Comparison of Leaf Gas Exchange and Water‐Use Efficiency in Two Eastern Gamagrass Accessions1

Abstract

Eastern gamagrass [Tripsacum dactyloides (L.) L.] is a highly productive, C4, perennial forage grass that is native to the eastern half of the USA. This species is most adapted to subirrigated sites or areas with at least 875 mm annual precipitation. Physiological data useful for improvement of this species are lacking. This paper documents photosynthetic potential and water‐use efficiency (WUE) of two greenhouse‐grown accessions: WW‐1318, a narrow‐leaf and WW‐1462, a wide‐leaf genotype. Carbon dioxide exchange rate (CER) and transpiration (E) were monitored in a temperature‐controlled, steady‐state leaf chamber from which stomatal (g′s) and residual (g′r) conductances for CO2 were calculated. Carbon dioxide exchange rate approached the highest reported values for C4 species under optimum conditions and ranged from 50 (WW‐1318) to 61 (WW‐1462) μmol m‐2s‐1. Although E was relatively high (5 to 6> mmol m‐2s‐1), transpiration efficiency (CER/E) reached 10 mmol mol‐1 which is among the highest reported values. For full sunlight conditions, optimum temperature for CER (Topt=35°C), an external CO2 (Ca) of 13 mmol m‐3, and a vapor pressure difference (VPD) of 2 kPa, these grasses were fixing C at about 0.65 (g′r/g′t g′t=g′s+g′r) of their potential maximum if limited only by CO2 diffusion through the stomata. Eastern gamagrass was highly efficient for a range of environmental conditions because g′r exceeded g′s whenever light was greater than about 0.15 of full sunlight and temperature was between 20 and 40°C. Although CER/E and g′r/g′t increased to maximum levels at one‐third full sunlight at Topt in both accessions, WW‐1318 maintained this efficiency to full light compared to an approximate 15% loss in efficiency in WW‐1462. The response of CER vs. g′s, as light was increased at constant T, VPD, and Ca, was linear for WW‐1318 but curvilinear for WW‐1462. This, together with the decline in WUE for WW‐1462 at higher light levels, suggested better control of stomatal conductance relative to that required to sustain but not limit CER in WW‐1318. Stomata in both grasses tended to function in a manner that stabilized internal CO2, (Ci) at about 2.6 to 4.6 mmol m‐3 for Ca=13 mmol m‐3, once light was above 0.25 full sunlight. The Ci/Ca stabilized at 0.3 to 0.4 at full sunlight once Ca exceeded 12 mmol m‐3 (35°C). Stomatal function appeared more nearly optimized in WW‐1318 in that dCER/dE was much less sensitive to increasing light or temperature than in WW‐1462. These data indicate that Eastern gamagrass has comparatively high photosynthetic and WUE potential when soil water is not limiting. The differences in the measured physiological parameters between these gamagrass strains suggest potential exists for germplasm enhancement with respect to maintaining optimum performance in variable environments.