Lorna Woodrow

ETHYLENE AND CARBON DIOXIDE EXCHANGE IN LEAVES AND WHOLE PLANTS.

The stimulatory effect of CO2 on ethylene evolution from photosynthetically active excised and intact leaf tissue of both C3 and C4 plants is now well documented (Dhawan, Bassi, and Spencer, 1981; Fuhrer, 1983; Grodzinski, Boesel, and Horton, 1982a, b; 1983; Grodzinski, 1984; Kao and Yang, 1982; Woodrow and Grodzinski, 1987). The range of CO2 concentrations over which ethylene release is most responsive, 50 to 1500 ul L−1, coincides with those encountered by leaf tissue in sealed greenhouse environments under conditions of CO2 depletion or CO2 enrichment (Porter and Grodzinski, 1985; Woodrow and Grodzinski, 1987). Interestingly the current global CO2 levels and the predicted future elevated concentrations also fall within this range. Because the stimulation of ethylene release by elevated CO2 is direct and readily reversible (Grodzinski et al., 1982a, b; 1983) in both C3 and C4 plants we originally suggested that CO2 may stimulate the synthesis of ACC or ethylene and/or affect the retention and metabolism of ethylene (Grodzinski et al., 1982a,b). Nilsen and Hodges (1983) and Fuhrer (1985) also present data which support the concept of retention as a mechanism of the CO2 effect. Alternately Kao and Yang (1982) favour the view that the ACC to ethylene conversion step is directly affected by CO2. Incubations of leaf tissue at high CO2 concentrations have also resulted in increased ethylene-forming enzyme content (Philosoph-Hadas, Aharoni, and Yang, 1986).

Ethylene- and submergence-promoted growth in Ranunculus sceleratus L. petioles: the effect of cobalt ions

Abstract Petioles of the celery-leaved buttercup, Ranunculus sceleratus L., elongate when the leaf blades are submerged. This elongation has been correlated with a rise in internal C 2 H 4 levels. Treatment with C 2 H 4 in air will also enhance petiole growth. The submergence-induced growth is inhibited in the presence of CoCl 2 solutions. Cobalt treatment does not inhibit petiole elongation when the leaves are exposed to C 2 H 4 in air or when the tissues are submerged in C 2 H 4 -saturated CoCl 2 solutions. Thus, the physiological effect of CoCl 2 is completely mitigated by treatment with C 2 H 4 . Cobalt can directly inhibit C 2 H 4 production by leaf tissue. There is no evidence of the effect being dependent on changes in CO 2 metabolism.

Partitioning and Metabolism of Photorespiratory Intermediates

Many external as well as internal factors control the partitioning of metabolites from source leaves to developing sinks. Initially, however all carbon in the plant and all energy used for the growth and development are derived through photosynthesis. With respect to net assimilation of CO2, photorespiration (ie. the release of CO2 in the light) is frequently viewed by breeders as an antagonistic process possibly even wasteful (1,2,3,4,5,6). However, in terms of maintaining whole plant growth in the natural environment photorespiration may have several homeostatic roles (7,8,9) which provide clear advantages to a plant facing adverse conditions. With the view to a better understanding of photorespiration and the role(s) it might preform, our studies have been focussed on the identification of the ‘end products’ of photorespiration at the cellular level and the relationship of their metabolism to the growth and development of the whole plant. As indicated in Fig. 1, the products of the glycolate pathway which actually leave the cell are CO 2, the transport sugars (primarily sucrose), and amino acids (eg. glycine, serine, glutamate, glutamine).

Photosynthetic Gas Exchange, Photoassimilate Partitioning, and Development in Tomato under CO2 Enrichment

Interest in CO2 enrichment has focussed primarily on its application in horticultural greenhouse production (1). CO2 may be added to the greenhouse atmosphere to maintain a level of approximately 330ul/L or enriched to higher levels (eg. 700–1500 μ1/L) to further stimulate growth. Elevated CO2 levels are also experienced by crops in some outdoor production situations. Recently Wallis and Grodzinski (2) reported that sweet pepper plants grown inside polyethylene tunnels on mulched soil experienced CO2 levels exceeding 3000 μ1/L due to soil respiration and restricted gas exchange. Potential global increases in CO2 concentration have also stimulated interest in plant responses to altered CO2 levels (3). Growth and development of plants is associated with photosynthetic activity and partitioning of carbon (4) as well as with the action and integration of plant growth regulators (5). In this study determinate tomato plants were grown as transplant stock at CO2 concentrations of 300, 1000, and 3000 μ1/L. Their growth and photosynthetic metabolism was studied during the period of early vegetative development. Leaf ethylene metabolism was examined as it relates to leaf photosynthetic activity and potential growth regulation.

An Assessment of Ethylene and Carbon Dioxide Exchange in Plants

Current evidence indicates that ethylene is a volatile by-product of amino acid metabolism. Ethylene synthesis is linked with nitrogen metabolism through methionine and S-adenosylmethionine and as a volatile emission may serve as a non-destructive probe of amino acid turnover during photosynthesis and photorespiration. Very little is known about the relationship between ethylene and CO2 gas exchange in photosynthetic tissue. CO2 enhances the rate of ethylene release from leaf tissue in the light (1–5) and it has been proposed (3) that ethylene synthesis and/or metabolism may be moderated by photosynthetic and respiratory processes in photosynthetically competent leaves through changes in the internal CO2 concentrations.

The Effect of Carbon Dioxide on Ethylene Release from Leaves: Photorespiration and Ethylene Release

Ethylene gas is a normal product of plant metabolism and is considered to be an important co-regulator of many developmental processes. In higher plants ethylene biosynthesis proceeds from methionine through S-adenosylmethionine (SAM) to 1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene (Yang, 1982). SAM functions as a methyl donor in many biochemical reactions in living tissue and is not restricted to the ethylene biosynthetic pathway. ACC however appears to have a unique role as the immediate precursor of ethylene. Green leaf tissue normally has a low endogenous ethylene production rate (20–200 pmoles/gm fr wt/h) and until recently studies with mature leaf tissue have been primarily concerned with leaf senescence and the role of ethylene in this process. Interest in leaf tissue has been increased by the observation that photosynthesis can play a role in ethylene metabolism (Gepstein, Thimann, 1980; de Laat et al., 1981; Grodzinski et al., 1982a,b; 1983; Horton et al., 1982). Several studies earlier suggested that light inhibits ethylene production in photosynthetically active leaf tissue when compared to rates obtained in the dark (Gepstein, Thimann, 1980; de Laat et al., 1981). However when leaf tissue is illuminated and the CO2 levels within the experimental system are maintained above the compensation point this inhibition is not apparent in either C3 or C4 plants (Grodzinski et al., 1982a,b;1983). Interestingly in the light ethylene release from Cl tissue never exceeds that in the dark even under conditions of high t02. The major inhibitory effect attributed to light is due to the depressed CO2 levels which are a result of the balance of photosynthetic and respiratory activity within the tissue (Grodzinski et al., 1982a). The difference between C3 and C4 plants may reflect the different carboxylation patterns in These two types and the relative ability to regulate internal CO2 concentrations. The amount of carbon flowing through the ethylene pathway is very small (pmoles carbon/mg Chl/h) in comparison to the flow through the major metabolic pathways (pmoles carbon/mg Chl/h). Ethylene metabolism and the resultant physiological effects may be subject to the many processes that can significantly modify the internal CO2 environment of the tissue. Ethylene release is responsive to changes in external CO2 concentration over the range extending from 0.0 to 0.10% which includes normal ambient levels (Woodrow, 1982; Woodrow and Horton, in preparation). Furthermore, a significant production of CO2 within the tissue from a photorespiratory intermediate (eg. glycolate) could represent a homeostatic mechanism by which internal CO2 pools may regulate ethylene metabolism. This would parallel the proposed role for photorespiration as a means of dissipating reducing energy under photo-oxidative conditions.