Fluctuation of non-structural carbohydrates in the stem and ears of maize (Zea mays (L.)) during grain fill as influenced by water stress.
Shanahan, Paul Edward.
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Stems of maize plants may serve as reservoirs for photosynthate produced in the leaves which may then be utilized for cell growth and maintenance requirements of the plants, and in particular for grain requirements during grain fill. Experiments were designed to ascertain the extent to which non-structural carbohydrates accumulate and are depleted in the stem and ears of locally cultivated maize hybrids during grain fill under conditions of water stress. Maize plants were grown: (i) under field conditions; (ii) under a rain-out shelter; and (iii) in pots placed inside a growth tunnel during grain fill. In the latter experiment whole maize plants were exposed to (14)C0(2) at selected intervals during grain fill. In the field trial large differences in the accumulation and depletion of total non-structural carbohydrates (TNC) were found between the six hybrids tested. The water stress conditions that prevailed from mid-grain fill (MGF) to physiological maturity (PM) resulted in TNC content levels being lower at PM than at anthesis in all hybrids except for SR 52. Total non-structural carbohydrate content in the whole stem of PNR 6427, CG 4602 and PNR 473 declined from anthesis to PM. In contrast TNC content in the whole stem of SA 60 and HL 1 declined from anthesis to MGF and then increased substantially in SA 60 and marginally in HL 1 from MGF to PM. In the rain-out shelter trial, water stress resulted in a 38 % reduction in final grain yield in SA 6 compared to 25 % in K78Y x I137TN. The greater tolerance to water stress of the more modern hybrid K78Y x I137TN compared to the obsolete hybrid SA 6 may be attributed to a number of factors, namely: (i) K78Y x I137TN recorded a higher leaf area index throughout grain fill under stress and non-stress conditions compared to SA 6; (ii) it did not partition as much non-structural carbohydrate to the stem during the first three weeks of grain fill as did SA 6 and did not markedly deplete stem non-structural carbohydrate pools to the same extent as did SA 6 under stress and non-stress conditions; and (iii) in the last week of grain fill as the leaf water potential of K78Y x I137TN increased sharply under stress conditions, it exhibited an ability to deplete stem non-structural carbohydrates to supplement the supply of current photosynthate to the grain. In the 14(C)-labelling pot trial, the maize single cross hybrid B254W X M162W generally depleted TNC in vegetative organs in the latter half of grain fill under stress conditions, while under non-stress conditions TNC continued to accumulate in vegetative organs until PM. Both stressed and non-stressed plants assimilated less 14(C) on consecutive labelling occasions during grain fill. The amount of 14(C) assimilated at six weeks after anthesis was only 12,1 and 16,3 % of that assimilated at anthesis in stressed and non-stressed plants, respectively. Stressed and non-stressed plants labelled at anthesis translocated a smaller proportion of assimilated 14(C) to the grain during grain fill than plants labelled later. Consequently, stressed and non-stressed plants labelled at anthesis recorded the highest proportion of whole plant 14(C) recovered in the whole shoot at PM compared to plants labelled on any of the other occasions. At anthesis the primary ear was not yet established as the major sink for photosynthate and much of the 14(C) assimilated at anthesis was utilized for final structural growth of the whole shoot including the cob and husks of the primary ear. Stressed and non-stressed plants assimilated similar amounts of 14(C) at anthesis and two weeks after anthesis, however, stressed plants assimilated less 14(C) than non-stressed plants at four and six weeks after anthesis. Forty-eight hours after each labelling occasion, the stressed plants had partitioned a higher proportion of assimilated 14(C) to the grain than the non-stressed plants. However, by PM the non-stressed plants had partitioned an equal or greater proportion of whole plant 14(C) recovered at PM to the grain compared to the stressed plants. Radioactivity associated with component non-structural carbohydrates, was determined using ion-exchange column chromatography and thin-layer chromatography. These procedures provided detailed data of the partitioning of 14(C) among glucose, fructose, sucrose and starch.