Physiological and anatomical factors influencing the pattern of carbohydrate accumulation in senescent and nonsenescent sorghum [Sorghum bicolor (L.) Moench] plants

Abstract

Two sorghum cultivars, ATx378 x RTx430 [(34), higher stem carbohydrates and slower rate of senescence] and ATx378 x RTx7000 [(37), lower stem carbohydrates and more rapid rate of senescence] were utilized in a study of factors to determine possible anatomical and physiological differences. Plant spacings of 10 or 40 cm apart within the rows, were used. Evaluations were performed at four selected maturity stages: preboot (PB), 15 days post anthesis (PA), upon completion of kernel black layer (BL), and 10 days post black layer (PBL). Cultivar 34 had approximately 45% of the bundle-sheath cell volume filled with chloroplasts, 2 - 3 mesophyll cells between veins and a shorter distance between veins. By contrast, cultivar 37 contained only 19% of the bundle-sheath cell volume filled with chloroplasts, 3 - 4 mesophyll cells between veins and a larger distance between veins. Stems of closer spaced plants contained more but smaller vascular bundles than wider spaced ones. Phloem vessels of cultivar 34 covered 23% of the total vascular bundle area, whereas only 18% was covered for 37. Cultivar 34 produced the highest carbon dioxide assimilation rates. PEP carboxylase enzyme activity, measured in the crude extract of the flag leaf, and the assimilation rates, decreased in a stepwise trend as plant maturity progresses. Cultivar 34 produced significantly higher stem nonstructural carbohydrates (NSC) for all fractions determined. Closer spaced plants contained significantly higher levels of sucrose and total NSC. The lowest concentration for all carbohydrate fractions, except glucose, generally occurred at PA, which coincided with maximum panicle growth and kernel filling. After completion of kernel filling, carbohydrates were again accumulated in the stem. Starch was present in higher levels at the earlier stages, decreased at PA and increased again at later stages, but at higher concentrations for the slower senescing than for the faster senescing cultivar. Higher carbohydrate accumulation of the cultivar 34 was evidently enhanced by the leaf characteristics. This is indicated by a larger volume of bundle-sheath cell chloroplasts, higher density of mesophyll chloroplasts, and a shorter distance between veins. This, plus a larger area of phloem vessels in the stem vascular bundles should result in a more efficient transport system.