Assimilate supply to the developing ear of maize (Zea mays L.) is an important determinant of grain yield. The objective of the current study was to determine the relative limitations of photosynthate and reduced N supply to the ear for determination of yield components, kernel number and kernel weight. Field‐grown maize plants on Dupo silt loam (Coarse‐silty over clayey, mixed, nonacid, mesic Aquic Udifluvents) were shaded during either vegetative growth, flowering, or grain fill. Control plants were not shaded. Photosynthesis was measured on plots from 9 d before flowering to grain maturity, and plants were sampled at intervals during this period for measurement of dry weight and reduced N content of plant parts of the aboveground vegetation (stover) and ear. When plants were shade during flowering, photosynthesis decreased during this period and kernel abortion increased relative to controls. However, N concentration was higher in aborting kernels than in nonaborting kernels through late flowering and early grain fill. The supply of reduced N to the ear during flowering was not a limiting factor for determination of kernel number. During grain fill, remobilization of N and dry matter from the stover of controls accounted for 46.5 and 4.7% of ear N and dry weight at maturity, respectively. Availability of newly reduced N was apparently more limiting than availability of current photosynthate for kernel dry weight accumulation. It is proposed that supply of newly reduced N to the ear may be limited by the amount of photosynthate partitioned for nitrate uptake and reduction during grain fill.

The steady state solutions of two mathematical models are used to evaluate Munch's pressure-flow hypothesis of phloem translocation. The models assume a continuous active loading and unloading of translocate but differ in the site of loading and unloading and the route of water to the sieve tube. The dimensions of the translocation system taken are the average observed values for sugar beet and are intended to simulate translocation from a mature source leaf to an expanding sink leaf. The volume flow rate of solution along the sieve tube, water flow rate into the sieve tube, hydrostatic pressure, and concentration of sucrose in the sieve tube are obtained from a numerical computer solution of the models. The mass transfer rate, velocity of translocation, and osmotic and hydrostatic pressures are consistent with empirical findings. Owing to the resistance to water flow offered by the lateral membranes, the hydrostatic pressure generated by the osmotic pressure can be considerably less than would be predicted by the solute concentration. These models suggest that translocation at observed rates and velocities can be driven by a water potential difference between the sieve tube and surrounding tissue and are consistent with the pressure-flow hypothesis of translocation.The generation of sufficient hydrostatic pressure to overcome the resistance to solution flow offered by the sieve tube and sieve plates remains a central problem in the consideration of Munch's pressure-flow hypothesis as the mechanism of translocation in the phloem. The hydrostatic pressure available to drive solution flow has been estimated from the concentration of solutes in sieve tube sap (22,25). However, owing to resistance to water flow offered by the membranes between the sieve tube and surrounding tissue, the hydrostatic pressure in the sieve tube could be considerably less than the osmotic pressure predicted on the basis of sieve-tube-sap solute concentration.A number of mathematical models have been formulated to describe translocation in the phloem (7, 9, 11, 17; 4 for review). However, most of these models have been concerned solely with the movement of radioactive tracers (2, 9, 11) and have not dealt with the osmotic and hydrostatic pressures in sieve tubes or the movement of water into and through sieve tubes. A recent attempt to quantify these aspects of the translocation process (8) failed to deal realistically with sieve tube anatomy, including the dimensions of the sieve tube, and ignored the presence of sieve plates. In addition, translocation is a continuous process, and a model attempting to simulate translocation should include continuous loading and unloading of translocate.This paper describes two mathematical models based on irreversible thermodynamics that attempt to quantify the pressure-flow hypothesis of phloem translocation. These models can be used to predict the osmotic and hydrostatic pressure required to drive solution flow in sieve tubes and to evaluate the pressure-flow hypothesis as a plausible mecha...

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Leaflets of Vicia laba L. were pulse labeled with "4CO2 and the kinetics of 14C-sucrose redistribution among individual tissues was followed. Sucrose specific activity in the whole leaf peaked about 15 minutes after labeling and declined with a half-time of about 80 minutes. In one experiment, leaflet discs taken at various times during the '2CO2 chase were quick frozen, freeze-substituted, and embedded in plastic. The tissue was sectioned paradermally and sections of palisade parenchyma, of spongy parenchyma, and of spongy parenchyma that contained veins were collected. Water extracts from these sections were assayed for sucrose specific activity. Sucrose specific activity in the palisade parenchyma was higher than that of the spongy parenchyma and reached a maximum in both tissues 9 to 15 minutes after labeling. Sucrose specific activity initially de- Two hours prior to an experiment, the shoot above the youngest fully expanded leaf was excised. The leaflet to be labeled was placed in an open Plexiglas chamber by sealing the stem immediately below the leaflet into an opening in the bottom of the chamber. Neither the leaflet nor the shoot below the leaflet were excised during these manipulations. The chamber was closed just before labeling. The chamber was connected in series with a circulating pump with sufficient capacity to give the chamber air a turnover time of 1 to 2 sec in the experiment reported in Figure 1. and 10 to 1 5 sec in the experiments reported in Figures 3 and 4