by
K . J. M I T C H E L L ,
Grassland Division, DSIR, New Zealand
One of the problems met in developing pasture in arid zones is the assessment of h o w far species and m a n -agement methods suited to higher rainfall areas have to be modified. This will apply particularly in the absence, but also in the presence, of irrigation. The average levels of temperature are frequently high in arid zones and, due to the high proportion of clear sunny weather, temperatures close to the soil surface often rise to particularly high levels during the day.
Some understanding of h o w well various pasture species can be expected to perform under such conditions can come from studies of the influence of various levels of temperature on the growth and the survival of these species.
Three interrelated approaches have been used.
Firstly, the habit of growth adopted by the plants, and the environmental conditions they live in, have been studied in the field. Secondly, there have been detailed studies of individual plants growing in pots either in heated glasshouses or under normal outside weather conditions. Thirdly, rates of growth have been measured in controlled environment equipment. In this, tempe-rature, intensity and duration of light, nutritional status and moisture supply are all held at predetermined levels.
F r o m these controlled environment measurements it became clear that the temperature optimum for growth of m a n y of the pasture species used in temperate zones is relatively low. Just h o w low is governed b y the criterion of growth adopted. There are considerable differences in the relative effects of changes in tempe-rature on the growth of the whole shoot, as compared with the growth of an individual tiller, or increase in number of tillers per plant. For pastures where a dense sward is established, the growth of the individual tiller (stem) has been considered the criterion of main importance, but for discussion related to arid zones, where plants more often grow as distinct units, the rate of growth of the whole shoot of the plant is probably a better criterion to use. The term "shoot" is here
used in the strict botanical sense where the parts of a plant are first classified as either shoot or root.
It can be seen from Table 1 that the optimum temperature for growth is about 65-70° F . for perennial ryegrass, cocksfoot and Yorkshire fog, grasses deve-loped in north-west Europe. It is considerably higher, 85° F . , for Paspalum dilatatum, a species originating in regions m u c h closer to the tropics.
T A B L E 1. Per cent increase per day in dry weight of shoot.
Plants grown under controlled environment
Temperature Plant
45» F. 55» F. 65» F. 75» F. 85» F. 95» F .
Perennial ryegrass
(Lolium perenne) 9.0 11.9 14.7 13.4 9.8 2.1 Cocksfoot
(Dactylis glomerata) 9.7 13.9 15.9 15.1 13.3 4.8 Paspalum dilatatum 1.8 7.1 14.4 20.5 22.1 19.2 Yorkshire fog
(Holcuslanatus) 10.1 15.3 18.6 16.6 12.3 — White clover
(Trifoliumrepens) 5.2 10.5 13.4 14.7 13.3 8.5 Lotus uliginosus 4.9 9.6 13.5 16.3 14.3 4.2 Subterranean clover
(Trifolium subterraneum) 7.6 10.8 11.8 11.5 8.1 4.0
The position with the legumes is of considerable interest. The optimum temperature for subterranean clover, a winter-growing annual of Mediterranean origin, is considerably lower than for white clover or Lotus uliginosus, both of which are summer-growing perennials of the moist temperate zones. W h e n tempe-ratures rise to over 85° F . , the rate of growth declines with all the species examined, and for m a n y of them the amount of growth at 95° F . is negligible.
Although levels of temperature well above 95° F . frequently occur near the soil surface during sunny days, particularly if the soil is dry, the density of the
Climatology and microclimatology / Climatologie et microclimatologie pasture cover a n d the character of the soil can greatly
change the effective temperatures m e t b y the plants (Table 2). O n a sunny s u m m e r d a y soil temperatures were found to be higher o n the sandy soil than o n a silty soil. H o w e v e r , even a relatively sparse cover of herbage, as with the 3-inch pasture on the sandy soil, substantially reduced the soil temperature, parti-cularly that near the surface.
T A B L E 2. Influence of cover and soil type on soil temperatures at approximately 2 p . m . on a sunny day in January.
Palmerston North, N . Z .
Coarse sandy loam Silt loam
D e p t h of
During their vegetative phase of growth those pasture species mentioned above all have their meristematic centres, from which all shoot tissue is formed and from which n e w roots are initiated, set close to and generally in direct contact with the surface layers of soil. M e a s -urements of the temperatures of these meristematic zones indicated that their temperatures are chiefly determined b y the temperature of the immediately adjacent soil. If the temperature of the soil rises to a high level, that of the meristematic zone rises also although not to quite as high levels. This emphasizes the value of a cover of vegetation for holding d o w n to reasonable levels the temperature of the meriste-matic zones. Deduction from the measurements reported in Table 2 and also direct measurements suggest that one of the major advantages of the tussock or tufted form of growth m a y be the m a n n e r in which a protective insulating cover is formed over the meristematic zones at the base of the shoot. These are therefore protected from diurnal extremes of high or low temperature.
The data presented in Table 1 describe results for plants growing without a n y defoliation, in relatively high light intensity and with adequate moisture and high nitrogen status. A s plants growing in the field are usually in competition with each other, or put to economic use b y the harvesting of their herbage, they are generally defoliated, or shaded. Furthermore at s o m e time during their growth they are likely to be short of moisture or nitrogen. It is therefore relevant to examine the m a n n e r in which temperature influences the response of the plants to these conditions.
At the higher levels of temperature defoliation definitely checks the development of tillers, and shading to 30 per cent of full daylight eventually halts tillering almost completely (Table 3). A t lower levels of t e m p e
-T A B L E 3. -Total number of tille s per plant of perennia ryegrass. Counts taken when the plants in each treatment had formed a mean of 8.0 leaves on their main stem. Plants grown in glasshouse and outside during spring
M e a n
temperature Full light Defoliated twice
Shaded to 30 per cent of full light
57° F.
rature similar defoliation has little influence o n the formation of tillers, and the effect of shading is m u c h less drastic.
At the higher temperature levels defoliation also considerably reduces the survival rate of plants grown under controlled environment conditions.
The effect of nitrogen status on the survival of plants under drought or high temperature conditions is m o r e complex. It has been widely noted that a n improvement in the general fertility level of a soil appears to enhance the growth and survival of pasture grasses under moderately dry conditions. Field experimentation at Grasslands Division has indicated that this is probably because the initial "drought" s y m p t o m s s h o w n b y pastures are due to a shortage of nitrogen, or probably of other minerals if they are limiting, rather than to a shortage of water available to the roots. This is because for a pasture the greater part of the limited supply of nitrogen available in the soil is in the surface layers, which dry out first. T h u s the plants are cut off from their supplies of nitrogen while reasonable supplies of water are still available in the lower layers of soil. T h e consequent baring of the soil surface, due to drying up of the vegetation, will then increase the heating u p of the general b o d y of soil, and lead to the dissipation of that moisture remaining in the soil after vigorous pasture growth has ceased.
In a m o r e fertile soil the a m o u n t of nitrogen available is greater a n d it is spread through a greater depth of soil. In these circumstances it is suggested that this T A B L E 4. Influence of nitrogen status of the soil on survival of plants grown under cool dry (mean 65° F.) and w a r m moist (mean 85° F.) conditions1
Plant
conditions all plants died, affected b y drought.
the high nitrogen plants being
The influence of temperature on the growth of pasture plants not only allows growth to continue longer, due to the
continued availability of nitrogen, but further that the continued maintenance of a green cover minimizes soil heating and moisture loss due to that cause.
However, a stage is reached in a drought when the moisture supply is used up despite such palliatives as those described above. Present experimental evidence suggests that in these circumstances a high nitrogen status can lead to poorer survival of plants (Table 4).
It is also apparent from Table 3 that the same applies if there is adequate moisture but temperatures are
Effet de la température sur la croissance des plantes fourragères (K. J. Mitchell).
Cette communication rend compte d'études relatives aux effets de fortes températures et de l'aridité sur le comportement de diverses espèces fourragères. Les résultats montrent que la température la plus favorable à la croissance de bon nombre des espèces fourragères utilisées dans les zones tempérées est relativement basse. Les conditions de température qui régnent dans
high. Hence the issue m a y also be of importance where irrigation is used in areas where m e a n temperatures are high.
In all of the issues mentioned more work is needed before the situation is clarified. T h e data already available indicate not only the manner in which the relative merits of some pasture species change with a rise in temperature, but also that the effects of various management practices on their productivity and survival change under high-temperature or low-moisture conditions.
les couches superficielles du sol dépendent de la couver-ture végétale et de la nacouver-ture du sol. L'auteur expose les effets de la défoliation et étudie l'influence de la teneur du sol en éléments azotés. Ces problèmes sont encore mal connus, mais les données recueillies montrent que les effets des divers traitements de pâturages sur la reproductivité et la vie des espèces fourragères se modifient lorsqu'on a affaire à des températures élevées et à u n climat aride.