The effects of salinity, irrigation and harvest regime on stem water potentials and leaf chlorophyll fluorescence parameters in Medicago sativa L

Background/Question/Methods

Uncertainties regarding future precipitation regimes in the United States have raised concerns about the impacts of drought on agriculture in the Midwest.  Irrigation may alleviate some potential impacts, but the quality of water may affect performance and yield. Data on water use of crops throughout the country are crucial for optimum management strategies, especially for cellulosic ethanol production.  Recently, alfalfa (Medicago sativa L.) has been proffered as a potential feedstock for cellulosic ethanol. However, alfalfa has a high water requirement compared to other crops and is only somewhat tolerant to moderate levels of salinity.  We examined the influence of irrigation and water quality on stem water potential and chlorophyll fluorescence in eight alfalfa varieties for two growing seasons in Southern Minnesota. Plants received weekly applications of (1) 1.27 cm of well water (0.75 dS/m), (2) 1.27 cm of saline (NaCl) water (5.0 dS/m) or (3) ambient precipitation. In addition, plants were repeatedly harvested at the full-bud and 50%-flower growth stages. We measured stem water potentials with a Scholander bomb and the quantum yield of photosystem II (PSII) electron transfer (Φ_PSII) and the ratio of the variable to maximal fluorescence (F_v/F_m) with a pulse-amplitude modulated fluorometer prior to each cutting.  

Results/Conclusions

Historical precipitation at our field site during the growing season (Apr – Oct) averages 70.9 cm.  Total precipitation received was 91.9 and 48.4 cm in the 2010 and 2011 seasons, respectively. In the wet-2010 season, alfalfa stem water potentials under the saline treatments were more negative towards the end of the growing season averaging -0.69 MPa compared to -0.64 MPa from plants in irrigated or control plots.   In the drier-2011 season the addition of irrigation and/or salinity reduced stem water potential in all eight varieties of alfalfa.  Stem water potentials ranged from -0.92 to -1.03 MPa in the control plots and -0.83 to 0.93 MPa under the irrigated and saline treatments.

There were no treatment effects on any of the chlorophyll fluorescence parameters during the 2010 season. However in 2011, irrigated plants had higher Φ_PSII values early in the growing season averaging 0.60 compared to 0.57 and 0.58 under the saline and control treatments.  There were no significant treatment effects on F_v/F_m during the 2011 season.  No growth stage effects were observed on any of the parameters we observed in this study.  Taken collectively, it appears that all eight varieties of alfalfa we examined are tolerant to moderate levels of salinity.