Relationship Between Carbohydrate Enzyme Activities and Carbon Partitioning in Cotton

Donald L. Hendrix


 
ABSTRACT

In mature leaves of greenhouse-grown cotton plants, leaf carbon exchange rate, starch accumulation rate and carbon export fluctuate diurnally. The sucrose synthesizing enzymes sucrose phosphate synthase and sucrose synthase also exhibit diurnal cycles of activity in cotton leaf. Cotton leaf is unusual in this respect since the activity of sucrose synthase, usually thought of as a degradative enzyme, is not only relatively high in mature cotton leaf but varies diurnally whereas its activity is low and invariant in the leaves of other species. The enzymic step preceding both of these enzymes, UDPG pyrophosphorylase, was found to exhibit constant extractable activity diurnally. Maximal carbon export from mature cotton leaves was found to be during the afternoon hours; significant export occurred at night as well but at a lower rate. Leaf starch mobilization during the dark exhibited an almost 'mirror image' of carbon export during this period. Leaf sucrose content increased strongly during the light period and fell rapidly with the onset of darkness. The ratio of hexose to sucrose decreased as the light period progressed. We find very little free fructose in cotton leaf at any time of the day, in either greenhouse, field- or growth chamber-grown plants. The extractable sucrose phosphate synthase activity, but not the sucrose synthase activity, in cotton leaf correlated with carbon export during the light period but not during the dark. This suggests that carbon export from mature cotton leaf is regulated by sucrose phosphate synthase or by sucrose levels in the light and by starch breakdown in the dark.

In developing cotton embryos and seed coats we find the accumulation of sucrose and the galactose containing sugars raffinose and stachyose. Phloem unloading into these structures occurs in the seed coat from which assimilates must diffuse to the embryo. The immature seed coat and embryo contain detectable levels of sucrose synthase sucrose phosphate synthase soluble invertase and insoluble invertase. In the developing embryo, sucrose synthase activity is higher than in the developing seed cost (prior to 30 days pos anthesis) and both structures contain insoluhl invertase which decreases rapidly in the embryo to very low levels after 20 days post anthesis. Soluble invertase persists at relatively constant levels in both tissues. These enzymic patterns may be related to phloem unloading in this structure. The developing embryo accumulates sucrose which appears to be converted to raffinose and stachyose in later stages of development (approximately 35 days post anthesis), following the appearance of the enzyme galactinol synthase, which is essential for this conversion. A maturity, these two soluble sugars are the major nonpolymeric carbohydrate compounds in cotton seed. Work by other authors show that they degrade rapidly during germination.



Reprinted from 1986 Proceedings: Beltwide Cotton Production Research Conferences pp. 77 - 78
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998