Sorghum morphology and anatomy

Introduction

The purpose of this page is to introduce the basic structure of the cultivated sorghum plant (Sorghum bicolor). Generally speaking, sorghum is organized like other grasses in tribe Andropogoneae (the Bluestem or Sorghum Tribe), except that it has been modified by the process of domestication. The ways in which it has been modified vary according to the variety of sorghum. Generally, domesticated sorghums have tall, thick, unbranched culms (stalks), inflorescences that do not break apart when mature, and relatively large grains (although still smaller than some other cereal crops).

Stems

Cultivated sorghum has a single thick stalk or culm that ends in an inflorescence; the length of the culm varies, but grain crops may reach about 3 feet (1 meter) whereas biomass types may reach 20 feet (more than 6 meters). The culm bears about 15 to 17 leaves, depending on the type of sorghum. Unlike in many other grasses, the interior of a sorghum stem is solid in both nodal and internodal regions. 

Roots

The root system of sorghum is similar in organization and development to the root system of maize (Zea mays). The first root, or primary root develops from the radicle (embryonic root) of the embryo. Shortly after the primary root begins elongating, seminal roots (seed roots) also develop from the embryo, emerging from the region above the radicle (primary root). The primary and seminal roots are temporary, eventually dying as their function is taken over by adventitious roots that develop from the stem.

Sorghum develops two types of stem-borne roots. Some adventitious roots develop near the base of the stem and are completely underground. Prop roots or brace roots begin development aboveground and grow into the soil. Prop roots help to support the aerial part of the plant; the aboveground parts of prop roots are green and carry out photosynthesis.

Leaves

Leaf morphology

Sorghum leaves are organized like the leaves of other grasses. Each leaf consists of a sheath that wraps around the stem and an elongated blade that sticks out from the side of the stem and intercepts sunlight. The place where the sheath and the blade come together is the collar. The major leaf veins are parallel. The leaves lack auricles but have ligules. The last foliage leaf on the stem (the highest leaf) is called the flag leaf. The flag leaf is the leaf directly below the inflorescence.

Leaf anatomy

Leaf epidermis

The epidermis of the sorghum leaf is similar to that of other grasses. It has elongated long cells and shorter short cells, which may contain cork or silica. The long cells have wavy side walls. The silica cells contain bilobate, trilobate, or cross-shaped silica bodies (phytoliths). The leaves also have prickle-like hairs that contain silica bodies. Stomata (pores) occur between some of the long cells. 

Leaf vascular bundles and mesophyll

Like other members of Tribe Andropogoneae, sorghum is a C₄ plant. Each vascular bundle has a bundle sheath made up of one layer of cells with large chloroplasts near their outer walls. As is common in C₄ plants, mesophyll cells are arranged concentrically around the bundle sheath.

Reproductive structures

Inflorescences, spikelets, and florets

Sorghum inflorescences are panicles. A panicle is an inflorescence with a central stem and lateral (side) branches. The lateral branches bear smaller inflorescence branches, called rames; each rame has between about two and seven groups (pairs and triplets) of spikelets. The inflorescence of cultivated sorghum varies in shape and size. 

The spikelets in sorghum are mostly in pairs, although the spikelets at the ends of branches are in groups of three. Each spikelet pair includes one pedicellate (stalked) spikelet and one sessile (stalkless) spikelet; groups of three spikelets have one sessile and two pedicellate spikelets.

The sessile spikelets have one fertile floret. The fertile floret is bisexual, meaning that it has both stamens (male or pollen-producing structures) and a pistil (female or ovule-producing structure). The pedicellate spikelets are either sterile or fertile. If fertile, the pedicellate spikelets are male.

Fruits

Following pollination and fertilization, the fertile bisexual florets mature into fruits. Sorghum fruits are caryopses (grains) that come in a variety of colors. Inflorescences of cultivated sorghum do not disarticulate (break apart) when they are mature. Inflorescences that do not break apart when mature are one of the common features of domesticated grasses that are grown for their grains.

A sorghum grain consists of an embryo and endosperm, which serves as food for the embryo once it begins growing into a seedling. The outer layer of the endosperm is the aleurone layer. The embryo and endosperm are surrounded by a thin pericarp, or fruit wall. 

As in other grasses, the sorghum embryo is complex. The upper end of the embryo is covered by a coleoptile, or protective sheath. The radicle (embryonic root) at the opposite end of the embryo is also protected by a sheath, the coleorhiza. A cotyledon (seed leaf) called a scutellum is attached to the embryo.