GENETICS AND PHYSIOLOGY OF THE XANTHOPHYLLOUS MUTANT OF THE TOMATO

Abstract

Xanthophyllous is a dominant yellow-leafed mutant; the genotype Xa Xa is lethal in either the seed or the very early seedling stage, while the genotype Xa xa is viable but reacts differently to light intensity than does the normal xa xa plant. The 1: 2: 1 ratio is never realized because less than half the lethals appear as seedlings. The 2: 1 ratio is affected by germination so that under good conditions for germination a 2: 1 ratio is obtained, but under poor conditions the ratio may approach 1: 1. Maximum likelihood formulae are given for calculating linkage when one gene is a dominant with recessive lethal effects, and it is shown that the simple product moment method gives similar results for these data. Xa is in linkage group VII (chromosome 10), and its lethal effect disturbs the monogenic ratios of all genes in this group. The following crossover values between these genes and Xa are found: H 44.5%, pe 50%, t 35.5%, tv31.7%, and ag 46.5%.Under standard growing conditions the pigment concentrations of xanthophyllous contrasted with green is: chlorophyll, 790 instead of 3240 μg./g.; xanthophyll, 54 instead of 211 μg./g.; and carotene, 60 instead of 130 μg./g. Both green and yellow plants, when grown under various intensities and spectral compositions of light, have different responses. Green plants make only limited response to changes in light intensity, whereas Xa xa plants become green and cannot be distinguished from xa xa plants. From 600 to 8000 ft-c. the Xa xa plants show a linear trend of decreasing pigment (increasing yellowness) with increasing light intensity. The hypothesis is advanced that Xa xa plants make as much pigment as do green ones, but a light-dependent reaction breaks down the pigments faster than they are being formed.