Alfalfa Stem and Leaf Growth during Water Stress

Alfalfa (Medicago sativa L.) requires irrigation in semiarid and arid regions and improved yields often result from irrigation in subhumid and humid areas. Despite the importance of optimizing water management for alfalfa leaf and stem growth, no field study has been reported which relates alfalfa leaf expansion or stem extension to plant water deficits. The objective of our research was to determine how leaf expansion, stem elongation, and stomatal response of alfalfa related to the water potential in alfalfa, field-grown on Plainfield loamy sand (mixed, mesic Typic Udipsamment). Periodic measurements of alfalfa stem extension and leaf expansion (area derived from length and width measurements) were related to leaf water potential (ΨL) measured with a pressure chamber, soil matric potential (Ψm, measured with tensiometers, and stomatal conductance measured with porometers. For wellwatered alfalfa, expansion curves of individual leaves were sigmoid-shaped, while internode extensions were nearly exponential with time through about 95% of the total extension. The primary leaf attached at the top of an internode expands to nearly full size before the internode begins rapid extension. The growth of individual leaves and internodes collectively results in nearly constant rates of leaf expansion and stem extension during vegetative regrowth. When Ψm at 30 cm decreased to −0.35 bar, afternoon ΨL decreased to about −8 bars, about 2 bars below irrigated alfalfa, and both the growth rate (GR) and relative growth rate (RGR) of stressed leaves and stems were significantly less (p < 0.1) than well-watered leaves and stems. In stressed treatments, wilting, gray-green canopy color, decreased afternoon stomatal conductance, and a 50% decrease in RGR as compared to irrigated treatments resulted when afternoon ΨL decreased to about −15 bars even though Ψm exceeded −0.1 bar at 90 and 150 cm. Alfalfa extracted 85% of its total water from the 1st meter of soil prior to the onset of decreased afternoon stomatal conductance, but nearly equal percentages from each of the top 2 m of soil afterward. Little shoot growth occurred when ΨL dropped below −10 bars. Development of water stress 2 weeks after cutting reduced alfalfa leaf size, internode length, and dry matter production, but not leaf and internode number or stem density. The percentage of total forage dry weight in leaves (LPDW) decreased linearly with stem height resulting in higher LPDW and a better quality forage in stress-stunted forage. Cool night temperatures limited growth of well-watered alfalfa.