TIBS - February 1979 N 27

Can we control the water requirements of plants?

Terence A. Mansfield

Why is the natural protection offered by abscisic acid not adequate for crop plants? The simple answer to this question is that the hormone becomes operative only afier the onset of stress. Plants cannot forecast the weather, and their natural tendency is to grow at maximum rate, which involves

The stomata which perforate the surfaces of leaves are the pores through which gas exchange occurs, and their opening and closing is the principal means by which a plant controls its rate of water use. Plant physiologists and ecologists have identified several situations in which it would be advantageous to impose artificial controls on the movements of stomata. The most important of these are the improvement of the water economy of field crops, and the reduction of wasteful expenditure of water by natural and seminatural vegetation in water catchment areas [ 11.

Several chemicals are known which will induce stomata to close when sprayed in solution onto the surfaces of leaves. The properties of one of these, phenylmercuric acetate, were once widely acclaimed, and this compound might be of some practical value if it were not for the pollution prob- lems it would cause [2]. The search for other suitable chemicals has not been successful, and interest in the problem has abated, despite its considerable agricultural and general economic significance. This should, however, still be regarded as a promising area of research, and recent studies of plant hormones have suggested routes which should be explored in the future.

the opening of stomata. Abscisic acid is the an extravagent use of water as long as it is

most powerful non-destructive inhibitor of available. We ourselves can, however, pre-

stomata1 opening known (Fig. 2) and the dict future weather, and can anticipate the part it plays in regulating water consump- needs of a crop throughout its entire life tion under dry conditions is crucial for the cycle. External applications of abscisic acid survival of land plants [3,4]. It is arguable at appropriate times might, therefore, pro-

that without a stress hormone with the vide crops with better strategies of water

properties of abscisic acid, land plants as use, ensuring that a limited supply is we know them could not survive. rationed through the growing season.

Applications of abscisic acid to the sur- faces of leaves are highly effective in caus- ing stomata to close. A dose of only 0.02 pg cm-* is sufficient to reduce water consumption by over 50:(; for a few days (Fig. 3) and this is achieved with very little inhibition of CO, intake for photo- synthesis, or side effects on other growth processes.

Encouraging though these experiments have been, the effects of application of abscisic acid have proved too short-lived to be of practical value. When sprayed onto orchard trees of Golden Delicious apple, for example, the effects do not persist after

I 1 I 1

,o-ll ,o-Ao ,0-g ,o.-* 48 h [5]. A consistent suppression of

1oe7M stomata1 openings for 2 or 3 weeks after Abscisic acid cont. application will be necessary if the tech-

Fig. 2. The relationship between opening of stom- nique is to be economically acceptable.

ata and concentration of abscisic acid. Data from a We have perhaps been somewhat naive

bioassay developed in the author’s,laboratory using in supposing that the naturally-occurring isolated epidermis of Commelina cornmu& hormone will itself be of use in agriculture.

Physiological role of abscisic acid

Plants which are short of water, and therefore under physiological stress, con- tain greatly increased amounts of abscisic acid (Fig. l), a hormone which has several physiological roles, one of the most im- portant of which seems to be the control of

(+)-Abscisic acid

Fig. 1. The naturally occurring form of abscbic acid, which is a sesquiterpenoid thought to be formed in chlorop!asts from farnesyl pyrophos- phate

T. A. Mansfield is Professor of Biology in the University of Lancasier, U.K. _i

0 Elsevicr/North-Holland Biomedical Press 1979 a

N 28

c I I I I

2 4 6 6 days

Fig. 3. Water consumption, as 9 percentage of that of untreated controls, of young plants of barley sprayed once on day 0 with a solution of IO-% abscisic acid. The dose was approximately 0.02 pg cm+ of leaf. From an experiment in Laneaster by Dr Ruth .Moulton

The experience with another plant hor- mone, indoleacetic acid, is that it is necess- ary to explore chemical analogues to find compounds that are agriculturally useful. The hormone itself is too readily destroyed by the plant to exert persistent effects, but analogous molecules have proved invalu- able ,and are widely used as herbicides, in regulation of rooting of cuttings, control of fruit set and abscission, and for many other purposes.

Analogues required

These prodigious practical successes with analogues of indoleacetic acid suggest that exploration of molecules related to abscisic acid should be a profitable area of research. Chemists have, however, been slow to enter this field, and in writing this article I am hoping to stimulate interest in a topic that could be of much benefit to agriculture and horticulture, not only in drier parts of the world, but also in temper- ate regions where periods of drought can seriously affect crop yield.

Consider the data in Fig. 4. Non- irrigated plants of coffee were sprayed once only with a lO-4 M solution of a simple derivative of abscisic acid, its methyl ester. For the next IO days they used water more economically than the untreated controls, after which the stomata were closing owing to water stress (presumably because of endogenous abscisic acid formation). At the end of the experiment, after 17 days, the treated plants still had enough water available to permit a high rate of transpira- tion, and this was clearly the result of economy enforced upon them during the first 10 days.

We have so far explored just a small number of analogues and derivatives of abscisic acid [1,6], and the methyl ester is the one with the most prolonged action. The number of compounds of comparable structure that should be tried is, however,

TIBS - February 1979 w ‘c

N

I

ABA methyl ester - - - -

‘5 Control -

-- _ I \

I I 1 I I 1 I I I I 1 I I I 1

1 2 3 4 5 8 7 8 9 10 11 12 13 14 1.5 16 17

DAYS

Fig. 4. An experiment carried out on non-irrigated co#ee plants in West Africa by the author’s collabora- tor, T. J. S. Moreira. The leaves were sprayed either with water (-) or a IO-% solution of the methyl ester of abscicisic acid (- - - -) on day I. From reference [4], reproduced by permission of the Royal Society of London

very large and beyond the scope of a uni- versity physiology laboratory. Can I urge those with suitable facilities and expertise to try their hand at solving this problem? 2

Simple derivatives of abscisic acid come so close to providing the necessary control that it will be surprising if further studies do 3

not uncover a compound with the required activity, which should approximate only 4 to a doubling of that shown in Fig. 4.

References 5

1 Davies, W. J., Mansfield, T. A. and Orton, P. J. 6 (1978) in Opportunities for Chemical Plant

Growth Regulation, proceedings of joint BCPC and BPGRG Symposium, pp. 45-54, British Crop Protection Council Waggoner, P. E and Turner, N C. (1971) Transpiration and its Control in a Pine Forest pp. l-87, Bulletin 726, Connecticut Agricul- tural Experiment Station, New Haven Hiron, R. W. P. and Wright, S. T. C. (1973) J. Exp. Bot. 24, 769-781 Mansfield, T. A., Wellburn, A. R. and Moreira, T. J. S. (1978) Phil. Trans. R. Sot. Lond. B275, 35-46 Goode, J. E., Higgs, K. H. a&l Hyrycz, K. J. (1978) J. Hort. Sci. 53, 99-103 Orton, P. J. and Mansfield, T. A. (1974) Planta, Berl. 121, 263-272

EMBC’s future confirmed By the unanimous vote of its seventeen member states the European Molecular Biology Conference (EMBC) has decided to extend the life of the Conference until April 1988.

EMBC is the intergovernmental organ- ization which provides the budgets for workshops, fellowships, lectureships and courses administered by the European Molecular Biology Organization (EMBO). Until the recent vote taken at the Con- ference’s latest meeting in December the future of EMBC was uncertain. The current arrangements for EMBC, which is composed of representatives from 16 west European states plus Israel, were due to expire in April next year. However, with the eight year extension recently agreed, the future of EMBO’s programme seems to be secure for most of the next decade.

Decisions on the size of the budget

will be made during the Conference’s next meeting at Heidelberg in late May. It is likely that EMBO’s programme will be maintained at its present level and indeed there is a good chance that some expansion may be possible.

EMBO’s main objective is to promote the molecular biological approach to the study of biological problems. With this aim in mind EMBO awards an average of 215 short-term and 135 long-term fellow- ships each year, in addition to organizing over 30 workshops and advanced practical and lecture courses. Each y r EMBO also awards about ten lectureshi s to distin- guished scientists.

-:;

SP

0 An article on the roles EMBC, EMBd and the European aboratory for Molecular Biology ( BL) can be found in TIBS (1976) N 28.

0 EkeviefjNorth-Holland Biomcdicpl Pram 1979