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Abstract
Human existence and civilization have always depended on an adequate supply of food. Throughout history, agriculturists have been at the helm of a remarkably successful enterprise to increase the quantity and improve the quality of the world food supplies. The giant achievements of the past, however, are dwarfed by the tasks that still lie ahead. As a reminder, the 5 billionth, currently living, individual of the human race was born in July 1986.
Abstract
Sugars are a major storage carbohydrate and a primary component of carrot root quality. The objective of this study was to determine the characteristics of sugar transport into the storage cells of carrot [Daucus carota (L.)]. Tissue disks were incubated in a buffered solution (pH 6.5) containing various concentrations of sucrose, glucose, or fructose, 1 mm CaCl2, and 100 mm mannitol. Passive uptake was defined as uptake in the presence of 5 μm carbonyl cyanide, - m chlorophenyl hydrazone (CCCP). Active uptake was the difference between total (–CCCP) and passive uptake. Characteristic, biphasic kinetics were observed for all sugars. At sugar concentrations below 10 mm, a saturating active component was operating. Above 10 mm, the influx was a nonsaturating, linear transport system. Active transport was pH dependent, showing high rates of uptake at low pH. Glucose and fructose did not inhibit sucrose influx and vice versa, but they did compete with each other. The kinetics of the hexose competition was noncompetitive inhibition. The competition studies suggested the presence of a separate carrier for each sugar. The evidence indicated that sugar transport across membranes of carrot storage cells is a combination of active and passive transport, consistent with transport kinetics observed in other crops. Active sugar uptake is a significant part of uptake, doubling influx at low apoplastic concentrations. At maturity, sucrose is the major transport and storage sugar. Glucose and fructose however, are transported at considerable rates in vitro, when they are present in the free space.
Abstract
Celery (Apium graveolens L.) is a favorable plant material for phloem loading studies, because vascular bundles and phloem tissue can be isolated from fleshy petioles with minimal manipulations. Uptake kinetics for sucrose loaded into phloem are different than uptake by phloem-free tissue. In isolated vascular bundles, uptake of phloem sugars (sucrose and mannitol) were biphasic kinetics, consisting of an active, saturating component operating at low sugar concentrations, and linear kinetics at higher sugar concentrations. Glucose uptake exhibited linear kinetics. However, when the glucose analog, 3-0-methyl glucose (unmetabolized) was used, biphasic kinetic profiles were obtained. Sucrose and mannitol uptake in isolated phloem tissue consisted of one saturating component showing Michaelis-Menten kinetics. In the homogenous storage parenchyma tissue, sucrose uptake kinetics were by diffusion and insensitive to carbonyl cyanide m-chlorophenyl hydrazone (identical to that of L-glucose). The data suggested the presence of different mechanisms for sucrose uptake across the sieve-tube companion cell complex and nonphloem cells. It is proposed that celery may be a useful system to obtain direct evidence on mechanism of solute loading. Chemical names used: carbonyl cyanide m-chlorophenyl hydrazone (CCCP).
Abstract
Advances in plant biochemistry depend on new technologies for the detection and quantification of small molecules. Plant biochemists must be able to detect and quantify low molecular weight metabolites and hormones in specific tissues or even single cells if they hope to relate biochemical function to physiological response. In many cases, new technology has to precede significant research. Phytohormone research offers such examples.
We investigated the effects of N nutrition on growth and carbohydrate partitioning of pepper (Capsicum annuum L., cv. Maor) seedlings in the greenhouse and on their subsequent recovery and development after transplanting. Seedlings received 0, 30, 100, or 200 mg N/liter for 14 days, after which they were transplanted and received 100 mg N/liter. Nitrogen levels below 100 mg·liter−1 inhibited shoot growth and leaf chlorophyll content; both were severely inhibited in the absence of supplemental N. Root growth had a negative relation with N supply; an enhanced root: shoot ratio was observed under low-N regimes. On a unit-leaf-area basis, CO2 fixation was not affected when N was present; however, it was greatly inhibited in the absence of N. Changes in the leaf starch and soluble sugar concentrations occurred as a function of N supply and leaf age. In the roots, low N led to lower sucrose and higher levels of hexose and starch. More sucrose was transported and accumulated into leaf veins of low-N tissue. Exogenously supplied 14C-labeled sucrose was rapidly converted into starch in the low-N tissue. Seedlings that received 100 mg N/liter had the highest post-transplant growth rate and flowered earlier. Carbohydrate status of young pepper seedlings influenced their post-transplant recovery. Optimal N supply is essential for full recovery and development of transplants.