Williams, W.
It is known that exsertion of the stigma in the tomato is influenced by short-day conditions, and that it varies in populations derived from different regions (Howlett, J. Agric. Res. 58, and Rick, Econ. Bot. 12). Exsertion of the stigma is an important component in determining the outbreeding behaviour of the species; when the stigmas are exserted, inbreeding is almost entirely excluded. Experiments, under controlled conditions, have shown that both day length and temperature have a critical influence on the position of the style in relation to the anther column in each of six varieties studied. The effect was measured from the ratio of the total carpel length to stamen lenght in the first two flowers of the first inflorescence; values above and below unity denoting exserted and inserted stigmas respectively. At 65 deg. F, short days (8 hours) as compared with long days (16 hours) increased the ratio slightly, but significantly, from 0.96 to 1.00, while high temperature, 70 deg. F compared with 57 deg. F at 16 hours day length, changed the ratio from 0.84 to 1.01, an increase of 20 per cent. The pronniloced effect on self-fertilisation of increasing the carpel length relative to stamen length was clearly evident in the number of fruit set by the different variations N\36, a variety with a carpel/stamen ratio of 1.12 sat only 16.2 per cent of its flowers, while in D\36, which had a lower carpel/ anther ratio (0.96), 60 per cent of the flowers were set. In the remaining varieties which fell between these two extremes, there was a high correlation between the carpel/anther ratio and fruit setting following autonomous selfing. The practical implications of this result for tomato production in heated glasshouses is obvious.
It has also become clear, from observations on hybrids between forms with inserted and exserted stigmas, that the position of the stigma in relation to the anthers in the mature flower has a strong genetic component with incomplete dominance. Such hybrids have frequently produced only parthenocarpic fruit under short-day conditions in a heated glasshouse, and the cause of infertility will now be clear from the above results.
In view of these findings, the use of the genetically-controlled, longstyle character in combination with "self-emasculating", recessive mutations such as "positional sterile" ps, can no longer be considered a satisfactory technique for the commercial production of hybrid seed. Furthermore, these results indicate reasons vhy genotypes with exserted stigmas are largely confined to the warmer, sub-tropical regions of the range of distribution of the species, where natural selection, exercised through active populations of pollinating insects and genotypic plasticity in response to short days and high temperature, has encouraged an outbreeding floral mechanism. In temperate latitudes, such as in middle and north Europe, as a result of the same forces acting in reverse, the inbreeding mechanism has become predominant.