Understand heat stress, develop specialty lima beans (The Vegetable Grower)
(Editor’s note: Gordon C. Johnson is an Extension vegetable and fruit specialist with the University of Delaware.)
(Oct. 10, 2017) Heat stress reduces yields of May and early June-planted lima bean on the Delmarva Peninsula.
High night temperatures during flowering and seed development can reduce or delay pod set, resulting in delayed harvest, lower yield and split pod sets.
Breeding heat tolerant baby and Fordhook type lima beans is one goal of the University of Delaware lima bean breeding program.
Greenhouse experiments were used to characterize the response of several lima bean varieties and selections to high versus ideal nighttime temperatures in order to better understand the mechanism by which high night temperatures reduce yield.
Past experiments had indicated that higher amounts of pollen shed under heat stress are correlated with higher yield under heat stress, and that there is variation between varieties or breeding selections for this trait.
In more recent experiments, heat sensitive lima beans exhibited a number of physiological changes while under heat stress, some of which may interfere with reproduction and ultimately yield.
Heat sensitive types had abnormal flower development inhibiting pollination and pollen collected from heat sensitive lima beans had lower viability. Therefore, fewer pods set in heat sensitive types.
However, when pods did set, seed number in the pod was not affected by heat stress, indicating that some aspects of reproduction are not affected in heat sensitive types.
Other aspects of reproduction, such as female flower part receptivity, may be problematic in some heat sensitive genotypes, but not others. As evidence, in one heat sensitive type grown under heat stress conditions, mature seed was obtained in 40 percent of flowers hand-pollinated with pollen from a heat tolerant type that had been grown in ideal conditions.
Only 3 percent of self-pollinated flowers on the same heat stressed plants produced mature seed.
In a different, more heat sensitive genotype, mature seed was produced in only 10 percent of similar hand pollinations, compared to no mature seed from self-pollinated flowers.
Vegetative growth was not reduced by high night temperatures.
Plants grown under stressed and unstressed conditions produced similar shoot dry weights.
Heat sensitive plants produce more leaves and stems under high temperature conditions, compensating for the reduction in seed weight.
In the University of Delaware lima breeding program headed by Emmalea Ernest, characterization of some of the physiological changes associated with heat sensitivity is being used to screen diverse germplasm and breeding lines in order to select for heat tolerance.
The University of Delaware initiated a breeding program 12 years ago, focusing on green baby lima beans and green Fordhook types.
In the breeding program, diverse lima bean germplasm is used for crosses and a wide variety colors, forms, and qualities result in the progeny.
While the focus of the program is on green types, there is potential for developing and introducing new specialty types adapted to Delmarva
We currently have a project seeking to identify and evaluate new specialty lima beans from the Delaware breeding program and from U.S. and international germplasm collections.
Currently we are evaluating an improved speckled type, a mixed green and speckled type, a white type, and a red type.
We are also evaluating a collection of diverse lines maintained by the UD lima bean breading program for cooking and eating characteristics for use in further breeding of specialty limas.
Types that are non-bleeding (that do not release soluble colors into the cooking water) are sought as well as those with desirable texture (that are not grainy or mealy) and taste (buttery, nutty, other positive flavors) for use in breeding new succulent specialty lima beans.
This material, because of its diverse sources from germplasm collections, consists mainly of vining pole types.
An offshoot of this objective would be the identification of pole lima types from this diverse material that could be grown as specialty lima beans by small farmers.
We are also evaluating consumer acceptance of specialty succulent lima beans from the UD breeding program.
A current limitation to the growth of the lima bean industry in Delaware, and the United States as a whole, is overall demand.
Quality characteristics of identified specialty succulent lima beans impacting consumer acceptance will be evaluated such as color bleeding, texture, and taste parameters using sensory and taste panels.
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