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a British Columbia Agroforestry Industry Dev. Initiative, c/o P.O. Box 4261, Quesnel, BC, Canada V2J 3J3
b Dep. of Agricultural, Food, and Nutritional Science, 4-10 Ag/For Centre, Univ. of Alberta, Edmonton, AB, Canada, T6G 2P5
* Corresponding author (george_powell{at}uniserve.com)
Received for publication February 10, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: APL, area per leaf • C3 plant, plants exclusively utilizing the Calvin cycle for photosynthesis • D, leaf/atmosphere vapor pressure difference • LS, leaf/stem ratio • N–P–K, nitrogen–phosphorus–potassium • PAR, photosynthetically active radiation • R/FR, ratio of red to far-red wavelengths • RGRH, relative height growth • RH, relative humidity • SLW, specific leaf weight • SML, stem mass/length ratio • SR, shoot/root mass ratio • T, temperature
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Modifications to temperature, airflow, and relative humidity, however, potentially benefit alfalfa and other plants utilizing the Calvin cycle for photosynthesis (C3 plants) by decreasing the leaf/atmosphere vapor pressure difference (D). The benefit is conveyed through improved stomatal conductance, which is necessary in C3 plants to exchange gases for active photosynthesis. As a result, photosynthesis in C3 plants is proportional to transpiration (Jones, 1992), with the latter influenced by light intensity, D, temperature, wind, as well as plant and soil water status (Black and Kelliher, 1989). Stomatal resistance to transpiration is the primary mechanism controlling water use in C3 plants, with stomatal conductance generally decreasing linearly with increasing D (Black and Kelliher, 1989). A large D creates a strong gradient for water movement from plant to atmosphere, which unchecked would result in plant desiccation or xylem cavitation. To conserve water, a large D initiates stomatal closure to slow or stop transpiration. This process simultaneously restricts or suspends gas exchange and photosynthesis (Dang et al., 1997), even when other conditions (PAR, soil moisture and nutrient levels) are optimal for plant growth. Recurring D-induced suspension of photosynthesis can ultimately reduce total annual production.
Alfalfa has well-adapted varieties for northern conditions but is generally intolerant of extensive shading (Wolf and Blaser, 1972). Alfalfa also exhibits declining midsummer growth rates compared with early or late-season production. Although slowed growth has been attributed to elevated temperature (Al-Hamdani and Todd, 1990), the concurrent effects of temperature and relative humidity on D have not been explored. As with most C3 species, stomatal conductance in alfalfa is positively correlated to production (Forde et al., 1977). Indeed, reduced stomatal conductance was responsible for 50% of the overall decline in net photosynthesis of drought-stressed alfalfa (Nicolodi et al., 1988). Indirect evidence suggests increasing D has negative consequences for alfalfa growth and survival. For example, alfalfa exhibits sun-avoiding movement of its leaflets under conditions of increasing atmospheric vapor pressure deficit (Reed and Travis, 1987), potentially as a mechanism to reduce leaf temperature and the associated D.
To date, the balance between facilitative and competitive effects of an aspen canopy on understory plant growth are unknown. If better understood, an optimal level of aspen cover could be prescribed for agroforestry applications that balance the negative effects of PAR reductions with the potential facilitation of understory growth through lower D. The general objective of this research was to isolate and compare the effects of variable PAR levels from the potentially facilitative effects of increased humidity (resulting in a lower D) on the early growth and development of alfalfa. Controlled-environment growth chambers were used to simulate contrasting two levels of PAR and relative humidity found under relatively dense boreal aspen canopies in central Alberta, Canada. The following hypotheses were tested:
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MATERIALS AND METHODS |
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Growth Chambers
Treatments were applied in controlled-environment growth chambers (Conviron CMP 4030). To simulate midsummer boreal conditions, growth chambers were set with a 16-h photoperiod, with day and night temperature of 22 and 17°C, respectively; the day temperature was within the optimum range for alfalfa photosynthesis (Brown et al., 1972). Field measures near Lac La Biche, AB, Canada (54°33' N lat; 112°05' W long) around midday (±2 h of solar noon) at midsummer, found a PAR range of approximately 1600 to 1800 µmol m–2 s–1 in the open under clear sky conditions. Only 11 ± 4% of full PAR was recorded in the understory of a dense boreal aspen stand (16319 ± 367 stems ha–1, height of 5.7 ± 0.2 m, and basal area of 22.7 ± 1.7 m2 ha–1) and 28 ± 4% of full PAR under the same aspen stand thinned to 6770 ± 640 stems ha–1. Other environmental parameters in the growth chambers were varied as per the treatment applications that follow, and approximated values observed previously under boreal aspen canopies at midsummer with a midday temperature of 20 to 25°C:
Treatments were assigned to growth chambers in a split-plot design with a factorial arrangement of subplots. Each humidity treatment was randomly assigned to a separate growth chamber (main plot) and was applied with the growth chambers' internal humidifiers. Within growth chambers, partitions constructed from white plastic tubing were arranged to separate the area into two equally sized compartments. A wood-framed center divider covered in two layers of black landscape fabric was placed to block lateral light transmission between compartments. Photosynthetically active radiation treatments were randomly assigned to these chamber compartments (subplots) and applied using either filtered or unfiltered light. Unfiltered light (high PAR treatment) emanated from a combination of florescent and incandescent bulbs suspended from movable ballasts. Filtered light (low PAR treatment) was applied by placing a frame covered with a combination of charcoal-colored fiberglass screening (New York Wire Co.) and acetate film bonded with a translucent layer of silver (3M Scotchtint Plus All Season Low E Window Film, LE50AMARL) on top of the chamber partitions, 60 cm above the pots. The film selectively filters red (R) wavelengths (655–665 nm) and simulates the effects of boreal aspen canopies removing more R than far-red (FR) wavelengths (725–735 nm), resulting in a decreased R/FR ratio (Ross et al., 1986).
During a test run of the experiment, temperature (to the nearest 0.01°C) and relative humidity (to the nearest 0.1%) within each chamber compartment were recorded at 1-min intervals with data loggers (Onset Computer Corporation, HOBO H8 Pro RH/Temp). Growth chamber environmental controls were calibrated to these actual measures by trial and error. Maximum daily variations of up to 1.5% RH and 1.4°C from prescribed treatment levels were observed. Continuous forced air circulation provided for relatively homogeneous environmental conditions throughout the chamber. However, due to the heat load emanating from the lights, an unavoidable 1.1°C difference in day temperature was noted between the filtered and unfiltered compartments. No difference in night temperature was observed.
Levels of PAR (radiation in the 400–700 nm bands) were confirmed with a LICOR LI-190SA quantum sensor placed in the center of each empty chamber compartment, 13 cm above the floor at a height corresponding to the top of the pots. Photosynthetically active radiation levels were monitored twice weekly, and recalibrated to the treatment specifications as necessary by raising or lowering the light arrays and replacing older bulbs. During five runs of the experiment (the test run and four experimental runs), high and low PAR levels averaged 240.5 ± 2.3 and 75.2 ± 0.8 µmol m–2 s–1, respectively.
In each run, 25 pots (subsamples) were placed in each chamber compartment in an equidistant 5 by 5 arrangement. Pot locations within each treatment combination were initially determined randomly and were re-randomized weekly when the chambers were emptied for PAR calibrations. Re-randomization minimized the potentially confounding effects of location differences within chambers. Pots were watered every 3 to 4 d to field capacity and fertilized weekly with a dilute solution of 20–9–17 (N–P–K) water-soluble fertilizer (0.8 g applied to each plant at each fertilization). Treatments were applied for 30 d, with the termination of each run coinciding with the early bud stage of alfalfa development in the high PAR treatment.
Measures
Daytime leaf temperature was measured (to the nearest 0.1°C) with an infra-red thermometer (0.2°C resolution ± 1% of reading) on upper canopy alfalfa leaflets. Thermometers were held 1 cm from the surface, perpendicular to the leaf. These measures required opening the growth chambers, which initiated air temperature changes in the chamber. For this reason, measurements were made in rapid succession, with a total of <30 s time required for measuring plants within a given compartment. Measures were conducted on two sets of 10 plants within each treatment combination of one experimental run. There was a 1-h lapse between measures on plants within a chamber to allow the internal temperature to fully stabilize. Alfalfa D was calculated from leaf temperature, prescribed air temperature and relative humidity, corrected for the conversion of the saturated pressure of pure water vapor to the saturation partial pressure of water vapor in moist air (Jones, 1992).
At the commencement of treatments and weekly thereafter, the number of leaves and shoot height (to the nearest 0.1 cm on the tallest stem) were recorded for each plant. Relative height growth (RGRH) was determined for each weekly period by dividing the change in height from beginning to end of each period, by the beginning height for that period. At the termination of each 30 d experimental run, plants were harvested and separated at the root crown into above- and belowground components. Leaves (including petioles) were separated from stems, and leaf area determined for each plant (to the nearest 0.01 cm2) by direct measurement on a LICOR, LI-3100 area meter. Area per leaf (APL) was estimated by dividing the total leaf area by the number of leaves at harvest for each plant.
Roots were extracted from the soil in a three-phase process. First, fine soil particles were separated from the roots by washing the pot contents with low-pressure through a 1.70-mm Canadian Standard Sieve (10-mesh Tyler equivalent, no. 12 U.S. equivalent). The remaining sieve contents were floated in clean water and extraneous material was discarded. The remaining material was strained from the water through a piece of black landscape fabric, and root segments were recovered by hand.
Yield components (leaves, stems, and roots) for each plant were determined by drying at 70°C to constant mass, and weighing to the nearest 0.01 g. Various indices of plant mass and size were calculated to examine the effects of PAR and relative humidity on alfalfa growth form and potential changes in photosynthate allocation. Specific leaf weight (SLW) was estimated by dividing the total leaf mass for each plant by its total leaf area. The leaf/stem ratio (LS) was calculated by dividing leaf mass by stem mass for each plant. Stem mass/length ratio (SML) was calculated by dividing stem mass by the sum of the lengths of all stems on each plant. Finally, the shoot/root mass ratio (SR) for each plant was determined by dividing the sum of leaf and stem masses by their respective root mass.
Analyses
Environmental controls failed during one run of the experiment in one chamber causing internal temperatures to briefly rise to 40°C; desiccation of the majority of alfalfa was noted in this chamber. As a result, data from this replicate (high humidity treatment, both high and low PAR) were not used in the analyses, leaving four replicate runs of the low humidity treatments and three runs of the high humidity treatments.
An analysis of variance of the treatment effects on leaf temperature, D, leaf area, APL, mass, SLW, SML, LS, and SR was conducted with mixed-models (Littell et al., 2002) for a split-plot design. Relative humidity and PAR treatments were assumed to have fixed effects, while the variation between each experimental run was assumed to introduce random effects. A Kenward-Roger correction was applied to the degrees of freedom to eliminate potential sample size bias. Comparisons of relative humidity means within PAR levels were obtained from F tests on the least-squares means partitioned from the main PAR effects (Littell et al., 2002).
The influence of relative humidity and PAR on weekly measures of alfalfa height, RGRH, and leaf and stem numbers were analyzed using mixed-models for repeated measures (Littell et al., 2002). A Kenward-Roger correction was applied to the degrees of freedom. The covariance models used in the analyses were selected iteratively for each response variable by testing several structures and comparing their Schwarz's Bayesian information criterion. This criterion test is based on the maximum likelihood fit corrected for the number of parameters in the model, analogous to the adjusted R2 employed in multiple regression analyses. For the analysis of leaf and stem numbers, first-order auto-regressive structures were employed. An auto-regressive covariance structure reflects the fact that observations on the same unit are more highly correlated to those taken close together in time than those measured farther apart in time. For the analysis of height and RGRH data, first-order ante-dependence covariance structures were utilized. Ante-dependence models can be regarded as an extension of the autoregressive model (Littell et al., 2002) with the covariance between observations taken at two points in time being the product of variances at both points and the correlation between the two sampling intervals. Simple effects in the interaction of PAR or relative humidity at each weekly interval were obtained from F tests on the least-squares means partitioned from the main analyses.
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RESULTS AND DISCUSSION |
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Although PAR had a significant positive influence on RGRH over the full period of alfalfa growth (p < 0.01), this was due largely to a strong influence (p = 0.01) on growth rates between the second and third week after the commencement of treatments (see separation of high and low PAR treatments in Days 7–14 and 14–21; Fig. 2) . This coincided with a period of major expansion in stem length, as well as the number of leaves and secondary stems of all plants (Fig. 1). Thus, high PAR appeared to accelerate growth during an important development period. There was also a difference in the temporal development of new stems between the two PAR treatments. Alfalfa in the high PAR treatment began expressing secondary stems 7 to 14 d after the onset of treatments (Fig. 1). In contrast, alfalfa at the lower PAR level generally did not form new stems until after the second week (14–21 d from the onset of treatments).
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Combined Effects of Photosynthetically Active Radiation and Relative Humidity
The PAR and relative humidity treatments interacted to influence several attributes of alfalfa growth. Alfalfa leaf area (p = 0.03), leaf mass (p = 0.03), stem mass (p < 0.01), shoot mass (p = 0.02), and total mass (p = 0.04) increased with greater relative humidity at high PAR, but did not differ (p = 0.55 to 0.88) within the lower PAR treatment (Table 1). Likewise, the combined effects of relative humidity and PAR strongly affected the number of alfalfa stems per plant (p < 0.01) between Day 7 and 14 (Fig. 1). Unlike the mass and leaf variables, the number of stems per plant showed inconsistent effects at high PAR, but was greater with elevated relative humidity at low PAR (Fig. 1). Interactions of light and humidity have also been observed in the growth of other species (Roberts et al., 1984).
In general, the influence of relative humidity in this study had consistent, positive effects on alfalfa growth but was only strongly expressed on variables that were also positively affected by PAR. This is consistent with the theory that the primary benefit of elevated relative humidity is through enhanced photosynthetic gas exchange by lowering D, which, in turn, facilitates photosynthesis provided that PAR is at adequate levels. The divergent response to relative humidity at different PAR levels is also consistent with this theory. At the lower PAR level, the magnitude of the positive relative humidity effect would be minimal due to the lower potential for photosynthesis. Conversely, at higher PAR, greater photosynthetic activity is augmented by a larger positive effect of relative humidity.
Significance to Annual Production and Agroforestry Design
These data demonstrate that sustained, elevated relative humidity has a positive effect on some aspects of alfalfa growth at a D typical of summer conditions in north temperate and lower boreal ecosystems. However, these effects were generally expressed only from the middle to the end of the 30-d growth period. This may indicate that relative humidity must remain elevated for an extended time to produce measurable positive effects. It is important to note, however, that PAR also tended to have its greatest effects on alfalfa during the second week of growth. Therefore, the observed effects of relative humidity may be limited to specific early development periods in alfalfa, particularly if its primary mechanism for enhancing growth is by augmenting the effects of PAR.
It is important to note that growth chamber conditions cannot fully reflect the patterns of light interception under natural field conditions. Under field conditions, relative humidity and temperature are highly dynamic and display strong diurnal variation, as well as daily and seasonal fluctuations. Boreal and subboreal climates are restrictive for plant growth, in part because of the brief annual growing season (Bonan and Shugart, 1989). Within this short growth period, recurring interruptions to photosynthesis could have a strong effect on total annual production. However, the prevalence or restriction of a large D to plant growth has not been well documented in these ecosystems, and the limited information available does not demonstrate a strong role of D in long-term production (Hogg et al., 2000).
The results of this study indicate that the elevated relative humidity common under aspen canopies can compensate for some of the lost growth potential due to concomitant light reductions, but cannot fully counteract the reduced photosynthetic potential of alfalfa at the two PAR levels tested here. At PAR levels closer to light saturation of photosynthesis in alfalfa, however, diminishing returns in growth from additional PAR increments may result in relative humidity, having a proportionately greater overall effect on growth. Further testing of relative humidity and PAR interactions across a broader range of conditions is necessary to fully assess this potential. Thus, designing temperate agroforestry systems solely to elevate relative humidity in the understory is unlikely to compensate for reductions in PAR unless light levels are close to saturation for a given understory species. Finally, the role of other growth-promoting factors associated with an overstory—such as reduction of water evaporation from the soil or enhanced nutrient cycling—may also contribute to a greater net facilitative effect within integrated tree–forage productions systems and require testing.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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