SOURCE AND SINK RELATIONSHIP IN CROP PLANT
Nweke Jerry Anayo
Department of Crop Production and Landscape Management
Ebonyi State University Abakaliki
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SOURCE AND SINK RELATIONSHIP IN CROP PLANT
Source and sink relationship is studied to get better yield , to understand the relationship between the source of assimilation and the sink in which dry weight is accumulated as economic yield.
The organ that synthesizes food materials is called a source, in crop plants the mature leaf is always a source. Then the organ that accumulate the synthesized food material is known as a sink. The source is the potential capacity for photosynthesis (Po) and the sink is the potential capacity to utilize the photosynthetic products of the source. For example in wheat grain in the part of starch is the photosynthetic product of the leaves which then trans located directly to the fruiting grain after flowering
The ripening of wheat grain can be considered a process of accumulation of content. The grain yield may be limited by either the amount of its contents. If the contents are small the yield cannot be high. On the other hand, even if the contents are large, the yield cannot be high if the content is limited. Their procedure and active appear to determine the nature of crop performance .beevers (3) and Evans (15) gave some broad indication of source and sink. Molecular level transformation s involving volume flow of assimilates , phloem loading and unloading , general assimilates pools, and metabolic step in multiple enzyme systems with coupled biochemical processes were covered in a very basic way by Gifford et al ( 17) , Hendrix et al (21), the scope of this review is limited to structural and productivity level of source and sink.
SOURCE AND SINKS
The continued growth of any plant depends primarily on photosynthetic activity in the leaves and the transport of organic compounds from the leaves to hetertrophics cells. The source supplies assimilates (C compound) to the sink. The sink accepts and consumes the assimilates for its own growth or accumulates them for a certain period . the sink source relationship resembles the demand –supply relationship in economics (52). In general , any plant part that has photosynthetic ability is considered a source , and non green cells , tissue , and organ are considered sinks. Thus , leaves are typical source , and stem , roots and tubers are typical sinks. The sink and source functions of any plant part may chage depending on its developmental stage for example , in rice , before flowering , the leaf sheath and culm accumulates sugars and starch i.e. act as sinks ( 79). After flowering , most of the accumulated carbohydrates move into spiklets (10).
Developing organs, tubers, nongreen aeriel plant parts, bud , flowers , fruits , and most of the cells in stems and petioles also constitute a drain on photosynthetic products and are thus sink. Even within leaves there are many cells without chloroplasts. And the autotrophics cells themselves consume photosynthate in their own growth and respiration.
Developing buds and meristematic regions in root place demands on the available assimilates and competes successfully as sinks with developing leaves (3). The onset of flowering and subsequent fruit development have a marked effect on the redistribution of assimilates . fruit develop at the expense of vegetative growth , and at this time the growth of roots may be restricted.
In vegetative plant the developing green leaves are sinks not only for their own photosynthate but also for that produced in the older leaves. The oldest leaves near the base of the plant, provided they receive adequate illumination export sugars to the roots. Waredlaw (70) reported that once a leaf is mature , it is no longer a sink , even when it is made hetertrophics by natural or experimental shading.
DIVISION OF FUNTION
I n several crop plants, and particularly in rice, there is a division of function among the leaves on a Culm after the elongation of internodes. The sink for photsynthates from the upper two or three leaves is the panicle . that for the lower leaves is the root system system (51) , leaves in an intermediate position may send assimilates in either or both. maize ,the source for repiening grain is the leaves above the ear, and the source for roots is the leaves below the ear. Except for several leaves at the baseof the stem, which snd their photosynthates to the roots, the tomato plant is composed of several units , each of which has three leaves , a truss and a bud (54). This unit is the source sink unit and the photosynthates of the leaves of a unit go preferentially to the sinks within the the unit. However , this source –sink unit is not an absolute one . there is an interunit translocation of photosynthates , the extent of which depends on the condition of the plant . there are sources –sink partnership between certain leaves and organs , as in tomato, where several trusses containing fruits are fed by specific leaves positioned around them e.g. for such a system is hardly seen in higher animals because assimilation is organ – oriented . there are no special storage structures. But fat deposits are concentrated at some vulnerable point like the stomach , chest , and hips. However , animals like camels store some items like water for use both in imes of need. While in a developing plant embryo. The embryo is the sink and the endosperm is the source , in a chicken egg, the growing chick is a sink. Source and sinks are highly interdependent.
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