by
J. N . B L A C K ,
Waiie Agricultural Research Institute, University of Adelaide
In recent years, increased interest has been shown in the role of light intensity in the growth of plants, and as a result it is n o w possible to put forward some overall account of its importance in crop production. Of all the major environmental factors affecting plant growth, light is probably the least understood, firstly because variations in light intensity are apt to be ignored—the h u m a n body being m u c h less sensible of changes of light than of, say, temperature or humidity—secondly because simple and inexpensive instruments for meas-uring, and more particularly for recording, changes in light intensity have only recently become available, and thirdly because normal daylight cannot be reproduced artificially under controlled conditions to enable detailed physiological studies to be m a d e . A s a result, agronomists and ecologists studying the growth of field crops have not had at their disposal sufficient information on the light factor or its variations caused by seasonal or ecological factors and have tended to overlook it.
Clearly the light factor of plant environment comprises a number of components; quality and dura-tion, as they affect plant growth, are better understood than others, and the present contribution will be concerned solely with light intensity and with the total amount of light energy available to the growing plant.
In a recent review, Stoughton [14]1 points out that in general usage the term "growth" is used in two different ways. Firstly, "elongation growth", which is suppressed by high light intensities, so that growth habit is markedly affected—in general it m a y be said that the less the fight available to the plant, the more spindly and elongated it will become, so that plants normally pros-trate become erect when subjected to shading; secondly,
"growth" is used to m e a n "dry weight increase", corres-ponding to the generally accepted scientific definition of plant growth, which Stoughton concludes to be determined primarily by the total amount of light energy available to the plant. F r o m the extensive inves-tigations of Blackman at Oxford [6] it is clear that
species differ in the fight intensity at which growth rate is maximal: a number of species m a k e fastest growth at about 80 to 85 per cent full summer daylight; a few, which occur naturally in shaded habitats, for instance as the ground flora in woods, have an optimum fight intensity of less than 80 per cent, while some species, which Blackman designates as "sun" species, have theoretically optimum fight intensities greater than (in some cases, very m u c h greater than) full daylight.
A review b y the present author (in press) has examined in detail the literature on the growth of pasture grasses and legumes in relation to light intensity, and the conclusion was reached that, while the pasture grasses as a group m a k e best growth at or even above full daylight, the pasture legumes are intolerant of even slight shade and would make best growth at very high levels of light intensity indeed. The conclusions reached by both Stoughton and Black are in general agreement, and indicate that the quantity of fight energy available is a major factor determining the rate of dry weight increase of plants. It is of interest to note that in Adelaide the growth rate of subterranean clover was found [3] to be determined solely by the amount of light energy received, being independent of temperature, and was highest in summer, when more than 400,000 foot-candle-hours were received in each of several 7-day experimental periods; and there was no evidence that light saturation had been reached even at these high values of light intensity. Similarly, Benedict [1], working in W y o m i n g , where, according to his paper, light inten-sities of 14,000 foot-candles were measured, found that shading to 70 per cent full daylight reduced the growth of three species of range grasses; his investigation was undertaken with the knowledge that light intensities in W y o m i n g were in general higher than those recorded elsewhere in the U . S . A . , and it had been suggested that they might be too high for optimum growth; this was not confirmed, however, by the experimental results.
1. T h e figures in brackets refer to the bibliography at the end of this chapter.
Light intensity and plant growth A number of experimental techniques are available
for the study of the light factor—the use of suitable shading materials [6]; artificial illumination [11], the correlation of measured seasonal fluctuations in light energy with growth [3]; and the correlation of the penetration of light into a plant association with the growth of the constituent species. This last approach has recently been used by Brougham [8] in N e w Zealand to show that the rate of dry weight production of a pasture following defoliation was at first slow, but acce-lerated until at about the 20th day a constant m a x i m u m rate was achieved; at that time, the area of leaves was about five times the area of the ground on which they were growing. A t this stage, the interception of light, as measured b y the intensity of light reaching ground level in the pasture, reached 100 per cent. It would thus appear that the attainment of m a x i m u m dry matter production is related to the elaboration of a leaf system capable of intercepting all incoming light energy, and it follows that the rate of growth is thereafter dependent on the amount of light energy available. Similarly, indirect evidence on the importance of light energy as a determinant of pasture yield was obtained by Donald [9] on the southern tablelands of N e w South Wales, in an experiment b y which subterranean clover was sown at a range of densities, herbage yields being taken on three occasions during the growing season.
It was shown that m a x i m u m yield was obtained at a density of 60 plants per square link, and that there was no further increase in yield w h e n this density was exceeded. This was ascribed to a restriction of growth rates once a full canopy of leaves had developed, so that interception of incoming light energy was complete.
Since competition for water and nutrients had been eliminated, it was concluded that competition for light was the factor involved, and that the amount of light energy available set a ceiling yield for any particular plant association; this ceiling yield can only be attained w h e n light is the sole factor limiting growth per unit area and w h e n complete interception of all incident light energy has been obtained. While the significance of light energy in determining crop yields has not been exten-sively examined, it is clear that in climates where water does not limit growth, and where soil deficiencies m a y be corrected, the ability of the plants to intercept all available light energy is of major importance.
O n the other hand, extension of this argument from climates where water is non-limiting to those of an arid or semi-arid type is clearly not justified, since in the latter environments soil moisture replaces light as the major factor controlling plant growth. Indeed, some desert plants have been shown to possess morphological characteristics reducing the absorption of radiation within the visible as well as in the infra-red wave-lengths [2], in addition to those characteristics more obviously associated with a reduction in water loss b y transpiration—reduced stomatal frequencies, w a x y cuticles etc.—or an ability to escape periods of drought
by ephemerality—flowering and seed-setting following rapidly upon germination. Similarly the existence of physiological mechanisms restricting the germination of ephemeral and shrub desert vegetation and the intense inter-plant competition for soil moisture result in so sparse a population of plants under arid conditions that a total leaf area capable of intercepting all the available light energy can rarely if ever be attained.
Indeed, W e n t [15] has stated: " T h e spacing of shrubs is rigorously controlled in the desert and ultimately only one n e w shrub can be established for every one that dies".
It must not be overlooked, however, that the amount of light energy potentially available for plant growth in the arid and semi-arid areas of the world is greater than in other districts better favoured in relation to soil moisture. This is, of course, scarcely surprising, and the correspondence between the distribution and m o v e m e n t of zones of high incoming solar radiation and the occurrence of the major arid regions has been noted b y Black [4] in a publication in which were presented monthly m a p s of the global distribution of solar radiation. Since inability to use the high values of light energy available in these regions is due to the extreme deficiency of soil moisture, it follows that, if this limi-tation can be removed, plant growth would be expected to proceed at a high rate. This is, in fact, the general experience where irrigation has been practised in arid or semi-arid environments. Furthermore, it is of interest that what is probably the most rapid rate of plant dry weight increase ever recorded (24.7 per cent per day) was obtained by the author with sunflower plants under irrigation in Adelaide in January 1953, a time of year during which the climate is not unlike that of the arid regions.
While it is certain that restrictions in plant growth in the arid or semi-arid areas are due to lack of soil moisture, it is possible that other factors m a y also be involved; indeed, high light intensity, high tempera-ture, low humidity and low soil moisture are so highly intercorrelated in these environments that their separate effects on plant growth could only be assessed in con-trolled conditions where each factor m a y be inde-pendently varied. However, it seems unlikely that artificially produced light intensities of the required order (at least 10,000 foot-candles) will be achieved in the near future. Until improved techniques are available, it is improbable that the relative importance in plant growth of these various aspects of the arid environment can be satisfactorily determined.
It was suggested earlier that one reason for the lack of appreciation of the role of the light factor in crop production w a s our limited knowledge of the micro-climatology of light, and this is no less true of the problems of plant growth in the arid environments.
Firstly, it is thought that the network of stations recording solar radiation should be greatly extended, so that detailed surveys, such as are at present in
Climatology and microclimatology / Climatologie et microclimatologie progress in certain countries (for instance, the U . S . A . ,
the U n i o n of South Africa a n d Australia) m a y replace estimates [4] based o n a n empirical relationship between radiation a n d other better k n o w n climatological factors.
Secondly, the relationship between solar energy a n d light energy requires examination in desert environ-m e n t s , since, at least till the recording of incoenviron-ming light energy is widely undertaken, ecologists interested in the wider understanding of the light factor of plant
RES
Intensité lumineuse et croissance végétale (J. N . Black).
Des études récentes sur le rôle de la lumière dans la production végétale ont m o n t r é q u e la croissance des plantes dépend de la quantité totale d'énergie lumineuse qu'elles reçoivent. L e s différentes espèces végétales ne réagissent pas toutes de la m ê m e façon a u x diminutions de l'intensité lumineuse. D a n s l'ensemble, la croissance des graminées fourragères paraît atteindre son m a x i m u m dans la pleine lumière d u jour; les légumineuses fourra-gères n e supportent pas l'ombre la plus légère et leur croissance est o p t i m u m pour des niveaux très élevés d'intensité lumineuse. D u point de v u e de la production agricole, il importe donc a u plus haut point d'obtenir
BIBLIOGRAPHY/
1. B E N E D I C T , H . M . "Growth of some range grasses in reduced light", Bot. Gaz., vol. 102, 1951, p . 582-9.
2. B I L L I N G S , W . D . ; M O R R I S , R . J. "Reflection of visible and infra-red radiation from leaves of different ecological groups", Amer. J. Bot., vol. 38, 1951, p . 327-31.
3. B L A C K , J. N . " T h e interaction of light and temperature in determining the growth rate of subterranean clover (Trifolium subterraneum L ) " , Aust. J. Biol. Sci., vol. 8, 1955, p . 330-43.
4. . " T h e distribution of solar radiation over the earth's surface", Arch. Met. Wien. (B), vol. 7, 1956, p . 165-89.
5. . " T h e influence of varying light intensity on the growth of herbage plants: a review", Herb. Rev. (in press).
6. B L A C K M A N , G . E . ; W I L S O N , G . L . "Physiological and ecological studies in the analysis of plant environment.
7. A n analysis of the differential effects of light intensity on the net assimilation rate, leaf area ratio and relative growth rate of different species", Ann. Bot., Lond., N . S . , vol. 15, 1951, p . 373-408.
7. B O Y K O , H . " O n the role of plants as quantitative climate indicators and the geo-ecological law of distribution", J, Ecol., vol. 35, 1947, p . 138-57.
environment are forced to rely on records of solar radiation, using some such relationship as that of Kimball [10]. Lastly, there seems to be an almost unexplored field of study in the micro-topographical variations in light and radiation and their effects on biological systems, to extend for instance the work of Boyko [7] on aspect, or that of Shreve [13] on phy-sical conditions in sun and shade.
ME
u n e surface de feuillage suffisante pour intercepter toute l'énergie lumineuse disponible.
L a répartition des grandes régions arides coïncide avec celle des zones d'intensité m a x i m u m de l'énergie solaire incidente, mais la croissance végétale y est limitée par la faible teneur d u sol en eau ; lorsque la sécheresse disparaît, le taux de croissance est très élevé. D'autres facteurs q u e la sécheresse d u sol contri-buent toutefois à réduire la croissance végétale, et il ne sera pas possible de déterminer leurs effets respectifs tant q u ' o n ne pourra pas atteindre, e n milieu expéri-mental, des intensités lumineuses égales à celles q u ' o n enregistre dans ces milieux naturels.
BIBLIOGRAPHIE
8. B R O U G H A M , R . W . "Effect of intensity of defoliation on regrowth of pasture", Aust. J. agrie. Res., vol. 7, 1956, p. 377-87.
9. D O N A L D , C . M . " Competition among pasture plants.
I. Intra-specific competition among annual pasture plants", Aust. J. agrie. Res., vol. 2 , 1951, p . 355-76.
10. K I M B A L L , H . H . "Records of total solar radiation intensity and their relation to daylight intensity", Mon. Weath.
Rev. Wash, vol. 52, 1924, p . 473-79.
11. M I T C H E L L , K . J. "Influence of light and temperature on the growth of ryegrass (Lolium spp.). 1. Pattern of vegetative development", Physiol. Plant., vol. 6, 1953, p. 21-46.
12. R O Y E N , W . V A N . The Agricultural Resources of the World N e w York, 1954.
13. S H R E V E , F . "Physical conditions in light and shade", Ecology, vol. 12, 1931, p . 96.
14. S T O U G H T O N , R . H . "Light and plant growth", J. R. hort.
Soc., vol. 80, 1955, p . 454-66.
15. W E N T , F . W . "Ecology of desert plants. 1. Observations on germination in the Joshua Tree National Monument, California", Ecology, vol. 29, 1948, p . 242-53.