Studies on the branching in Pisum sativum. I.

Effects of photoperiod and seed-vernalization on the branching

Abstract

Intending an ecolo-physiological study on branching in pea (pisum sativum L.), the authors have first tried to examine influences of photoperiod and seed -vernalization on branching.
Varieties used were GW (early-flowering), Shirobana-Zairai (middle-flowering) and Aka-Endo (late-flowering) for the strain of arvense and Alaska (early-flowering), Alderman (middle-flowering) and Usui (late-flowering) for the strain of hortense.
Seeds were sown in the field of Kusatsu City, Shiga Prefecture, on May 24, September 5, and October 20 in 1960 and on April 25 in 1961. The photoperiodic treatment (long and short day) was started as soon as the seedlings expanded their first leaves.
In the experiment beginning in April or May, plants receiving a short photoperiod (SD) were given 8 hours of day light between 9.00 a. m. and 5.00 p. m. They were covered with light-tight white-vinyl screen from 5.00 p. m. to 9.00 a. m. Plants receiving a long photoperiod (LD) were given the natural long-day in the spring. In the experiment from September or October, a long photoperiod of 16 hours was made supplementing the natural daylength with artificial lighting in the evening. Plants receiving a short photoperiod were placed under the natural short-day in the autumn.
Besides, in April and September, vernalized seeds (V20 or V30) were sown along with untreated (V0). The seed-vernalization was performed at 1-3°C for 20 or 30 days in a dark electric refrigerator.
Each treatment with two replications contained 20 plants, and some 7-8 plants out of them were sampled for measurement. The further details of the experiment are arranged in Table 1.
1. Under short-days, the internode elongation of main shoot and branches of pea is remarkably inhibited. Influence of seed-vernalization on the growth of main shoot was not verified.
2. Generally, the total number of branches per plant increases under short phtoperiod, and hereby the increase is more remarkable in the secondary than in the primary branches. Further, varieties belonging to the strain of arvense, in particular its late varieties respond more decisively to the photoperiod than those belonging to the strain of hortense. Their response is, however, makedly weakened when they are grown in a warmer season.
The influence of seed-vernlization on the total number of branches per plant is so varied with varieties and the sowing time that any conclusive remark is hard to make.
3. Branches of pea may be divided into two groups with reference to their position on main shoot, that is, groups of low- and high-nodal branches. These two groups are separated from each other on the main shoot by some blank nodes inserted between them.
Our experiment has made it clear that photoperiod and seed-vernalization produce a considerable influence on the distribution of branches. Short-days encourage the branching at basal nodes, while under long-days high-nodal branches are always more than under short-days.
The influence of seed-vernalization on the distribution of branches is comparable to that of long-day in the way that plants grown from vernalized seeds produce more high-nodal branches than control plants.
4. With all the varieties used in our experiment, the influence of photoperiod and seed-vernalization on the position of the first flower on main shoot is quite evident. The number of nodes to the first flower decreases on long photoperiod and by seed-vernalization. Here also, the response is more remarkable in late varieties than in early ones.