Radiation use efficiency and leaf CO₂ exchange for diverse C₄ grasses

Abstract

Biomass accumulation of different grass species can be quantified by leaf area index (LAI) development, the Beer–Lambert light interception function, and a species-specific radiation-use efficiency (RUE). The object of this field study was to compare RUE values and leaf CO₂ exchange rates (CER) for four C₄ grasses. Biomass, LAI, and fraction of photosynthetically active radiation (PAR) intercepted were measured during three growing seasons. CER was measured on several dates and at several positions in the canopies. Switchgrass (Panicum virgatum L.) had the greatest RUE whereas sideoats grama [Bouteloua curtipendula (Michaux) Torrey] had the lowest. Big bluestem (Andropogon gerardii Vitman) and eastern gamagrass [Tripsacum dactyloides (L.) L.] values were intermediate. The two species with the greatest differences in RUE, switchgrass and sideoats grama, had similar relative amounts partitioned to roots. Likewise differences among species in the accumulation of soil carbon showed trends similar to total shoot biomass production. The light extinction coefficients (k) of switchgrass, big bluestem, and eastern gamagrass were smaller than for sideoats grama, implying that light was more effectively scattered down into the leaf canopy of the first three grasses. Whole canopy CER values were calculated with a stratified canopy approach, using LAI values, the Beer–Lambert formula with appropriate extinction coefficients, and CER light response curves. Differences among species in RUE were similar to these values for estimated whole-canopy CER divided by the fraction of light that was intercepted. High LAI along with low k contributed to higher RUE in switchgrass, in spite of lower values for single-leaf CER.

Introduction

Modelling grass biomass accumulation requires knowledge of rate-limiting processes to accurately simulate growth. Increased understanding of factors controlling grass biomass production will help define productivity of different environments. Thus, limitations due to stress or due to canopy architecture and leaf area can be better understood.

Radiation-use efficiency (RUE) is an effective and efficient approach to quantifying plant biomass accumulation. A species’ RUE may be defined as the aboveground dry weight increase per unit intercepted photosynthetically active radiation (PAR). Values for C₄ grasses such as bluestems, indiangrass [Sorghastrum nutans (L.) Nash], johnsongrass [Sorghum halepense (L.) Pers.], and elephantgrass (Pennisetum purpureum Schum.) can be similar to those of the C₄ crops maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench]. A grassland in Kansas dominated by big bluestem, little bluestem (Schizachyrium scoparium (Michaux) Nash) and indiangrass, had RUE values of 1.4–3.4 g MJ⁻¹ absorbed PAR [1]. This is equivalent to 1.1–2.7 g MJ⁻¹ of intercepted PAR, assuming 80% of intercepted PAR is absorbed [2]. Johnsongrass in Texas had an RUE of 2.3 g MJ⁻¹ intercepted PAR [3]. In Florida, elephantgrass RUE [4] was 2.9 g MJ⁻¹ intercepted PAR when forage yields [5] were 4700 g m⁻². Mean RUE of maize for several locations, using intercepted PAR, was 3.5 g MJ⁻¹ and the mean for sorghum was 2.8 g MJ⁻¹ [6].

The relationship between forage RUE and leaf CO₂ exchange rate (CER) is interesting from an application viewpoint and for the basic understanding of the physiology of biomass production. Close correlation between RUE and total canopy CER per unit intercepted PAR would provide better understanding of interspecies differences in RUE. Differences in canopy structure, as described with the extinction coefficient in the Beer–Lambert formula [7] and the leaf area index (LAI) could help explain species differences in RUE.

Efforts at finding a relationship between plant biomass productivity and single-leaf CER frequently fail. This was the case for comparisons among cultivars within C₃ species [8], [9], [10], for comparisons among different C₃ species [11], [12], [13], [14], and for comparisons between C₃ and C₄ species [14], [15]. Even Zelitch, who argued that such studies did not adequately integrate CER over the leaf canopy and over the season [16] admitted that the difference in productivity between two tobacco (Nicotiana tabacum L.) cultivars was primarily due to different leaf areas and thus different photosynthetic area and sink size [17]. When comparing big bluestem, switchgrass, and indiangrass, Polley et al. [2] concluded that differences in total canopy CER among species were more related to morphological differences than to single-leaf CER. Similarly, when Wullschleger et al. [18] studied several switchgrass populations at three locations, they found no relationship between single-leaf CER and forage yield. However lowland (tetraploid) switchgrass populations had 10–15% greater photosynthetic rates than upland (octoploid) switchgrass populations.

One trait contributing to differences among species for plant growth rate or RUE is the leaf orientation. More erect leaf types might cause

spreading of the incoming light over more leaf area, decreasing the average light intensity intercepted by an individual leaf, [resulting in] more efficient conversion [of light] and greater yield [19].

Using the light extinction coefficient value (k) in the Beer–Lambert formula to quantify efficiency of light interception per unit leaf area index, more erect leaf types have smaller k. Growth rate was negatively correlated with k for different C₃ grass species [11], [13] and for different perennial ryegrass selections [20], supporting the erect leaf advantage concept. Techniques of combining the Beer–Lambert formula to determine leaf illumination down in the canopy, with functions for CER response to PAR have provided realistic estimates of whole canopy productivity [21], [22].

Leaf nitrogen (N) concentration has been described as a factor in plant biomass productivity, especially in N-deficient conditions [23]. Leaf CER was shown to be related to leaf-N concentration for guineagrass (Panicum maximum) [24], for big bluestem and indiangrass [2], and for new and old rice cultivars [25].

Differences in partitioning to roots and shoots could cause erroneous conclusions when comparing productivity among species if only the aboveground biomass is measured. Ideally, productivity should be analyzed in terms of shoots, roots, and soil carbon to compare leaf CER measurements with plant productivity.

The objective of this field study was to quantify the RUE of four C₄ grasses: sideoats grama, big bluestem, eastern gamagrass, and switchgrass. The LAI and light extinction coefficients were determined for these species to quantify how the grasses intercepted PAR. Single-leaf CER was measured and root:shoot ratios and specific leaf nitrogen were measured to assess how these contributed to differences in RUE among species. Total canopy CER was calculated by stratifying the leaf canopies into layers. Thus canopy CER was related to RUE without undertaking the complexities of diffuse:direct beam radiation [26] or the distribution of leaf nitrogen [27]. These results should help simulation modellers focus their efforts on predicting biomass production at sites dominated by C₄ grasses.