Guar responses to temperature: Estimation of cardinal temperatures and photosynthetic parameters

Temperature is the primary environmental determinant of geographic distribution and productivity of plant species. There has been increased interest in the cultivation of guar [Cyamopsis tetragonoloba (L.) Taub.] in many countries including the United States due to the high demand of guar gum in the oil fracking industry. However, information on gaur response to temperature is still lacking in the current literature. An experiment was conducted using six different day/night temperature regimes (20/12, 24/16, 28/20, 32/24, 36/28 and 40/32 degrees C) maintained inside walk-in growth chambers of the Controlled Environment Research Facility at Oklahoma State University. The objectives of this study were to evaluate the temperature responses of vegetative growth, development, and reproduction of guar to estimate cardinal temperatures, and to evaluate photosynthetic responses to temperature gradient. The vegetative growth of guar was sensitive to low temperature ( < 20 degrees C), and has high mean cardinal temperatures (T-min, T-opt, and T-max) for biomass accumulation (14.2, 34.1 and 48.2 degrees C). Plants exhibited reproductive development within a narrow range (19-31 degrees C) with maximum pod yields observed at a T-opt of 25 degrees C. Higher temperature regimes (3 6 / 2 8 and 40/32 degrees C) completely suppressed flower initiation in guar. Responses of photosynthesis to both light and internal CO2 concentration (A-C-i) suggested guar photosynthetic rates increased under high temperature through an increased rate of electron transport. The identified cardinal temperatures and photosynthesis parameters can be used to develop mechanistic crop models to simulate adaptation strategies for guar. Further research is required to investigate the basis for suppression of flower bud initiation under heat stress.