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CROP PHYSIOLOGY & METABOLISM

Gas Exchange and Transpiration Ratio in Sorghum

^a Texas AgriLife Research and Extension Station, Bushland, TX 79012
^b current address, Tidewater AREC, 6321 Holland Road, Virginia Tech, Suffolk, VA 23437
^c Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
^d Texas AgriLife Research and Extension Station, Lubbock, TX 79403. Mention of a trade name or product does not constitute endorsement

^* Corresponding author (m-balota{at}tamu.edu; mbalota{at}vt.edu).

Decreased water availability and increased food demand worldwide require development of more water-efficient crops. This study was conducted to examine preflower transpiration ratio (CO₂ assimilation rate [A]/transpiration rate [E], A:E) in four inbred sorghum [Sorghum bicolor (L.) Moench] lines and 12 of their F₁ hybrids in terms of phenotypic and genetic variation, inheritance (i.e., additive and nonadditive genetic effects), and its relationship to water use efficiency (WUE) (i.e., the ratio of whole-plant biomass to cumulative transpiration). Lines were selected with contrasting A and A:E in a field trial and then crossed in a full diallel pattern. All hybrids and lines were further analyzed for A, E, A:E, biomass, and WUE in a greenhouse. Variation among hybrids and lines was significant for all traits evaluated. In both environments, the most contrasting lines for A:E were Tx430 and Tx7078. Average A:E was 3.10 mmol CO₂ mol^–1 H₂O for Tx430 and 2.91 for Tx7078. Both Tx430 and Tx7078 had highest A. Genetic effects resulting from general (GCA) and specific combining ability (SCA) for A, E, and A:E were significant for all traits. Tx430 had positive and Tx7078 negative GCA effects on A:E. Tx430 hybrids had the highest A:E values, and Tx7078 hybrids the lowest. The A:E ratio was correlated with total biomass and WUE for all crosses. Data suggest that concomitant selection for high A:E and A at preflowering could result in improved WUE and biomass of mature sorghum.

Abbreviations: A, CO₂ assimilation rate • A:E, transpiration ratio • E, transpiration rate • GCA, general combining ability • HW, high soil water content • LSD, least significant difference • LW, low soil water content • PPFD, photosynthetic photon flux density • RH, relative humidity • SCA, specific combining ability • SES, standard error of the slope • T_a, air temperature • VPD • vapor pressure deficit • WUE, water use efficiency (dry matter/cumulative transpiration)