Weed Physiology

Volume I: Reproduction and Ecophysiology

Stephen O. Duke, Duke

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315898643
• eBook ISBN: 9781351094641
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Weeds are plants existing at places and/or times at which they are considered undesirable by man. Thys, man's primary interest in weeds is in dinging methods for eliminating their presences. Understanding the physiology of weeds and how it differs from that of crop plants is becoming increasingly important in discovering new chemical, genetic, and cultural methods of controlling weeds. The two volumes of this book will aim to discuss the following; the physiology of weed production the ecophysiology of weeds, the mechanisms of herbicide action, and the mechanisms of herbicide resistance and tolerance.

Chapter 1

FLOWERING, SEED DEVELOPMENT, AND THE INFLUENCES DURING SEED MATURATION ON SEED GERMINATION OF ANNUAL WEEDS

Yitzchak Gutterman

TABLE OF CONTENTS

I.	Introduction
II.	Flowering A. Portulaca oleracea (Portulacaceae) B. Amaranthus (Amaranthaceae) C. Chenopodium album (Chenopodiaceae) D. Conclusions
III.	Seed Development and Maturation
IV.	Environmental and Maternal Influences during Maturation which Affect Seed Germination A. Influences of Daylength B. Daylengths and Hormonal Applications C. Temperature Effects D. Red and Far-Red Light E. Light Intensity F. Altitude Effects on Germination G. Senescence Effects H. Position Effects I. Conclusions
V.	Summary
Acknowledgments
References

I. INTRODUCTION

This chapter deals with environmental influences upon flowering mechanisms, seed maturation, and germinability determined during seed maturation of annual weeds. To what extent is the fate of the future generation or generations set by conditions during seed maturation on the mother plant? We have tried to emphasize what is known of the answer to this question, using some of the prevalent types of the most widespread weeds in the world.

The central question examined in this chapter is: What are the ecophysiological adaptations which enable these plants to live in agricultural fields, to persist, and to spread to new areas despite the war waged on them by man? The conditions for survival of the annual weeds are similar in many respects to those of the annual desert plants. Therefore, it is interesting and valuable to compare the annual weeds with the annual desert plants. The flowering mechanisms that enable flowering to occur within a wide range of daylengths, adapting the length of the life cycle according to the daylength, and the maternal and environmental influences on seed development and subsequent germinability are all mechanisms that allow for the distribution of seed germination over time. Even under optimal conditions, only a portion of the seeds germinates at once. This reduces the risk of destruction.

II. FLOWERING

What are the survival mechanisms of some annual weeds that enable them to spread over cultivated fields in very large areas of the world in spite of agricultural efforts to destroy them? Some of the most widespread species will be mentioned here to show their amazing survival potential and their ability to inhabit quite varying habitats in different parts of the world.

A. Portulaca oleracea (Portulacaceae)

This is an annual succulent weed found in irrigated fields. It reproduces by seeds and is widely distributed over temperate and tropical regions of the world. It is one of the eight most common weeds on earth.¹

As reviewed by Kigel and Rubin,² the P. oleracea species is a polyploid complex. The tetraploid and hexaploid subspecies are usually weeds, whereas the diploid is mainly a halophyte found in coastal areas. P. oleracea requires high temperatures and light intensities for its normal development and dies under conditions of shade and frost. Depending on the weather, the complete life cycle can take from 5 weeks to 4 months. In the tropics this plant flowers all year round. The terminal cluster of yellow flowers are usually self-pollinated and develop a globular one-celled dry capsule containing many small black seeds.

P. oleracea under white light was found to be a day neutral (DN) plant in a range of daylengths of 4 to 24 hours (h) of light per day, whereas under extended daylengths with low intensity of white light, this plant was found to be a facultative short-day (SD) plant (Table 1).²

In a study by Gutterman,^3,4 the treatment was as follows: 8 h of natural daylight intensity extended to photoperiods of 11, 13, and 15 h with low-light intensity. In a later experiment, the photoperiods were for 16 and 24 h. The plants under 11 h of light develop flower buds in less than 10 days and after the appearance of about three leaves. The longer the daylengths, up to 16 h, the longer the time and the greater the number of leaves until the first flower bud appears (Table 2).^3,4 These experiments had 27°C day and 22°C night temperatures. At lower temperatures (16/11 °C) the life cycle is slightly longer.³

A short life cycle is a very important survival mechanism of a weed in cultivated areas. An additional mechanism is found in P. oleracea. The plant is able to continue seed maturation and produce mature seeds even when it is cut after flowering.

Table 1
ENVIRONMENTAL CONTROL OF FLOWERING OF PORTULACA OLERACEA

Locality	Light treatments	Photoperiodresponse	Ref.
Michigan	White light of 4—24 h	DN	62
Veranesi, India	White light of 4—24 h	DN	1
Israel	Extended daylength with low-intensity white light	FSD	3, 4
India	Same as above	FSD	63

Note: DN = day neutral: FSD = facultative short day.

Adapted from Kigel, J. and Rubin, B.. Handbook on Flowering, Vol. 4, Halevy, A., Ed., CRC Press, Boca Raton, Fla., 1985. With permission.

Table 2
THE INFLUENCE OF THE DAYLENGTHS ON THE NUMBER OF LEAVES UNTIL FLOWER BUD APPEARANCE AND THE AGE OF THE PLANT AT THE FIRST FLOWER BUD APPEARANCE IN PORTULACA OLERACEA PLANTS^a

Photoperiod treatment (hours of light per day)	Number of leaves until flower bud appearance	Age at first flower bud appearance (days)
8^b	4.4 ± 0.5	12.0 ± 0.7
11^b	3.2 ± 0.4	9.7 ± 0.2
13^b	7.0 ± 0	14.0 ± 0
15^b	10.5 ± 0.2	19.0 ± 0
16^c	11.1 ±0.3	19.0
24^c	11.1 ± 0.2	18.6

^a Adapted from Gutterman.^3,4

^b Study started on 16-8-66.

^c Study started on 28-11-67.

A night break of red light (see Figure 1, treatment G) caused a delay of flower bud appearance. Flower buds appeared after 13.8 leaves, a delay in comparison to long-day (LD) conditions (11.3 leaves, treatment B). However, a far-red night break accelerates the appearance of the flower buds. They appear after six leaves (treatment H) in comparison to SD of 8 h of daylight in which the flower buds appear after 7.9 leaves (treatment A). Eight hours of red or far-red light following the 8 h of a dark period...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Flowering, Seed Development, and the Influences During Seed Maturation on Seed Germination of Annual Weed
• Chapter 2 Physiology of Weed Seed Dormancy and Germination
• Chapter 3 Asexual Reproduction of Weeds
• Chapter 4 Comparative Ecophysiology of Weeds and Crops
• Chapter 5 Weed Allelopathy
• Index