The Gardeners’ Chronicle February 7, 1908
Inheritance of pigment in Pisum sativum.
Mr. C. C. HURST, F.L.S., communicated the following note from Mr. E. A. BUNYARD, of Maidstone, on "The Inheritance of Pigment in Pisum sativum":—
"The green and yellow colours of the cotyledons of Pisum sativum were selected by Mendel as one pair of constant differentiating characters, and from their apparent alternative inheritance the theory of gametic purity was deduced to explain results obtained in this species. The lack of any definite examination of the two colours in question led me to make some chemical and photochemical experiments as below, the pigments of the cotyledons alone being considered. The greet pigment is, of course, chlorophyll, and thin sections mounted in glycerine show the chloroplasts well, and give the well-known 'hypochlorin reaction' when treated under the cover glass with glacial acetic acid. An alcoholic extract also gives the well-known bands in the red when spectroscopically examined, and the fainter bands in the blue and violet. The yellow colour is due to a pigment of the xanthophyll series, pigments which are always found in association with chlorophyll in the green parts of plants. The point, however, which is of importance is the gradual fading of the green (Sachs' 'Degradation of chlorophyll'), and the presence of the xanthophyll. The green chloroplasts, as the seeds attain maturity, gradually lose their green pigment, and when it has entirely vanished they are left in the cell as pale yellow globules. The yellow xanthophyll has, however, been present from the beginning, and the disappearance of the chlorophyll green has merely rendered it visible. The simultaneous presence of the two colouring matters can be demonstrated in this way. When a number of green cotyledons are steeped in alcohol a green extract, as referred to above, is obtained, and this fluid retains its green colour only so long as it is kept from light. Then it is exposed to daylight, or even gaslight, it rapidly loses the green colour and fades to a yellowish tint. Upon examining this spectroscopically, it is found to have absorption bands in the blue and violet identical with that of an alcoholic extract made from yellow cotyledons. This fading of the green is seen in the autumnal colouring of leaves, and in an inverse order the slow development of chlorophyll when etiolated plants are exposed to light. These facts, I venture to think, render it necessary to modify Mendel's original conception in this special case, as it is evident that the conception of a factor for green and one for yellow, and the alternative inheritance of each is hardly in harmony with the facts. As all cotyledons pass through the green stage, and certain only pass through to the yellow, I would suggest that the factor may be not a factor of 'quality,' but a factor which extends or limits development."
Commenting upon the foregoing, Mr. HURST wrote:—"The above note by Mr. E. A. Bunyard is a valuable contribution to our knowledge of the nature of Mendelian characters in Peas. According to Mr. Bunyard's results, green Peas contain invisible yellows at all stages of their development, while yellow Peas contain green in the early stages only. A yellow Pea may, therefore, be regarded as due to the presence of a factor which causes the green to fade at an early stage of development, while in the green Pea this factor is absent. The Mendelian units concerned are therefore not simply yellow and green, as Mendel supposed, for all green Peas contain the yellow element as well as the green, but would appear to be rather the presence and absence of a factor which causes the green to fade."
CybeRose note: Yellow-pod peas are of a similar nature. The pods, sepals and stems just above the blooms are yellow, but become greenish later in development. The chlorophyll production is delayed, in this case, rather than cut off prematurely.