Evaluating Salt Tolerance: Some Experiences
with Nondormant Alfalfa.
Steve Smith
Department of Plant Sciences, University of Arizona,
Tucson, AZ 85721
Without elaborate irrigation or drainage systems, accumulation of excessive levels of soil salinity (salinization) is inevitable in most environments where irrigated alfalfa is grown. Worldwide, nondormant alfalfas are often grown in warm desert environments where irrigation is essential and the potential for salinization is high. Alfalfa is considered moderately sensitive to salinity and yield reductions of about 15% will result with electrical conductivity (EC) in the soil solution of only 4 dS m-1. EC's of soil solutions on over 40% of the cropland in the Imperial Valley of California average above 4 dS m-1. Changes in cropping practices or improvement of crop salt tolerance through plant breeding may offer temporary solutions to salinization problems in these environments.
Breeding for improved salt tolerance in alfalfa is complicated by a variety of factors, most notably the very low heritability of tolerance. Different growth stages in alfalfa also appear to respond independently to salinity. Mass selection at increasing levels of NaCl has been successful in producing a population that germinates in very saline environments (35 dS m-1). Populations with less germination salt tolerance have been produced by commercial plant breeders. Describing germination salt tolerance has been problematical. Using data from trials conducted using a range of salinities, we have shown that probit analysis permits calculation of the level of salinity where a certain proportion of seeds in a sample (e.g., 50%) would be expected to germinate.
We have developed and refined a greenhouse procedure to evaluate forage yield of single plants under moderate salt stress. This technique involves plants grown in 23-cm conetainers in artificial soil. They are irrigated with water having EC=7.3 (or 9.1) dS m-1, and forage is harvested and weighed three times. By growing populations under these conditions but using non-saline irrigation it is possible to measure salt tolerance per se by the saline/non saline yield ratio. Mass selection using this protocol has resulted in populations with increased forage yield under these experimental conditions. Some populations produced using other selection procedures exhibit both increased yields and salt tolerance. Selection using more saline irrigation water ( 10 dS m-1) may be worthwhile. Relationships between performance in this greenhouse trial and in the field are not well known. While economical, visual selection appears unlikely to be as efficient as the protocol outlined above for increasing either forage yield with saline irrigation or salt tolerance.
Mechanism(s) of tolerance to salinity utilized by alfalfa are unknown. A better understanding of tolerance mechanisms could permit more deterministic approaches to plant breeding. However, this appears unlikely in the near future.
Reference
Al-Doss, A., Smith, S.E. 1998. Registration of AZ-97MEC AND AZ-97MEC-ST very nondormant alfalfa germplasm pools with increased shoot weight and differential response to saline irrigation. Crop Sci. 38:568.