Pratt, David : David Pratt, Professor, Bacteriology, U.C., Davis.

Thomas, Bruce R. : Bruce R. Thomas, formerly graduate student, U.C., Davis.

to suboptimaltemperatures is among the neg - Scanning electron micrograph of pollen grain germinating lected areas of potentiallyuseful research and of stigma . on surface development . Sensitivityto chilling in many important crop plants limits their climatic distribution for economic production . Even in areas normally suited to these crops , this sensitivity causes losses through injury suf - fered from sporadic low temperatures . Our laboratory has been exploiting the ex - isting genetic diversity in wild tomato germ - plasm forplant breeding to reducecrop losses and allow expansion into environments too As a result , hostile for economicproduction . we have improved chilling tolerance in the of cultivated tomato through the technique and pollen tube elongation under low tem - vanced backcross progeny carrying selected pollen selection . peratures would effect fertilization . There - genetic characters that enable them not only Our strategyhas been to test whether selec - fore , only the selected pollen genotypes to grow and reproduce in environments tion among haploid male gametophytes ( pol - would contribute their genes to the subse - where cultivated tomatoes would experience len grains ) might have a positive , correlated quent generation . We have demonstrated , in severe chillinginjury , but also to survivetem - effect on the vegetative ( sporophytic ) genera - fact , that the haploid ( pollen ) genome can peratures just above freezing that kill culti - of genes tion resultingfrom directed selection modify the fertilization ability of individual vated types . expressed in both stages . The critical require - gametes in a manner reflecting their geno - The correlated response established be - type . Furthermore , the â?? low - temperature â?쳌 ment of this overlapping model is that speci - tween chillingtolerance in tomato pollen and fic genes that are expressed in the pollen genes amenable to pollen selection also con - the sporophyte is the first of its kind and per - grains and confer a particular physiological fer low - temperature tolerance to the resul - mits the plant breeder to select with great trait also function in the vegetative plant . tant sporophytic progeny . economy in the haploid generation . The pol - Using the pollen selection technique , we Hence , selection for these genes in the pollen len selection technique developed in this pro - phase could have a positive effect on the suc - have successfullytransferred chilling tolerance gram may prove useful in selecting for im - cess of the sporophytic phase . from a wild tomato species to the cultivated proved physiologicalefficienciesin numerous In practice , virgin stigmas are pollinated tomato . This was accomplished , without re - crops under a host of stress conditions . and exposed to a low temperature challenge . course to laborious and expensive progeny Under thesecontrolledconditions , only those testing , by applying pollen selection during RichardA . Jones , Assistant Professor , Vegetable pollen genotypes conferring germinability Crops , each subsequentbackcross . We now have ad - U.C . , Davis . Embryo callus hybrids only with great difficulty by embryo culture . and not maternal seed tissues or maternal Bruce R . Thomas The undeveloped seeds produced were care - self - pollinated embryos . David Pratt fully dissected to search for the rare embryos The embryo callus method may become a ( approximately 1 in l0 , OOO ) that had devel - valuabletool for tomato breeders . F , hybrids oped enough to then continue their develop - 8 out of 11 L . have been obtained easily for Hybridization between cultivated species ment into plants on appropriate culture peruvianum strains tested . More work will be and related wild species has been of great media . needed to determinemore precisely what pro - value for cultivar improvement . Genes that L . esculenturn and L . Hybrids between L . peruvianurn strains are suscep - portion of enhance the survival of the wild species by peruvianum can be obtained much more effi - tible to hybridization by this technique . The providing disease or insect resistance , salt tol - ciently when whole , undeveloped seeds are first backcross of some F , hybrids to L . escu - erance , cold tolerance , and the like , often placed on a culture medium suitable for the has also been achieved via embryo lenturn confer the same trait on the cultivar to which growth of callus . Under these conditions , up callus . Subsequent backcross generations are they are transferred . Fertility barriers restrict to 1 in 10 ( but sometimesonly 1 in 200 ) of the expected to arise by means of normal seeds , the number of wild species that can contri - All plants regenerated seeds form callus . so the embryo callus method appears suffi - bute genes to any particular cultivatedspecies from this callushave clearlyderived from hy - cient to completelyovercomethe fertility bar - through normal sexual crosses , but these fer - brid embryos rather than endosperm or ma - riers between the two species . tility barriers can often be overcomethrough ternal cells in the seed . All of the regenerated Although the success rate is somewhatvar - the use of special procedures . plants were diploid or tetraploid , whereas en - iable , indicating that more work will be The cultivated tomato , Lycopersicon escu - dosperm cells are triploid . Traits of the regen - needed to determine the optimum plant lenturn , hybridizes with all but two of the erated plants that resemble the male parent growth conditions to use , the embryo callus wild tomato species in the same genus by nor - ( L . peruvianurn ) , such as anthocyanin con - technique is clearly more efficient than em - Lycopersicon peruvia - mal sexual crosses . tent ( the female parent used was anthocya - bryo culture for wide crosses between wild nurn , the most distantly related species in the ninless ) , form of leaves , flowers , and fruits , and cultivated tomato species . genus , does not produce viable seeds when inability to self - pollinate , and the isozyme Bruce R . Thomas , formerly graduate student , hybridized with cultivatedtomatoes . Hybrids migration rate in gel electophoresis , proved and David Pratt , professor , ~ ~ ~ u , c . , ~ ~ ~ i ~ l ~ ~ ~ , between thesetwo species have been obtained that the plants derived from hybrid embryos Davis . CALIFORNIA AGRICULTURE.AUGUST 1982 27