The branch roots of Zea. II. Developmental loss of the apical meristem in field‐grown roots

Abstract

SUMMARYMost of the first-order branches of field-grown maize roots are short (around 10 mtn). An anatomical and morphological survey of the tips of branches from nodal roots showed that almost all branches lacked a normal apex. .Most tips were broken off (e.g. 74 % in the mid-regions of roots from node 1, 95 "^o in the same region of node 3 roots), with living, mature tissues right to their end. The remaining tips were of 3 types: {a) Determinate tips with rounded offends, no root cap, and mature tissues right at the tip; {b) Normal tips which were miniaturized versions of main root apices with a meristem, root cap and long zone of elongation ; (c) Intermediate tips in which cap loss, meristem outgrowth and differentiation were incomplete. Normal tips were mainly on branches of the youngest roots and determinate tips were the predominant intact tips of old roots. This distribution, together with anatomical features of the three tip types indicates that Determinate tips form from Normal tips by a controlled pattern of cell enlargement, differentiation and, possibly, cell division. Many tips must be broken accidentally by excavation, or earlier by browsers or pathogens. There was some evidence, however, that some broken tips are formed by developmental!y controlled abscission both before and after the formation of determinate tips.

Apical meristems of tomato roots and their modifications induced by arbuscular mycorrhizal and soilborne pathogenic fungi

“…Active apices are formed by a distal cap that protects the apical meristems, and are mainly composed of cells with intense metabolism, where chromatin replicates, nuclei divide, cytoplasm is synthesized and new walls are elaborated (Barlow, 1994). However, only in seedlings or very young plants do root apices possess active meristematic tissues : subsequently the meristematic cells frequently lose their mitotic activity, undergo a limited cell distension and vacuolization and the apices become ' parenchymatized ' or ' determinate ' according to Berta et al (1993) and Varney & McCully (1991) respectively. Many gymnosperm and dicotyledonous trees have a root system formed of long and coarse supportive *Author for correspondence (fax j39 11 6707459 ; e-mail fusconi!bioveg.unito.it).…”

“…Herbaceous plants, too, possess roots with a determinate type of growth. The morphological and anatomical modifications that root apices undergo when they become mitotically inactive have been studied in detail for some monocotyledons, including Zea mays (Varney & McCully, 1991), Ornithogalum umbellatum (Berta et al, 1993) and Allium porrum (Berta et al, 1990).…”

Cluster root development in Grevillea robusta (Proteaceae). I. Xylem, pericycle, cortex, and epidermis development in a determinate root

“…Root thorns in palms fall into this category (McArthur & Steeves, 1969). Fourthly, some roots appear determinate, but are in fact dormant (Varney & McCully, 1991, Couot-Gastelier & Vartanian, 1995. Fifthly there are roots whose meristem undergoes differentiation but in which there is no abscision.…”

“…Secondly, symbiotic relations can lead to the production of determinate roots such as in certain mycorrhizas (Harley & Smith, 1983 ;Fitter, 1985 ;Berta et al, 1990) and in actinomycete infections of Myrica, Casuarina and Comptonia (Torrey & Callahan (1978) and references therein). Thirdly, some species seem to undergo a programmed abscission of the apical meristem, such as in the water fern Azolla (Gunning, 1978), Zea (Fusseder 1987 ;McCully, 1987 ;Cahn, Zobel & Bouldin, 1989 ;Varney & McCully, 1991) and Allium (Berta et al, 1990). Root thorns in palms fall into this category (McArthur & Steeves, 1969).…”

“…Firstly, there are those that become ' determinate ' owing to damage inflicted on their meristems (Varney & McCully, 1991) either from biotic or abiotic sources. Secondly, symbiotic relations can lead to the production of determinate roots such as in certain mycorrhizas (Harley & Smith, 1983 ;Fitter, 1985 ;Berta et al, 1990) and in actinomycete infections of Myrica, Casuarina and Comptonia (Torrey & Callahan (1978) and references therein).…”