Whiteman, P. C. (Peter Carlile)

Wilson, Graham Langdon

University of Queensland eSpace

Effects of Water Stress on the 
Reproductive Development of 
Sorghurn vulgare Pers. 
by 
P. C. WHITEMAN and G. L. WILSON 
University of Queensland Papers 
Department of Botany 
Volume IV Number 14 
THE UNIVERSITY OF QUEENSLAND PRESS 
St. Lucia 
4 June 1965 
]D( 
WHOLLY SET UP AND PRINTilD IN AUSTRALIA BY 
WATSON l'llRGUSON AND COMPANY, BRISBANE, QUEENSLAND 
1965 
SUMMARY 
EfFECTS Of WATER STRESS ON THE 
REPRODUCTIVE DEVElOPMENT Of 
SORGHUM VULGARE. PERS.
Sorghum vulgare plants were subjected to various droughting treatments, followed 
by rewatering, around the time of floral initiation. The consequences, particularly in 
respect of leaf numbers, time of initiation, and early inflorescence development, 
were observed. 
Severe water stress caused cessation of morphological changes, vegetative and 
reproductive . 
Plants droughted early resumed vegetative growth on rcwatering and flowered 
at a slightly reduced leaf number. 
Later drought al lowed the more vegetatively advanced plants to accomplish the 
early changes of the floral transition without visible change. Rewatering led to 
immediate floral development without further leaf initiation. 
The floral apex may develop at a water stress sufficient to prevent leaf expansion. 
INTRODUCTION 
Differentiation of the floral apex in sorghum occurs at an early stage of vegetative 
growth and at the expense of further leaf initiation . Consequently, water stress early 
in the life of the plant may not only affect growth directly, but in other ways through 
an influence on reproductive behaviour. 
Although it is commonly believed that low soil moisture affects flowering, there is, 
as Hagan et al. (1957) have pointed out, little exact infon ation . The data of these 
233 
234 P. C. WHITEMAN and G. L WILSON 
authors, together with that of .Phillis (1956) and Alvim (1960) for various crops, 
suggest influences both on initiation and subsequent development of the floral 
structures. Scott (1956) discusses the belief that under adverse soil-moisture conditions, 
grasses seed at the expense of leaf production. On the other hand, Phillis (1956) 
claims that under moisture stress, sorghums remain vegetative, producing leaf nodes 
until favourable moisture conditions cause the apex to become reproductive. H.e 
regards as part of its drought adaptation the ability of sorghum to enter the reproduc­
tive phase only when sufficient moisture is available to ensure some seed production. 
These latter ideas were based on field observations which did not include examination 
of apices and the relationship of drought conditions, both in degree and timing, to 
floral differentiation and subsequent development. The present study was undertaken 
to provide more precise information. 
MATERIALS AND METHODS 
Sorghum vulgare (cv. Saccaline) was grown in the glasshouse in pots containing 
6.5 kg of black earth at field capacity and, before floral initiation, exposed to various 
droughting treatments. Two plants were grown in each pot, one being removed at an 
earlier sampling and the other left for later observations. 
Three replicate plants were sampled from each treatment at 2-day intervals prior 
to initiation, and at longer intervals thereafter until floral emergence. Plants were 
dissected and the initiation and subsequent growth in length of the inflorescence 
followed. Plant height to uppermost emergent leaf base, and number of emergent 
leaves, were recorded for all plants in each treatment at regular intervals. 
RESULTS 
Experiment I. Tlu� effect of increasing duration of wilting
Pots were watered to field capacity at sowing, and the control plants were 
maintained at adequate water levels throughout. Watering was restricted in the other 
pots to cause wilting just prior to the normal time of floral differentiation. Wilting 
durations t)f 2, 3, and 4 weeks were imposed. Small additions of water were given during 
these periods to prevent death of the plants, which however always remained wilted. 
The design was a randomized block, with sufficient plants in each treatment to allow 
fifteen sampling occasions, leaving thirty plants to flower. 
· 
Height increment and leaf emergence show that the plants in all droughting 
treatments suffered from water stress about a week before actually wilting, when the 
first observations were made. Height growth virtually ceased until rewatering, atld 
emergence of leaves was affected in a similar way (Fig. 1). The first lloral differentia­
tions were observed in the control plants on the 24th day, when there were six fully 
emergent leaves, and were completed on the 26th day, without further leaf emergence. 
Wilting had begun in the other plants on the 25th day, initiations were first seen on the 
26th and completed on the 28th. These plants had only five emergent leaves and 
remained at this stage, except for a gain of one leaf in the 2-week-drought plants, 
TABLE 1 
Tml EFFECl' 011 INCREASING DURATIONS OF WATER STRESS ON PLANT HEIGHT, STEM DIAMETER, LHAF 
NUMBER, TIME TO FLOWER EMERGENCE, AND SEED NUMDER PER HEAD 
--- . 
-·- -···-·-· - -··
. 
-----·----· --
·
·-· ·-·--------·--
.... -. 
- ·- ····-·-·· ---· ·-· ··- ---·-·-····-··. Plant Height Stem Diameter Number Time to 50% IN umber of Seed··· s .. -... Treatment (em) (mm) ofLeaves Flowering (days) per Head --·-·- -----�--- · -·------· �-� 
Control 84 9.0 12 64 · 1,300 
2 weeks' wilting 74 7.4 11 74 830 
3 weeks' wilting 78 6.1 11 88 1,190 
4 weeks' wilting 78 4. 7 11 . 94 I ,340 I
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.EfFECTS OF WATER STRESS 235 
DAYS FROM SOWING 
FIG. }.-Experiment I. Effect of increa.�ing duration of wilting on (a) number of emergent leaves, 
(b) plant height, and (c) inflorescence length. X, control; 0, 2 weeks' wilting; /::,, 3 weeks' wilting; 
0, 4 weeks' wilting; W, onset of wilting; and R, time of rewatering. 
until rewatered. At floral emergence the con trol plants had a mean leaf number of 
twelYe, compared with only eleven in the droughted. Therefore water stress, applied 
immediately prior to and during the floral differentiation period, reduced the number 
of leaves and delayed initiation slightly. 
Adequately watered plants, in keeping with their greater node number and 
probably their continued expansion of both stem and leaf sheaths, were taller at 
initiation (Fig. 1) and remained so at flower.ing {Table 1), although the diffe;reuce is 
not sigt)ificant. The stem diameter of droughted plants decreased significantly with 
increasing duration of wilting. 
236 P. C. WHITEMAN and G. L. WILSON 
Inflorescences of control plants elongated rapidly from the time of differentiation 
and, in the 2-week-drought plants , immediately on rewatering. The others however 
commenced elongation prior to full rewatering, probably due to the small applications 
of water made to keep the plants alive. Emergence of flowers was delayed by periods 
corresponding fairly closely to the duration of drought, viz. delays of 10, 24, and 30 
days for 14, 21, and 28 days drought (Table 1 ) . The rate of extension after rewatering 
was therefore little influenced by the previous drought history. Final inflorescence 
length is also independent of the earlier delays, and this is reflected in the number of 
seeds set per head (Table 1). 
Experiment II. The effect of time of onset of wilting 
In this experiment the effect of a constant duration of wilting applied at different 
stages prior to initiation was studied. In addition to control plants, three droughting 
treatments were applied, viz. wilted from day 15 until day 21, from day 19 to day 25, 
and day 27 to day 35 (early, intermediate, and late drought respectively). There were 
twenty sampling occasions during the growing period and thirty plants were allowed 
to flower. 
Again, water stress effects were apparent before wilting, as shown by reduced 
height growth, while during wilting height increment and leaf emergence ceased 
(Fig. 2). 
Water stress delayed floral initiation, and morphological change at the apex 
occurred only after supply had been restored. Irrespective of age when wilting was 
imposed, all plants differentiated floral apices between days 34 and 37, compared with 
23 to 26 days for control plants (Fig. 2). Although the late drought plants were wilted 
only after the normal time of floral initiation, increasing water stress leading up to 
wilting sufficed to prevent the transition. 
Mean leaf number was reduced as the onset of water stress approached the 
nonnal time of floral initiation. As in the previous experiment, reduction in node 
number was accompanied by a reduction in mean plant height at !lowering (Table 2). 
The time of floral emergence was delayed by about the same period (correspond­
ing very approximately to the length of drought) in all wilting treatments, probably 
because of the similar duration of all three. 
Inflorescences in all treatments elongated at a similar rate and attained the same 
final sizes (Fig. 2). 
TABl,E 2 
THE EH'ECf OF WILTING PERIODS ON FINAL PLANT HEIGHT, LEAF NUMBER, 
AND TIME TO FLOWER EMERGENCE 
Drought Period \ .Plant Height (em) IN-i�------�---�-11-72 __ _ 
Day 15--Day 21 
Day 19-Day 25 108 
Day 27-Day 35 j 104 
DISCUSSION 
Number of Leaves � - Time to 50 'Yo Fl�::ring
. 
(days) 
----·· - -·- --··--· 
14.8 
14.1 
13.8 
13.2 
58 
68 
68 
65 
The changes at the apex depend on the interaction of two factors; firstly whether 
or not there is sufficient vegetative development to allow the formation of a Horal 
stimulus, and secondly the extent to which water stress may prevent changes occurring. 
In the second experiment early drought stopped growth before the plants had grown 
sufficiently, and these plants remained vegetative, returning to leaf initiation after 
rewatering . During the development of the floral stimulus, leaf initiation continued 
and ceased only when the apex became floral. Therefore, more leaves were produced 
than in plants exposed to late drought which had already attained the critical stage. 
::! 
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DAYS FROM SOWING 
FJO. 2.-Ellperiment 2. Effect of time of onset of wilting on (a) number of emergent leaves, (b) plant 
height, and (c) inflorescence length. X, control; 0. wilted from day 15 to day 21; 6., day 19today25; 
O, day 27 to day 33. 0-0, 6.-6., and 0-0, wilting periods . In (c), 6. follows very closely tho 
line for 0. 
238 P. C. WHITEMAN aod G. L. WILSON 
In these latter plants, notwithstanding the cessation of growth which prevented leaf 
initiation, the floral stimulus developed, and on rewatering the morphological changes 
of floral initiation occurred without further leaf initiation. In the early drought plants 
there was a cessation of progress towards ripeness to flower, whereas with late drought 
the delay was in the morphological changes which follow the perception of the 
flowering stimulus. The constant length of the delays in each treatment, although on 
different processes, led to a similar retardation of the appearance of a floral apex in 
each case. Therefore all plants exposed to water stress differentiated flowers at the 
same time, although leaf numbers varied according to the time of droughting. 
In the first experiment, the various durations of droughting were all imposed at 
the one age, very close to floral initiation by the controls, although again height 
increments and leaf emergence reveal earlier water deficits. All drought treatments 
were comparable with the late drought of the second experiment. Vegetative develop­
ment was adequate for the fotmation of the floral stimulus at the same time as in the 
controls, but leaf production ceased, resulting in a reduced leaf number. Evidently the 
small additions of water to the droughted plants sufficed to allow, with a slight delay 
of about 2 days , the visible transition to a floral apex and some subse'luent elongation 
of this structure. Meyer & Anderson (1952, p. 260), Kramer (1959), and Gwynne 
(1960) draw attention to the ability of apical tissue to obtain water at the expense of 
older tissue, and Wilson (1948) showed that stem tips of tomatoes continued growth 
when the stem was shrinking under a water deficit. Increasing duration of drought 
in this experiment had no effect on initiation and leaf number, as all relevant changes 
were completed early in the period. However, duration of stress influenced subsequent 
development of the structures initiated. Leaf and flower emergence and plant height 
increment were delayed, although the constant node number led to constant final leaf 
numbers and heights. ·n1e greater height of control plants is due to the higher number 
of nodes. Likewise in Experiment ll, heights follow node numbers, but the delay in 
floral emergence, which was the same for all droughting treatments, corresponds to 
tl1e delay in initiation. This is in contrast to Experiment I, in which the delay was due 
to varying post-initiation dormancy resulting from the varying duration of drought . 
The reproductive behaviour in these two experiments agrees with earlier views 
that both initiation and subsequent floral development can be affected by drought. 
Drought, prior to initiation, exerts an influence before wilting. In the pre-"ripeness to 
flower" stage, water stress leads to apical dormancy as judged by external observation, 
and the restoration of water supply leads to a resumption of leaf production. With 
drought at later stages, the floral stimulus may develop while the apex is dormant in 
respect of leaf initiation, and while leaf expansion and internode extension have 
ceased. The morphological change to a floral structure and its subsequent growth 
may occur while the plant is still wilted. 
Phillis' (1956) observation that droughted plants produce additional leaves is 
only understandable in the case of water stress less severe than those employed here. 
Unpublished tesults show that at the usual ages of floral initiation in Sorghum 
vulgare plant survival below wilting point, without water additions, is less than 2 
weeks. Plants could perhaps remain near wilting point, continuing leaf initiation 
without sufficient vegetative development for floral initiation. If leaf area is the facet 
of vegetative development which is important, the progressive death of leaf tissue 
might keep plants vegetative. There is also the possibility that prolonged retardation 
could carry plants which are normally sown before midsummer into longer days 
which in turn leads to greater leaf number {Quinby & Martin, 1954, and unpublished 
data of the present investigations). . 
ACKNOWLEDGMENTS 
This work was carried out during the tenure, by P. C. Whitem.an, of a Genenil 
Motors-Holden's Linlited Post-graduate Research FeJlowship. Appreciation is 
expressed for this support. 
· 
EFFECTS OF WATER STRESS 
REFERENCES 
ALVIM, P. T. (1960). Moisture stress as a requirement for flowering of coffee. Science 132: 354. 
GWYNNE, M. D. (1960). Drought effects on plants. New Scientist 8: 795. 
239 
HAGAN, R. M., PETERSON, M. L., UPCHURCH, R. P., & JoNES, L. G. (1957). Relation of soil moisture 
stress to different aspects of growth in Ladino clover. Soil Sci. Soc. Am. Proc. 21: 360. 
KRAMER, P. J. (1959). Transpiration and the water economy of plants. In: Plant Physiology: a Treatise, 
ed. F. C. Steward. vol. II, p. 607. N.Y.: Academic Press. 
MEYER, B.S. & ANDERSON, D. B. (1952). Plant Physiology. N.Y.: D. van Nostrand Co. Inc. 
PHILLIS, E. (1956). Moisture in relation to reproduction. Proc. C.S.I.R.O. Canberra Sym. on water 
relations of plants. 
QUINBY, J. R. & MARTIN, J. H. (1954). Sorghum improvement. Adv. Agron. 6: 305. 
Scon, J.D. (1956). The study of primordial buds and the reaction of roots to defoliation as the basis 
of grassland management. Proc. 7th Int. Grass. Gong. N.Z., p. 479. 
WILSON, E. C. (1948). Diurnal fluctuations of growth in length of tomato stem. Plant P/zysiol. 23: 157. 


Effects of water stress on the reproductive development of Sorghum vulgare Pers.

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Effects of water stress on the reproductive development of Sorghum vulgare Pers.

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