The Manifestation of Dehydration Avoidance in Wheat Breeding Germplasm 1

The genetic improvement of wheat for conditions of drought stress requires an evaluation of specific physiological attributes related to drought resistance, as a basis for developing a working screening procedure. An important component of drought resistance is dehydration avoidance. Dehydration avoidance is defined as plant ability to maintain a relatively higher leaf water potential under conditions of soil moisture stress. The purpose of this investigation was to document the existance of and variability in dehydration avoidance within a random sample of advanced and well-adapted wheat (Triticum sp.) genotypes. Midday leaf water potentials (Wi), as a measure of dehydration avoidance, were monitored with a pressure chamber in the various wheat genotypes, simultaneously subjected to soil moisture stress in three different field tests. Leaf diffusive resistance (R,) was monitored with a diffusion porometer in two of the tests. Genotypes varied significantly in their midday leaf water potentials. About 2 bars of ^i were sufficient to differentiate significantly between genotypes. Variations in Wi were greater than variations in RI among genotypes. The largest difference in "^i between extreme genotypes, as observed in these tests, reached 12.9 bars. In one test, where drought stress developed during the spike development stage, leaf water potential (but not leaf diffusive resistance) was correlated with the final number of grains per spike across genotypes of similar phenology. Linear regressions developed for each genotype between Wi and Ri revealed large variations among genotypes in stomatal response to Wi. These relationships lead to the conclusion that dehydration avoidance in only a few of the genotypes may have resulted from stomatal sensitivity and closure at low stress levels. Most of the avoidant genotypes, however, maintained relatively open stomata. Additional index words: Triticum aestivum L. , Drought resistance, Water stress, Leaf water potential, Stomata, Leaf diffusive resistance. S for drought resistance in plant breeding programs has been undermined by the lack of appropriate strategies and selection techniques. It was proposed (Blum, 1979) that as the best available approximation, testing, and selecting for drought resistance should be performed at least among advanced breeding lines otherwise excelling in all breeding requirements. Such selection may reduce the amount of subsequent yield testings that are required for the derivation of yieldstability parameters. The total drought resistance of a genotype cannot yet be defined physiologically (Blum, 1980), and most probably it does not exist as a unique plant trait. However, two major components of the resistance have been defined (Levitt, 1972): dehydration avoidance and dehydration (or desiccation) tolerance. Dehydration avoidance is characterized by maintenance of favorable plant water status and is measured by the relative maintenance of a higher (less negative) plant water potential in a given genotype, under conditions of soil moisture stress. Plant water potential displays a daily curve with minimum (most negative) values at midday. Comparative plant responses in dehydration avoidance are therefore evaluated by midday measurements of water potential. Plant water stress at the boot-to-anthesis plant development stage has the greatest effect on wheat yield, as compared with stress at other growth stages (Fischer et al., 1977). Intergenotypic variations in dehydration avoidance at this stage should be important. Genetic variations in dehydration avoidance have been recorded, among the cereals, in sorghum (Blum, 1974) and rice (O'Toole and Maya, 1978). Limited observations (Jones, 1977; Morgan, 1977; Quarrie and Jones, 1979) indicate that this may be true also in wheat. Indirect con' Contribution from the Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan, Israel, No. 177-E, 1980 series. This work was supported by a US-Israel Binational Science Foundation (BSF) grant no. 1654/78. Received 30 June 1980. 2 Senior research agronomist, technician, and agronomist, respectively. The Division of Field Crops. 496 CROP SCIENCE, VOL. 21 ,JULY-AUGUST 1981