76 mm diameter filled with six-mesh indicating Drierite (W.A. Hammond Drierite Co. Ltd., Xenia, OH) into the containers at 2–3 L min−1 . All flow rates were monitored using flow meters (Model FL-2043, Omega, Stamford, CT), one for each chamber. Measurement of transpiration rate at each VPD level was done after allowing the plants and air in the container to equilibrate for 30 min. VPD was calculated for each chamber based on the measured temperature and relative humidity. During each humidity treatment, air temperature and relative humidity were used to calculate an average VPD based on three measurements taken every 20 min during the 60 min measurement periods. At each setting, the humidity levels in the chambers proved to be stable over the 60 min period within individual chambers, although the VPD varied somewhat among chambers.
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For each temperature and humidity setting the transpiration rate per plant was calculated based on the change in mass between the start and end of each VPD measurement period. Mass was determined by weighing pots on a balance with a resolution of 0.1 g (Model SI-8001, Denver Instrument, Denver, CO). For all treatments, transpiration rates were measured over 60 min periods. Data were analyzed by regressing transpiration rate against VPD using observations from individual plants within a species. For each species and experiment, the data were analyzed using both linear and quadratic terms (PROC REG, SAS). Slopes and intercepts for individual experiments were compared to one another and no differences were found (˛ = 0.05), indicating that the relationship between VPD and transpiration did not differ over experiments within a species. Data were then pooled for a species for further analysis using GraphPad Prism 2.01 (Graph pad Software Inc., San Diego, CA, 1996). Pooled data were analyzed by plotting all individual transpiration rate (Tr ) data against VPD for each species. Initially, a two-segment linear regression using GraphPad Prism software was applied to the data: If VPD < X0 ,
Tr = S1 (VPD) + C1
(1)
If VPD = X0 ,
Tr = S2 (VPD) + C2
(2)
where X0 is the breakpoint between the two line segments, S1 and S2 the slopes of the first and second line segments, respectively. In the regression, the second line segment is constrained to intersect with the first line segment at X0 . The software uses all the data obtained for a species and solves for the regression coefficients of both linear segments including the breakpoint. The slopes of the two linear regressions (S1 and S2 ) were statistically compared to determine if they differed significantly (P < 0.05). If the slopes differed, the two-segment linear regression results were retained. When the slopes were not significantly different,
a simple linear regression was applied to all the data for that species. 3. Results Air temperatures inside the humidity containers for the data retained for this analysis were normally distributed with a mean temperature of 29.3 ± 1.5 ◦ C (S.D.). Hence, the main basis for variation in VPD in the chambers resulted from changes in humidity as a result of the air flow rate and the air source introduced into the chambers. Relative humidity in these tests ranged between 31 and 79%. Calculated VPDs were between 0.79 and 2.99 kPa. For all C4 species, a single linear regression provided the best fit (R2 = 0.57–0.79, all cases P < 0.0001) (Fig. 1). Only slight differences in water loss rates were detected among the C4 species. Slopes ranged from 0.27 mg H2 O pot−1 s−1 in C. traansvalensis and S. secundatum to 0.35 mg H2 O pot−1 s−1 in P. notatum and Z. japonica for each 1 kPa increase in VPD (Fig. 1). The two-segment linear regression provided the best fit for all C3 species (Fig. 2). (These data were also analyzed using a quadratic model but the R2 for these regressions were not superior to the two-segment linear regression.) As VPD increased within the low VPD range, transpiration of the C3 grasses increased sharply at rates ranging from 0.66 to 1.41 mg H2 O pot−1 s−1 (A. palustris and L. perenne, respectively) for each 1 kPa increase in atmospheric VPD. An apparent limitation on transpiration rate became evident at VPD in the range of 1.3 and 1.8 kPa. A. palustris had a higher breakpoint than the other three C3 grasses (Fig. 2). Noticeably lower rates of water loss rates were obtained for C3 species when VPD exceeded their VPD breakpoint. Limitation on transpiration rate occurring in response to increasing VPD for a given species could be seen by comparing slopes at low vs. high VPD (slope 1 vs. slope 2). As such, the most substantial constraints on transpiration rate among the C3 species occurred in A. palustris and L. perenne, while F. arundinacea showed the least sensitivity to increasing VPD.
Fig. 1. Transpiration rates of four C4 grasses in response to VPD. Each observation is shown as a single datum.
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Fig. 2. Transpiration rates of four C3 grasses in response to VPD. Each observation is shown as a single datum. Double-linear regressions provided a better fit for all C3 data than single-linear regression.
4. Discussion We are unaware of any previous attempts to evaluate the responses of C3 and C4 turfgrass species to VPD. These results demonstrated that as a group the C4 and C3 turfgrass species differed substantially in their response and sensitivity to VPD. While the C4 species appeared to lack sensitivity to VPD within the range studied, C3 species showed limitations on transpiration rate as VPD increased beyond about 1.3 kPa. Of course, this study was limited to a single cultivar per species so it is necessary to remember that variation in VPD sensitivity is certainly possible within a species (Ebdon et al., 1998). Nonetheless, differences between C4 and C3 turfgrass species in our study were striking. These observed difference in limitations on transpiration rate at high VPD, are likely to extend to decreased CO2 assimilation and growth with increasing VPD in C3 species due to decreased stomata conductance (deWit, 1958; Sinclair et al., 1984) at high VPD. These observed differences between the C3 and C4 grass species is consistent with observations in crops species responses to VPD. Pettigrew et al. (1990) found that grain sorghum (C4 ) gas exchange rates were less sensitive than well-watered soybean (C3 ) to increasing VPD. These results are also in agreement with to those of Kawamitsu et al. (1993), who found that low humidity (i.e., high VPD) caused lower leaf conductance, transpiration, and CO2 assimilation in the C3 Oryza sativa, but had little effect on stomatal responses and CO2 assimilation in the C4 P. maximum. Decreased humidity resulted in lower rates of photosynthesis in C3 Chenopodium album, but had no effect on photosynthesis of two C4 species Amaranthus hybridus and Portulaca oleracea (Bunce, 1983). Rawson et al. (1977) showed that photosynthesis and transpiration rate of whole barley (Horendum vulgare) plants (C3 ) decreased
across a range of VPD from 0.7 to 1.8 kPa (at a constant 26 ◦ C), indicating stomatal sensitivity to VPD. Unfortunately, it is difficult to compare our results to the few other VPD studies on turfgrasses, all of which involve C3 species, due to the limited number of species included in each study. Ebdon et al. (1998) studied evapotranspiration of only P. pratensis (C3 ), although they compared 61 cultivars. Exposure of plants to VPD treatments as high as 2.3 kPa, indicated substantial variability among cultivars in their response to high VPD. Gaussoin et al. (2005) evaluated carbon dioxide exchange rates of P. annua and A. palustris in response to VPD, but only two treatments at low VPD (0.5 and 1.0 kPa) were used. Our observations on F. arundinacea (C3 ) are in agreement with those of Sinclair et al. (2007) who found that shoot growth of this species declined at VPD exceeding ∼1.3 kPa with temperature held constant at 21 ◦ C. While the apparent threshold for regulation of transpiration we observed at 1.35 kPa for F. arundinacea was not as dramatic as with the other C3 species, the observed inhibition of gas exchange at high VPD is likely to contribute to the decreased growth previously reported at high VPD. Our results reveal unique differences in overall transpiration rates as well as sensitivity to VPD between the C4 and C3 turfgrass species. These findings are in agreement with previous research, which has shown that C4 grass species possess considerably lower evapotranspiration rates than C3 species (Beard, 1973; Huang and Fry, 1999). This has been attributed to the fact that C4 species contain the precursor organic acid pathway for capturing CO2 . Photosynthesis rates by C4 species similar to C3 species can be achieved at lower CO2 concentration within the leaves. Therefore, C4 grass species can have decreased stomata apertures and water loss rates while fixing CO2 at rates equal to or greater than those of their C3 counterparts (Taiz and Zeiger, 1998). Conversely, due to the inability
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to concentrate CO2 , C3 species require high stomata conductance. Therefore, C3 species may be more susceptible to encountering limitations on water transport in the plant to guard cells at high VPDs resulting in decreased stomata conductance. If VPDs were to be increased beyond the range studied here, a transpiration limitation might well be observed also in the C4 species. The breakpoint for the C3 species in terms of transpiration occurred in the range of 0.63–0.85 mg s−1 pot−1 (Fig. 2), which is the range of the maximum transpiration rates measured in the C4 species. Therefore, both groups of species may have the same limitation on water transport in the plant but in these experiments the transpiration rate, i.e., VPD conditions, were not sufficiently high to detect a possible breakpoint in C4 species at high VPD. Studies at higher transpiration rates are needed to fully resolve this possibility. However, a breakpoint in C4 species may be of less relevance than in C3 species for most environments, since VPD in many turfgrass would not greatly exceed the range of VPD tested. Acknowledgment This research was made possible in part through funding from the Southwest Florida Water Management District. References Anderson, D.B., 1936. Relative humidity or vapor pressure deficit. Ecology 17, 277–282. Aronson, L.J., Gold, A.J., Hull, R.J., Cisar, J.L., 1987. Evapotranspiration of cool-season turfgrass in the humid northeast. Agron. J. 79, 901–904. Beard, J.B., 1973. Turfgrass: Science and Culture. Prentice Hall, New York. Beard, J., 1997. Dealing with heat stress on golf course turf. Golf Course Manage. 7, 54–59. Beard, J.B., Daniel, W.H., 1965. Effect of temperature and cutting on the growth of creeping bentgrass (Agrostis palustris Huds.) roots. Agron. J. 5, 249–250. Bunce, J.A., 1981. Comparative responses of leaf conductance to humidity in single attached leaves. J. Exp. Bot. 32, 629–634. Bunce, J.A., 1983. Differential sensitivity to humidity of daily photosynthesis in the field in C3 and C4 species. Oecologia 57, 262–265. Carrow, R.N., 1996. Summer decline of bentgrass greens. Golf Course Manage. 6, 51–56.
deWit, C.T., 1958. Transpiration and Crop Yields. Institute of Biological and Chemical Research on Field Crops and Herbage. Wageningen, The Netherlands. No. 64.6. Ebdon, J.S., Petrovic, A.M., Zobel, R.W., 1998. Stability of evapotranspiration rates in Kentucky bluegrass cultivars across low and high evaporative environments. Crop Sci. 38, 135–142. Fletcher, A.L., Sinclair, T.R., Allen Jr., L.H., 2007. Transpiration responses to vapor pressure deficit in well watered ‘slow-wilting’ and commercial soybean. Environ. Exp. Bot. 61, 145–151. Gaussoin, R.E., Branham, B.E., Flore, J.A., 2005. The influence of environmental variables on CO2 exchange rates of three cool-season turfgrasses. Int. Turf. Soc. Res. J. 10, 850–856. Hartley, W., Williams, R.J., 1956. Centres of distribution of cultivated pasture grasses and their significance for plant introduction. In: Proc. 7th Int. Grassld Congr., Palmerston North, New Zealand, p. 190. Hirasawa, T., Hsiao, T.C., 1999. Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field. Field Crop Res. 62, 53–62. Howard, H., Watschke, T.L., 1991. Variable high-temperature tolerance among Kentucky bluegrass cultivars. Agron. J. 83, 689–693. Huang, B., Fry, J.D., 1999. Turfgrass evapotranspiration. In: Kirkham, M.B. (Ed.), Water Use in Crop Production. Food Products Press, Binghamton, NY. Huang, B., Liu, X., Fry, J.D., 1998. Shoot physiological responses of two bentgrass cultivars to high temperature and poor soil aeration. Crop Sci. 38, 1219–1224. Isoda, A., Wang, P., 2002. Leaf temperature and transpiration of field grown cotton and soybean under arid and humid conditions. Plant Prod. Sci. 5, 224–228. Kawamitsu, Y., Yoda, S., Agata, W., 1993. Humidity pretreatment affects the responses of stomata and CO2 assimilation to vapor pressure difference in C3 and C4 plants. Plant Cell Phys. 34, 113–119. Pettigrew, W.T., Hesketh, J.D., Peters, D.B., Woolley, J.T., 1990. A vapor pressure deficit effect on crop canopy photosynthesis. Photosynt. Res. 24, 27–34. Rawson, H.M., Begg, J.E., Woodward, R.G., 1977. The effect of atmospheric humidity on photosynthesis, transpiration, and water use efficiency of leaves of several plant species. Planta 134, 5–10. Sheffer, K.M., 1979. Response of three cool-season turfgrass species to heat and moisture stress. Ph.D. Diss. Univ. of Missouri, Columbia (Diss. Abstr. 80-07193). Sinclair, T.R., Tanner, C.B., Bennett, J.M., 1984. Water-use efficiency in crop production. BioScience 34, 36–40. Sinclair, T.R., Bennett, J.M., 1998. Water. In: Sinclair, T.R., Gardner, F.P. (Eds.), Principles of Ecology in Plant Production. CAB International, pp. 103–120. Sinclair, T., Fiscus, E., Wherley, B., Durham, M., Rufty, T., 2007. Atmospheric vapor pressure deficit is critical in predicting growth response of ‘cool-season’ grass Festuca arundinacea to temperature change. Planta 227, 273–276. Taiz, L., Zeiger, E., 1998. Plant Physiology. Sinauer Associates, Sunderland, MA. Wehner, D.J., Watschke, T.L., 1981. Heat tolerance of Kentucky bluegrasses, perennial ryegrasses, and annual bluegrasses. Agron. J. 73, 79–84. White, R.H., Stefany, P., Comeau, M., 1988. Pre- and poststress temperature influence perennial ryegrass in vitro heat tolerance. Hortscience 23, 1047–1051.