Abstract:
Water relations and drought resistance in vetiver grass [Vetiveria zizanioides (L.)
Nash.] were studied in laboratory and greenhouse experiments. Vetiver clones used in this study were obtained from the Plant Materials Center, Galliano, Louisiana. The plants were grown in plastic pots (25 em x 25 em, height x diameter) with Cornell mixture. My study focused on five important water-relations parameters: xylem pressure potential (XPP), relative water content (RWC), bulk modulus of cell wall elasticity (E), transpiration rate (q), and leaf diffusive resistance (LDR) to understand the plant-water relations. The measurement ofXPP under different water regimes was used to estimate theoretical limits of drought resistance in the experimental plants. All the water-relations parameters were measured in the laboratory, and experiments on drought resistance were conducted in the greenhouse.
The predawn xylem pressure potential of well-watered plants was recorded as -0.09 ± 0.005 MPa (SE). It decreased to -0.93 ± 0.036 MPa at midday, but the experimental plants fully recovered from the midday stress by the next dawn. Approximately 50% of leaves on plants dried when the mean predawn XPP was -4.4 ± 0.35 MPa (SE). Vetiver grass has elastic cell walls [E = 0.35 ± 0.076 MPa, (SE)] and low osmotic potential when compared to many forage grasses. The minimum
transpiration rate of the plants was 0.242 ± 0.04 ""g cm-2 S-I (SE), and a negative exponential relationship was found between transpiration rate and XPP. The minimum leaf diffusive resistance (LDR) of the experimental plants was calculated as 1.47 ± 0.27 s cm-I(SE) and the maximum LDR was greater than 127.3 s em-I. The LDR in vetiver grass appears to be more influenced by atmosphere vapor pressure deficit than by leaf water potential. Under well-watered conditions, the relative water content (RWC) of the experimental plants ranged from 9004 to 95.8%. The mean RWC at the point of turgor loss, as obtained from pressure-volume analysis, was 40.1 %. The RWC of the leaves decreased linearly with its xylem pressure potential. Results showed that vetiver grass behaves similarly to many drought-resistant C4 grasses in water relations and possesses most of the physiological characteristics that favor the plant's growth and survival in
semi-arid or arid environments.
