﻿ 4.3 Embryogenesis, fruit/seed development and seed dispersal - Office of the Gene Technology Regulator
4.3 Embryogenesis, fruit/seed development and seed dispersal - Office of the Gene Technology Regulator
Pollen grains germinate immediately after settling on the silk. The pollen tube takes between 12 and 24 hours to reach and fertilise the ovule (reviewed in Sleper & Poehlman 2006). Upon completion of fertilisation the silk detaches from the ovary and dries out.
In the first phase of embryogenesis, irregular cell divisions result in a proembryo consisting of approximately 12 – 24 cells within 100 hrs after fertilisation. The initial basal cell divides into a number of large, vacuolated suspensor cells. By the same time, the initial apical cell has given rise to 9 – 18 small cells that are densely filled with cytoplasm.
Starting approximately 8 – 9 days after fertilisation, the second phase of embryogenesis commences, which leads to the establishment of meristems and the embryonic axis by approximately 13 days after fertilisation (transition embryo). Approximately 14 – 15 days after fertilisation, the coleoptilar embryo is established, characterised by the differentiation and growth of scutellumf, coleoptileg, coleorhizah and root and shoot apical meristem (see Figure 3). By approximately 16 days after fertilisation, the first leaf primordium arises. After this time, the embryo is referred to as stage 1 embryo. It is now approximately 1 mm long and weighs approximately 1 mg.
During the last stage of embryogenesis (during 30 – 40 days after the first leaf primordium appears), embryo growth continues, more leaf primordia and a primary and one or more secondary root primordia develop, and the whole embryo is enveloped by the expanding scutellum. Storage products also accumulate within the embryo during the final phase of embryogenesis, especially in the scutellum. At kernel maturity the embryo may have a fresh weight of approximately 50 mg (Sheridan 1988).
The pericarp (ovary wall) and testa (seed coat) are fused to form the fruit wall and because of this tight adhesion between fruit and seed, the two structures actually appear to be a single structure. This structure is commonly referred to by a number of interchangeable terms – fruit, kernel, grain and seed. The kernels are composed of three main parts - the embryo, the endosperm and the fruit wall (see Figure 3). The number of kernels per ear and the number of ears that develop is established at, or shortly after, pollination (Duncan 1975).
The rate of cob development is dependant on environmental factors, such as climate (particularly temperature) and genetic factors, such as cultivar (see discussion in Norman et al. 1995; Sleper & Poehlman 2006).
The grain filling period in maize is approximately 8 weeks in length (Lee & Tollenaar 2007). As reviewed by Farnham et al. (2003), the following stages can be discerned during kernel development: blister stage, milk stage, dough stage, (dent stage in dent maize varieties) and physiological maturity. Physiological maturity is generally reached approximately 55 – 65 days after silking.
For the first 10 – 14 days after fertilisation, the husks, cob and shank develop rapidly and soon after, nutrients are accumulated in the developing kernels (Farrell & O'Keeffe 2007). The small blister-shaped kernels are filled with a clear fluid.
The milk stage is characterised by the presence in the kernels of a white fluid with high sugar content.
During dough stage, the kernels are filled with a white paste and, starting at the tip of the kernel opposite the embryo, starch is deposited. This starch deposition is visible, as a ‘milk line’ develops (soon after denting in dent maize varieties) between hard (starch-filled) and soft (paste-filled) phases of the endosperm. A milk line of 0 indicates that no starch deposition has occurred, whereas a milk line of 5 indicates that starch deposition is completed. Monitoring the progress of the milk line towards the base of the kernels allows estimating the time to physiological maturity.
Physiological maturity is reached once maximum dry matter has been accumulated in the kernels and their moisture content is approximately 30 – 38%. A ‘black layer’ is formed underneath the tip of the kernel that is attached to the cob. It seals off the kernel from the remainder of the plant.
Information on harvesting and post-harvest can be found in a number of sources (see Colless 1992; Lafitte 2000b; Farnham et al. 2003). The moisture content of the grain at physiological maturity is usually above 30%. After physiological maturity is reached, kernels continue to loose moisture and in moist tropical environments, harvest should not proceed until moisture content has been reduced to approximately 25% (Paliwal 2000e). Usually, mechanical harvesters perform best at a kernel moisture content of approximately 18 – 24%. After harvest, maize grains must be dried artificially to no more than 14% moisture in order to minimise infestation with pests and development of diseases during storage. Artificial drying should be carried out at temperatures below 49°C.
The rate of development of maize is linked to temperature (over 24 hours) rather than to photosynthesis which is governed by temperature only during daylight hours (Duncan 1975). The concept of thermal time (expressed in units of day degrees) can be used to categorise maize cultivars into early- and late-maturing (see also discussion in Section 2.3). Basically, thermal time is a measure of accumulated temperature that is required for a phenological character (such as flowering) to take place. Lafitte (2000b) and Norman et al. (1995) note that tropical maize cultivars have a lower yield than temperate cultivars because, while temperatures are higher in the tropics, the plants have a much shorter time to maturity.
Pollen can have an immediate effect on kernel characteristics, a phenomenon that is known under the name ‘Xenia’ (reviewed in Sleper & Poehlman 2006). The underlying cause for it is the fertilisation of the diploid polar nucleus by the haploid vegetative sperm nucleus, resulting in triploid endosperm cells. As the endosperm comprises approximately 80% of the mature maize grain (reviewed in Boyer & Hannah 1994), kernel characteristics depend on the genotypes of both female and male parent. Endosperm characteristics that exhibit Xenia include endosperm colour (eg yellow vs white), waxy vs non-waxy endosperm, aleurone colour (purple vs colourless), starchy vs sugary endosperm and non-shrunken vs shrunken endosperm.
The maize cob lacks any abscission layers between its basic units and therefore the cob remains intact at maturity (Doebley et al. 1990). Thus the tightly held grains are unable to be dispersed and confer a low survival rate to the maize plant in nature (Fedoroff 2003). The cob itself usually remains on the plant until harvested but if left on the plant or if damaged by insects or disease will eventually fall to the ground. This means that there may be localized dispersal of grains around the base of the plant. Harvesting activities and grain transport result in more widespread grain dispersal (see Section 8.1 for a discussion of volunteerism). Indications from the scientific literature would not suggest that dispersal of maize grain by animals, including birds is significant although there is the possibility that intact grain may be spread as the result of the activities of vertebrate pests (see Section 7.2.2).