Mobilization of fructan reserves and changes in enzyme activities in wheat stems correlate with water stress during kernel filling.

Triticum aestivum (wheat) plants grown at a day∶night temperature of 18∶13 °C from anthesis were held as well watered controls, or subject to either a mild (large pot volume) or a more severe (small pot volume) water stress by withholding water from the time of anthesis. Extracts from the peduncle (enclosed by the flag leaf sheath) and the penultimate internode were prepared to determine the activities of fructan exohydrolase and acid invertase and to assess the level of hexose sugars, sucrose and fructans. Measurements were made of ear and individual grain weights and stem fresh weight and dry weight. Plant water relations at the time of each sampling were determined as the flag leaf water potential and the water content of individual organs. Water stress resulted in a shorter duration of kernel filling, smaller kernels at maturity and an earlier loss of stem weight. There was an increase in stem fructose and a fall in fructan level that preceded the loss of dry matter associated with water stress. Coincident with the early fall in fructan content under water stress there was a rise in both fructan exohydrolase and acid invertase in the internodes of stressed plants. This correlation suggests that the conversion of fructans to fructose might have resulted from enzyme induction associated with water stress, but as this conversion occurs before the major export of reserves from the stem it might be only indirectly related to changes in the demand for reserves.

Phloem Transport and the Regulation of Growth of Sorghum bicolor (Moench) at Low Temperature.

Leaf expansion in Sorghum bicolor (Moench) was severely retarded by low night temperatures (5 C). However, this was not reflected in the early measurements of relative growth rate, indicating that the response was not associated with a deterioration of the photosynthetic system. For plants grown at 30/25 C (day/night) and subsequently held at an ambient temperature of 30 C, phloem transport, as measured either by the movement of (14)C-photosynthate through a zone of controlled temperature or by accumulation of dry matter distal to this zone, was inhibited by temperatures below 10 C. The speed of movement of (32)P through the temperature controlled zone was more sensitive to temperature with reductions apparent below 20 C. Although there was some recovery in the movement of (32)P following 3 days equilibration at low temperature (1 to 10 C), the new values (approximately 100 centimeters per hour) were still only about one-third of those obtained in the high temperature controls. For plants held at an ambient temperature of 21 C, which is well below the optimum for growth, translocation was only inhibited by temperatures below 5 C. Although low temperature may reduce the carrying capacity of the phloem of S. bicolor, this is unlikely to be an important factor in regulating the growth of the plants at low temperatures.

Source, sink and hormonal control of translocation in wheat.

An analysis of the pattern of movement of (14)C-labelled flag leaf assimilates in wheat (Triticum aestivum l. c.v. Gabo) during grain development, indicated that the greater the requirement for assimilates by the ear the more rapid was the speed of movement of these through the peduncle to the ear and also the lower their concentration. Experiments with [(14)C] indoleacetic acid ([(14)C]IAA) suggested that auxin production by the grains was not responsible for the control of assimilate translocation through the peduncle. Limiting the supply of available assimilates by shading the lower parts of the plant, did not significantly alter the speed of movement of (14)C-photosynthate through the peduncle, while severing half of the vascular tissue in the peduncle altered the pattern of movement of (14)C to the ear and enhanced the speed of movement of (14)C through the remaining functional conducting tissue. These results are discussed in relation to the mechanism of translocation.

Temperature and the translocation of photosynthate through the leaf of Lolium temulentum.

Localized cooling of a 2 cm length of the leaf blade of the grass Lolium temulentum to 0° C, had only a small effect on the mass transfer of assimilates from the distal part of the leaf. There was no evidence of a reduction in the velocity of movement of (14)C-labelled assimilates through the low temperature zone, and the retention of assimilates along the pathway of movement was reduced by low temperature. In contrast to the longitudinal movement, lateral movement of (14)C, induced by steam killing sections of the leaf, was extremely sensitive to temperature and showed no sign of acclimatization, even after 3 days at low temperature. These results suggest that, although loading sugars into the transport system was sensitive to temperature, the movement of assimilates was not directly controlled by metabolic processes along the pathway.

Effect of assimilate utilization on photosynthetic rate in wheat.

Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO2 dm(-2) leaf surface hour(-1) under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3-15 hours, and there was a marked reduction in the outflow of (14)C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and (14)CO2 fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the "sink" organs.

Independent translocation of (14)C-labelled assimilates and of the floral stimulus in Lolium temulentum.

It is widely accepted that the floral stimulus produced in leaves is carried to the shoot apex passively in the phloem with the assimilate stream. Three kinds of evidence presented here suggest that the floral stimulus moves independently of the assimilates.Simultaneous determination of the velocities of translocation out of the seventh leaf blade, in comparable plants under the same conditions, yielded estimates of 1-2.4 cm/hr for the floral stimulus, and 77-105 cm/hr for (14)C-labelled assimilates.The effect of the size of the seventh leaf on its ability to export assimilates or to initiate flowering was quite different. Leaves with only 14-26% of their final blade area emerged exported little assimilate, yet were highly active in inducing flowering.The effect of DCMU applications at a range of concentrations on the translocation of assimilates was quite different from their effect on the flowering response.