Regulation of Fruit Set of Phaseolus Crosses by Pollen Source and Environment

Abstract

Pollinating Phaseolus coccineus Lam. with P. vulgaris L. (PC X PV) predetermines that seeds will abort by the time they reach 10 mm in seed length, while P. coccineus selfed (PC X PC) develop normally. The overall objective of this work was to determine the causes of failure of the PC X PV seeds to develop to maturity. To meet this objective, the project compared pre-abortion seed development of PC X PV with normal seed development in PC X PC in the following areas; developmental patterns, ('14)C-photoassimilate partitioning, endogenous plant growth substances, and light effects.

Pod diameter was the only parameter measured that exhibited a linear relationship with seed length for both crosses. Seed length will then estimate embryo development. When the two crosses were compared, PC X PV showed (1) slower seed development with respect to days after pollination, (2) reduced pod length, (3) a higher (pod length)/(no. of seeds per pod) ratio, (4) slower embryo development, and (5) increased volume of liquid endosperm.

Partitioning of ('14)C-photoassimilates into developing seeds was studied at three stages of embryo development. When expressed as sink strength (% dpm) or sink activity (% dpm/d.wt.) there were no differences in partitioning between the two types of seeds. If the embryo is analyzed separately, the sink activity of PC X PV embryo was higher. Abortion of PC X PV embryos appeared not to be caused by a lack of newly fixed photoassimilates.

The concentration of four plant growth substances was determined in the seed coat, embryo, and liquid endosperm of PC X PC and PC X PV. High levels of zeatin riboside and dihydrozeatin riboside and the relatively low levels of indole-3-acetic acid (IAA) and abscisic acid (ABA) in the PC X PC embryo suggest functions for these compounds in normal embryo development. The timing of elevated levels of IAA and ABA in PC X PV embryos suggest that imbalances of these compounds may function in regulating embryo abortion.

Pod abscision of PC X PV occurs after 5 days in winter compared to 16 to 20 days during the summer. Supplemental lighting in winter had no effect on PC X PV pod abscision. Shading of plants in summer, to equal light intensities of winter, did not effect PC X PV pod abscicion.