The Physiology of Flowering Plants - KHAM PHA MOI

304 REPRODUCTIVE DEVELOPMENT(invertase and/or sucrose synthase) metabolize the sucrose, thusforming, and maintaining, a concentration gradient into the developingtissue. Maize provides an attractive experimental system inwhich to study seed development, as lesions affecting kernel developmentare simple to observe as well as maize being an agriculturallyimportant crop. Many such mutations have been identified andinclude miniature1 (mn1), shrunken1 (sh1) and shrunken2 (sh2). As theirnames imply, the kernels of maize plants bearing these lesions do notdevelop properly. The mn1 mutation results in a loss of invertaseactivity. This enzyme is located at the base of the endosperm whereit hydrolyses sucrose, supplied from the maternal tissue, to glucoseand fructose which are taken up by the developing endosperm. Theseare then used to synthesize starch and other compounds. In theabsence of invertase, nutrient flow to the endosperm is disrupted.The sh1 and sh2 mutations result in a loss of activity in sucrosesynthase and ADP glucose pyrophosphorylase, which are importantenzymes of starch biosynthesis.There is great diversity in the protein, lipid and carbohydratecontent of seeds (Table 11.2). Starch-storing seeds are of great economicimportance and include cereals (e.g. rice, maize, wheat andbarley) and dicotyledonous plants (e.g. pea and bean). In contrast theseeds of pumpkin, sesame and oilseed rape (known as canola in theUSA) are very rich in lipids. In most seeds a significant proportion ofthe total dry matter is protein – up to 40% in soybean – hence itsimportance in vegetarian diets. Seeds may also be important sourcesof dietary vitamins and minerals.Starch is stored as granules within amyloplasts, either in theendosperm (e.g. in the Poaceae or castor bean) or in the cotyledonsTable 11.2 Composition of different seeds. Data derived from the USDepartment of Agriculture. Values have been rounded to the nearest wholenumber.Species% dry weightCarbohydrate Lipid ProteinRice Oryza sativa 86 3 9Wheat Triticum aestivum 84 2 12Maize Zea mays 83 5 11Barley Hordeum vulgare 81 3 14Pea Pisum sativum 69 2 26Flax Linum usitatissimum 38 37 21Soybean Glycine max (soja) 34 21 40Sesame Sesamum indicum 25 52 19Almond Prunus dulcis 21 53 22Pumpkin Cucurbita sp. 19 49 26Oilseed rape Brassica napus 11 49 32