Field Crops Research, 9 (1984) 41--57 41 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
YIELD IN WATER-STRESSED MAIZE GENOTYPES: ASSOCIATION WITH TRAITS MEASURED IN SEEDLINGS AND IN FLOWERING PLANTS
A.J. HALL, C. CHIMENTI, N. TRAPANI, F. VILELLA and R. COHEN de HUNAU
Departamento de Ecologia, Facultad de Agronornia, Universidad de Buenos Aires, Av. San Martin 4453, 1417 Buenos Aires (Argentina)
(Accepted 9 January 1984)
ABSTRACT
Hall, A.J., Chimenti, C., Trapani, N., Vilella, F. and Cohen de Hunau, R., 1984. Yield in water-stressed maize genotypes: association with traits measured in seedlings and in flowering plants. Field Crops Res., 9: 41--57.
Thirteen maize (Zea mays L.) genotypes were evaluated for variability with respect to seedling traits reportedly associated with tolerance to water stress. Significant variability was found within this set of cultivars for chlorophyll loss under thermal stress, loss of intracellular electrolytes under thermal and desiccation stresses, and proline accumulation in response to desiccation. These attributes were not mutually correlated in the genotypes examined. A sub-set of seven genotypes was grown in a sand-nutrient solution culture in the field and subjected to stress at tasseling for about eight days. Yield was significantly reduced in all cultivars, but the magnitude of the reduction was variable. Relative kernel number of the uppermost ear (the major yield-determining factor in most cultivars) was strongly associated with duration of exposure to pollen of the same ear. Relative spikelet number of the uppermost ear was significantly reduced in two cultivars by stress, but this response appeared to be independent of mean exposure to pollen. Relative spikelet num- ber was associated with plant-leaf area: root weight ratio at tasseling. No association was found between seedling traits and relative yield, additional yield-related responses or other, presumably less complex, responses to stress such as loss of leaf area and fall in chlorophyll content.
INTRODUCTION
The identification of traits which contribute to tolerance to water stress in crop plants is an important objective for physiologists and plant breeders, and attempts have been made to establish the relationships between traits ex- hibited by seedlings or by fully grown plants and yield under stress in several species (e.g. Pint6r et al., 1978; Richards, 1978; Fischer and Maurer, 1978). The need to identify these traits is perhaps even greater in environments where drought is a frequent, but not certain, phenomenon. This uncertainty hampers attempts to breed for stress tolerance using yield-related properties
(Finlay and Wilkinson, 1963; Eberhardt and Russell, 1966; Fischer and Maurer, 1978). The main maize-growing area of Argentina is one such en- vironment, yield being frequently constrained by availability of water (Rebella et al., 1976).
We have evaluated the variability within a set of thirteen genotypes of maize (Zea mays L.), representing a diversity of types used in breeding and commercial production in Argentina, with respect to biochemical and phys- iological traits of seedlings which have been suggested as indicators of stress tolerance in maize and other crop species, namely: chlorophyll loss under thermal stress (Kaloyereas, 1958; Kilen and Andrew, 1969); resistance to loss of intracellular electrolytes under thermal and desiccation stresses (Blum and Ebercon, 1976, 1981); and capacity to accumulate proline in response to dehydration stress (Singh et al., 1972; Blum and Ebercon, 1976; Pinthr et al., 1978; Richards, 1978, but cf. Hanson et al., 1977}.
To examine the relationships between the above-mentioned traits, traits exhibited by fully grown plants, and responses of plants stressed at a develop- ment stage appropriate to the conditions of the maize-growing area in Argen- tina, a sub-set of seven genotypes was grown in sand-nutrient solution culture under field weather conditions and subjected to stress at tasseling for about 8 days. The use of semi-controlled conditions in the experiment allowed us to minimize the uncertainties arising from variation between genotypes in rates of development and the resulting interactions with timing of exposure to stress (Fischer and Maurer, 1978; Hanson and Nelsen, 1980). Traits and responses to stress examined in these plants were aerial biomass, relative grain yield (yield under stress/yield of controls), relative spikelet number of the uppermost ear, pollen availability, loss of leaf area, reduction in chloro- phyll content, leaf water potential, and leaf area:root weight ratio. These variables were selected because they represent the end results of the interac- tions between the complex of factors determining tolerance to water stress, their possible relationship with traits observed in seedlings, or because pre- vious work had shown them to be important or potentially important deter- minants of yield under stress.
The usefulness of seedling and flowering-plant traits as possible tools in breeding for tolerance to water stress at tasseling was evaluated through an analysis of their associations with relative yield. This analysis also throws some light on the possible origins of cultivar differences in response to water stress.
MATERIALS AND METHODS
Traits measured in seedlings
Seedling culture Seedlings of the set of thirteen maize genotypes examined (Table I) were
grown in flats {surface area 735 cm 2, substrate volume 7 1) containing a mix-
43
ture of equal parts of soil, compost and sand. Flats were placed in a glass- house fitted with heating and cooling (nominal day/night temperature re- gime 28 + 2°C/18 + 2°C) and received supplementary light (one mercury vapour HPLR-400 W lamp per 3 m 2 of bench space). Most of the growth of the seedlings took place in September and the period of supplementary light (12 h per day) was centred on the natural photoperiod (mean value 11.9 h per day). Flats were watered with nutrient solution twice weekly and with tapwater as necessary, and re-randomized on the glasshouse bench once a week. Twenty plants of a single genotype, arranged in four rows, grew in each fiat, with two fiats per genotype. Traits were determined using the cen- tral portion of the most recently expanded leaves taken from plants bearing eight to nine visible leaves and which grew in the central part of each fiat.
TABLE I
Brief description of, and coding for, maize cultivars examined
Inbred lines Code number Orange flint L870 1 Orange flint BC 100 3 Orange flint P 465 6 Orange flint P 341 11
Open pollinated, unselected populat ion Orange semident Poblacidn Salitrosa
Open pollinated, synthetic populat ion Yellow dent BSSS
Single hybrids Orange flint Dekalb 2FI0 I0 Sweet corn San Pedro II 12
Double hybrid Orange flint Dekaib 4F34 13
Measurement o f traits
(a) Chlorophyll loss under thermal stress Chlorophyll content of 1.5 cm diameter discs was determined using an
ISCO-SR spectroradiometer (Instrumentation Specialties Company, Lincoln, NE) to measure the apparent optical densities of the disc at 675 and 750 nm
44
(S~nchez et al., 1983). Discs were subjected to heat stress by floating them in distilled water held at 65°C during 80 rain. This interval was sufficient to produce an important drop in chlorophyll content while ensuring that the rate of loss of chlorophyll remained approximately linear. Determinations were made in octuplicate, on samples taken from four plants per flat.
Chlorophyll content of the discs was measured immediately after excision from the plant and again after exposure to thermal stress. Chlorophyll loss was expressed as a propor t ion of the initial value.
(b ) Resistance to loss of intracellular electrolytes Loss of intracellular electrolytes in response to thermal and desiccation
stresses (Blum and Ebercon, 1976, 1981) was measured using conductimetry to estabhsh, in stressed and non-stressed leaf discs, the proportion of the to- tal electrolytes present which diffused into the bathing solution when discs were held for 22 h at 10°C. Total content of electrolytes in the disc was determined by heating the discs in the bathing solution to 100°C for 20 min, cooling, correcting the volume and measuring the conductivity of the re- sulting solution.
Relative loss of electrolytes in response to stress was estimated using the equation (Blum and Ebercon, 1981):
where Ci and Cf represent conductivities measured before and after heating the disc to 100°C, respectively, and the superscripts c and s refer to control and stressed discs, respectively.
The loss of intracellular electrolytes under thermal stress was determined by subjecting replicated leaf discs to heat stress by floating them on distilled water and holding them at 47, 50, 52 or 55°C during 30 rain. Control discs remained in distilled water during this interval. Values for relative loss at each temperature were determined in triplicate. Relative loss in all genotypes went from low (ca. 20%) to high (ca. 80%) over a very narrow temperature range (ca. 3.3°C), following an S-shaped curve. The use of a single temper- ature for evaluating all genotypes (e.g. Blum and Ebercon, 1981) would have imphed a variable genotype separation depending on the temperature se- lected. To avoid this effect, curves were fitted by hand to data for each geno- type and the temperature causing a relative loss of 50% estimated from these curves.
Loss of electrolytes under desiccation stress was determined in discs har- vested from pre-washed leaves after they had been subjected to desiccation for 30, 34 or 40 h in a controlled environment cabinet under 50 W m -2 (400--700 nm), 25°C and 60.5+ 1.5% relative humidity. Determinations were made in quintuplicate. Rate of increase in relative loss was linear with time, so results were expressed as relative loss after 40 h desiccation, as this inter- val gave the greatest genotype separation. Differences were found between genotypes in the rate of fall of leaf water potential, (determined using a
45
Scholander pressure bomb, PMS Model 600, PMS Instrument Company, Corvallis, OR), but water potential after 18.5 h desiccation (range -2.64 to -4.0 MPa, LSD = 0.92 MPa (P = 0.05)) showed no correlation with the rela- tive loss produced by any of the desiccation times used (r 2 range from 0.04 to 0.22).
(c) Capacity to accumulate proline under desiccation stress Excised laminae were stressed by desiccation (Pint6r et al., 1978, 1979)
rather than by stressing rooted plants until they wilted, in view of the water potential and proline accumulation gradients which can form along the laminae when plants are subjected to the latter treatment (Hanson et al., 1977). Neither proline nor water potential gradients developed in excised maize leaves for at least 30 h after the start of desiccation. Laminae from five plants per treatment were desiccated under the conditions described above for 29 h and free proline determined on a sample of five 2.5-cm diam- eter discs taken from the central portion of one side of the laminae. An equal number of discs were taken from the opposite side of the laminae for dry weight determinations. Leaf discs were extracted in hot (80°C) ethanol, and free proline in the extract determined using the technique described in Setter and Greenway (1979), proline levels being expressed on a dry weight basis. Proline levels in non-stressed laminae of all genotypes were negligible, so capacity to accumulate proline is expressed as the proline concentration at the end of the desiccation period. The correlation b e t w ~ n proline levels and leaf water potential measured after 18.5 h of desiccation was weak (r 2 = 0.15), non-significant, and opposite in sign to that expected if proline level were related to rate of desiccation.
Yield and other responses o f plants stressed at flowering
Plant culture During the summer of 1979--80 plants of cultivars 1, 4, 5, 6, 8, 9 and 10
(cf. Table I) were grown in sand-nutrient solution culture under field con- ditions as described by Hall et al. (1980). Briefly, a single plant grew in each 40-1 container. The experimental crop had a density of 3.79 plants m- : and was surrounded by two guard rows. Plants in these rows also grew in sand- nutrient solution culture. All tassels and ears were bagged before pollen-shed- ding and silking, respectively. Pollen was harvested daily aad used to hand- pollinate ears, pollination being restricted within each ger:otype and treat~ ment. Daily observations of the progress of flowering in male and female in- florescences were made and stress was imposed by withholding water from the appropriate containers as each genotype reached 50% tasseling (apical portion of the tassel just visible). Suspension of watering continued until each plant showed signs of wilting when examined at 08.30 h. Thereafter each plant was watered daily with a volume of nutrient solution equivalent to one-quarter of the mean (all cultivars) water consumption of non-stressed
46
plants over the previous day, as determined by the changes in the volume of nutrient solution in the holding tanks. This watering regime was maintained until 75% or more of the stressed plants showed signs of wilting when exam- ined at 16.00 h, when normal watering was resumed. While plants were ex- posed to stress the experimental plot was sheltered from rain by a polyethyl- ene-film roof. This reduced incident illuminance (measured using a selenium photocell, Weston Instruments, Newark, NJ) by 18%. During the period plants were being subjected to stress, pressure bomb measurements of leaf water potential were made on three plants per occasion using random sam- ples taken from the three uppermost expanded leaves. Measurements were made only once a day to restrict loss of leaf area. Sampling for leaf water potential was carried out at nightfall (ca. 20.30 h), as it has been shown that the difference in water potential between treated and control plants mea- sured at this time of day is a more sensitive indicator of the onset of stress than that determined in the morning or at mid, lay (Hall et al., 1980). The period during which the treated plants were assumed to be under stress was taken as the interval between the day on which leaf water potential values of stressed plants began to diverge from control values and the day following resumption of normal watering. This normally coincided with the period in which some proportion of the treated plants showed signs of wilting when examined at 16.00 h. Figure 1 shows the changes in leaf water potential and wilting for cv. 8; these are typical of the pattern exhibited by the remaining genotypes. Because of differences in developmental rate between genotypes, the initiation of the corresponding intervals without normal watering was spread over 14 days. There were variations in the values of climatic variables
I I I
'7 9 1'1
0 , - J
m I,-,-
z -0.5 U J
O
a . o
n-~ -1.0 W
?~ - 1.5 LL
W
~ -2,0 I I I l l
1'3 lj5 1'7 19 21 DATE (day in January }
,oo a
7s ¢-,
50 "~
25 z
Fig. 1. Typical pattern of changes, during the period normal watering routine was sus- pended, for the nightfall leaf water potential in control (e) and stressed (o) plants, and the proportion of stressed plants showing signs of wilting on one or more morning (~) and mid-afternoon (A) observations. The data correspond to cv. 8, arrows marked S and R in- dicate days watering was suspended and plants were returned to normal watering routine, respectively. The vertical bar indicates least significant difference (P ffi 0.05) for water po- tential values.
47
(measured at a site 300 m from the experimental plot) likely to affect atmo- spheric demand for water vapour during the 22<lay interval in which the dif- ferent cultivars were stressed (Fig. 2). However when mean values of these variables were computed for the stress periods corresponding to each geno- type and then compared, there were no significant genotype-related differ- ences between means for any variable. With the exception of cv. 1, which reached 70% silking at the start of the stress period (Table II), the develop- mental stage reached by the remaining genotypes at the start and end of the stress period was reasonably alike. Mean leaf water potential during the stress period differed between some of the genotypes; and the duration of the stress period was 7 or 8 days for all cultivars except cv. 6, which was sub- jected to stress for 10 days (Table II).
"' 5.0
u~ u~ ~ 4.0 hJ o n,n n.~
3.0
2.o
z '~ 1.o w
0 w
= B
A o ,'%, o, ,P'°'o P~ °'° o-o' \ / °'o ; 'o ' %' ~ :
9 -6101
J~ I I I I S l I l I I l l l [
t t t t t t /, 8 10 I - 6 l i
lOO
8o ~ z T
60 tn z
40 w
I, , ,- 20 ~
w n~
o
,o, ~o.O " b. o
4 8 12 16 20 2'4 2'8 DATE {day in January)
F i g . 2 . A . Mean vapour pressure deficit for the 09,00 to 17.00 h interval (e) and relative sunshine (o) for the period during which the various genotypes were subjected to stress. Total radiant flux density measured outside the shelter at 1 3 . 3 0 h on January 21 was 9 9 0 W m -~. B. Daily temperature maxima (o) and minima (e). Numbered arrows over bar labelled S indicate dates of suspension of watering for the corresponding genotypes, while arrows over bar labelled R indicate dates of return to normal watering routine.
Measurements and observations
Plants were harvested at black layer maturity. Aerial biomass, kernel num- ber per ear, kernel weight and number of visible spikelets per ear were deter- mined after plant parts had been dried at 70°C to constant weight. Relative yield was taken to be the ratio between mean grain yields of stressed and control plants of each genotype, and relative spikelet number of the upper-
GO
TABLE II
Stage of development of stressed plants at the start (S) and end (E) of the stress period, mean nightfall leaf water potential and duration of the stress period
Plants with silks on apical ears (%) 70 100 0 0 0 0
Mean nightfall leaf water potential -1 .30 -1 .59 -1 .55 (MPa) a
6 8 9 10 S E S E S E S E
100 100 100 100 100 100 100 100
60 100 10 100 10 100 40 100
0 40 0 30 0 30 0 10
-1 .40 -1 .41 -1 .60 -1 .19
Duration of stress period (days) 8 7 7 10 8 8 8
aMean value for controls of all cultivars: -0 .62 MPa; least significant (P = 0.05) difference between culti- vats: 0.28 MPa.
49
most ear as the corresponding ratio between mean numbers of visible spike- lets. Leaf area per plant was estimated from periodic measurements of lamina length (Ginzo, 1969), and the loss of leaf area during the stress period estimated from determinations made on stressed and control plants immedi- ately after recovery from stress. Chlorophyll levels in the uppermost leaves of control and stressed plants were determined at the end of the stress period for each genotype. For this purpose discs were harvested from six plants per treatment and chlorophyll measured using the spectroradiometer technique mentioned above.
Statistical analysis
Standard analysis of variance techniques were used. Square root transfor- mations were applied, where necessary, to ensure homogeneity of variances for variables measured in flowering plants (Sokal and Rohlf, 1969). When significance of differences between cultivars and associations between traits were tested and a variable involved was expressed in relative terms, angular transformations were performed (Sokal and Rohlf, 1969).
RESULTS
Significant differences (P = 0.05) between extreme genotypes were found (or, in the case of loss of electrolytes under thermal stress, could be inferred) for all the traits measured in seedlings (Table III). Least significant differ- ences, for traits for which these could be computed, represented one-half or more of the difference between extreme genotypes, suggesting that inter- genotypic variability was not large with respect to experimental error. The ranking of the cultivars obtained using the four traits did not coincide and some complete inversions of rank-order were found (e.g. cv. 8 for loss of electrolytes under thermal and desiccation stresses; cv. 3 for chlorophyll losb and capacity to accumulate proline). Pairwise linear correlation between characteristics showed that the association between traits was always very weak and non-significant (Table IV), suggesting that they were mutually in- dependent.
Timing of stress in relation to development is important in determining which yield<letermining variables are affected (Hall et al., 1981). As cv. 1 was stressed at an earlier developmental stage than the remaining cultivars, data for these variables which correspond to cv. 1 are not considered here. Aerial biomass of non-stressed plants varied by a factor of about 2.5 from lowest (cv. 6, an inbred line) to the highest (cvs 4 and 5, two open-polli- nated varieties and cv. 10, a single hybrid) (Table V). Variations among cul- tivars in grain yield of non-stressed plants was greater; there was a five-fold increase from the lowest (cv. 6) to the highest (cv. 10) (Table V). Inspection of the inter-genotypic range for kernel weight and kernel number shows that variation in the latter was the most important factor determining cultivar
50
TABLE HI
Intergenotypic variability for traits measured in seedlings. Cultivars ranked in increasing order of putative tolerance to water stress
LSD (P = 0.05) 7.2 19.7 -- 2.89 Estimated SE a - - - - 0 . 6 - -
aEstimated standard error: estimated from mean standard error of arcsin(p)l/2relative in- jury, transformed to °C at the mean slope of the central portion of the relative injury vs. temperature curve. Traits are coded as follows: CL, chlorophyll loss; LEDS, loss of electrolytes under desic- cation stress; LETS, loss of electrolytes under thermal stress; CAP, capacity to accumu- late proline. For cultivar coding see Table I.
TABLE IV
Correlation coefficients for associations between seedling traits
Traits coded as follows: CL, chlorophyll loss; LETS, loss of electrolytes under thermal stress; LEDS, loss of electrolytes under desiccation stress; CAP, capacity to accumulate proline.
d i f f e r ences in y ie ld . I n u n s t r e s s e d p l a n t s o f cvs 4 a n d 10 t he s e c o n d ear
m a d e a p p r e c i a b l e c o n t r i b u t i o n s t o t o t a l g ra in y i e l d ; in t h e r e m a i n i n g cu l t i - vars, th i s c o n t r i b u t i o n was u n i m p o r t a n t ( T a b l e V).
TABLE V
Variat ion among cultivars o f yield, its c o m p o n e n t s and some yie ld-re la ted pa rame te r s
Cultivar 4 5 6 8 9 10
Aerial b iomass (g p lant -1) 492 ± 31 Grain yield (g p lan t -1) 157 ± 23 Kernel weight (mg kernel -I) 171 ± 11 Kernel number (kernels p lan t -~) 943 ± 160 P ropor t ion of kernels in s econd ear (%) 27.1 Visible sp ike le t s /uppermos t ear (no . ) 921 ± 50 Kerne l s /uppe rmos t ear (no . ) 687 ± 85 Dura t ion o f exposure to pol len o f u p p e r m o s t ear (days) 6.4 ± 0.58
For cultivar cod ing see Table I. Abso lu te values are means and s t andard errors.
5.7 + 0.42
Cu
52
Dry weights of the aerial portion and grain yield of stressed plants of all genotypes were significantly (P = 0.05) reduced, with grain yield showing the largest variations among genotypes in relation to the corresponding control values (Table VI). Mean kernel weight was practically unaffected by stress, and in stressed plants of all cultivars almost all the kernels came from upper- most ears. Kernel number of uppermost ears was reduced by stress in all genotypes, significantly so for cvs. 5, 6, 8 and 10. Spikelet number of the uppermost ear of cultivars 6 and 9 was significantly (P = 0.05) reduced by stress (Table VI), but this reduction did not limit grain yield as grain set (kernels per visible spikelet, data not shown) was reduced proportionally more than relative spikelet number in all cultivars.
Water stress reduced the availability of pollen for fertilization of the up- permost ears, the duration of exposure to pollen being significantly (P = 0.05) reduced for cultivars 5, 6, 8 and 10 (Table VI). Cultivars exhibiting large re- ductions in duration of exposure to pollen of the uppermost ear also showed large reductions in kernel number per uppermost ear and hence, yield per plant (Table VI).
TABLE VI
Variations between genotypes of relative effects of stress on yield, its components and some yield-related parameters. Values are expressed as % of control values
alnsufficient data in this category. * Value for stressed plants differs significantly (P ffi 0.05) from control. For cultivar coding see Table I.
Exposure to water stress brought about reductions in leaf area and chloro- phyll content of all genotypes (Table VII). While leaf area was significantly reduced with respect to controls in all cultivars, the reduction in chlorophyll content proved to be non-significant (P = 0.05) for cvs. 5 and 9. This sug- gests there may be inter-genotypic differences in the magnitude of this re- sponse to water stress.
53
TABLE VII
Reductions (expressed as % o f control value) in leaf chlorophyll content and leaf area produced by exposure to stress at tasseling in seven maize cultivars
Loss of leaf area 33.0* 33.1" 30.5* 22.8* 26.3* 34.1" 19.8"
* Stress values differ significantly (P = 0.05) from controls. For coding of cultivars, see Table I.
DISCUSSION
We e xa m ine d the associat ion b e t w e e n the t rai ts d e t e r m i n e d in seedlings and relat ive yield, o the r y ie ld-re la ted character is t ics and responses to stress whose d e t e r m i n a t i o n is less c o m p l e x than t h a t o f y ie ld (Table VIII) . In all cases the associat ions were weak o r non-ex i s ten t . We conc lude tha t fo r the subset o f ge no types stressed at tasseling, any con t r i bu t ions to the sum of p lant fac tors which de t e rmine to le rance to wa te r stress (Blum, 1979) associ- a ted wi th the seedling trai ts were t o o small to be de t ec t ed . O t h e r au thors , work ing wi th maize , have f o u n d associat ions b e tw een some o f these trai ts and yield or o t h e r responses to stress {e.g. leaf area loss) (Kilen and Andrew, 1969 , Pint6r e t al., 1978) . I t is n o t easy, par t icu lar ly in view o f d i f fe rences o f t e chn ique where appl ica t ion o f stress to ful ly g rown plants is conce rned , to
TABLE VIII
Correlation coefficients for associations between traits measured in seedlings and in flow- ering plants
Flowering plant traits Seedling traits RY RSN RKN DEP LAL* RCL*
* Correlations include data from cv. I ; LAL and RCL were assumed to be independent of timing of stress over the range of developmental stages involved (cf. Table II). Traits are coded as follows: RY, relative yield; RSN, relative spikelet number of upper- most ear; RKN, relative kernel number of uppermost ear; DEP, duration of exposure to pollen of the uppermost ear of stressed plants; LAL, leaf area loss; RCL, reduction in chlorophyll level; CL, chlorophyll loss; LETS, loss of electrolytes under thermal stress; LEDS, loss of electrolytes under desiccation stress; CAP capacity to accumulate proline.
54
discern why these associations were not reproduced in the material we used. Where relative yield and relative kernel number of the uppermost ear are concerned, these were largely determined in the cultivars we used by differ- ences in pollen availability. Grain set on ears increases with duration of ex- posure to pollen, reaching a maximum after about four days (ToUenaar and Daynard, 1978), and this pattern also appears to hold for stressed plants (Hall et al., 1981). There was a significant (P = 0.05) correlation between relative kernel number of the uppermost ear of stressed plants and duration of exposure to pollen (y = 1.10 + l l .2x , r 2 = 0.94, where y = arcsin(p) !/2 relative kernel number, and x = duration of exposure to pollen, assuming durations greater than 4 days to be equal to 4 days). This suggests that availability of pollen was an over-riding factor determining relative yield of the uppermost ear under the conditions of this experiment. Any contribu- tion to stress tolerance by traits examined in seedlings would have been masked by this pollen effect. In less diverse sets of genotypes, and those used by other authors may be such sets, differences in pollen availability between cultivars may have been smaller and relationships with traits measured in seedlings easier to detect. A detailed examination of the origins of cultivar differences in pollen availability for the genotypes used in this work has been published by Hall et al. (1982).
In their exhaustive analysis of associations between plant traits and drought susceptibility in wheat, Fischer and Wood (1979) found that sus- ceptibility (which they define as proportional to (1 - relative yield)) in- creased with potential yield (i.e. yield in absence of drought), and also showed that the grain yield of stressed crops was associated with above- ground biomass of stressed and non-stressed crops. These relationships do no appear to hold for the maize cultivars we subjected to stress at flowering (Table IX). We also found no evidence of association between relative yield and the estimator of plant water status during the stress period which we measured (nightfall leaf water potential). Some of the lowest water potential values (Table II) were found in cultivars with the highest relative yield (Table VI). This result is consistent with those of Fischer and Wood (1979, cf. other authors cited therein).
TABLE IX
Correlation coefficients between relative yield (RY) and stressed plant grain yield (YS) and some traits measured at flowering or at harvest
PY TDWC TDWS PWS
RY -0 .28 -- -- -0 .84 YS -- 0.09 0.12 --
Traits are coded as follows: PY, potential yield (i.e. yield of non-stressed plants); TDWC, aerial biomass of non-stressed plants; TDWS, aerial biomase of stressed plants; PWS, plant water status during the stress period.
55
Relative spikelet number was not a yield-limiting factor in this experiment because grain set was reduced in a proportionally greater measure by stress. Nevertheless, potential ear size can limit yield when stress occurs before an- thesis and pollen availability is adequate (Hall et al., 1981). Thus mainte- nance of ear size under stress is a potentially useful characteristic. In the sub- set of genotypes stressed at tasseling, pollen availability was not necessarily correlated with maintenance of ear size (i.e. number of spikelets per upper- most ear) (cf. cvs. 9 and 10, Table VI). This suggests that there may be some scope for improving tolerance to water stress through adequate combination of both characteristics.
It has been found that the root weights of non-stressed plants at black layer maturity do not differ from those obtained at tasseling (Trapani and Hall, unpublished). So root weights of control plants at harvest were deter- mined and the leaf area:root weight ratio at tasseling estimated. Relative spikelet number proved to be significantly (P = 0.05) correlated with leaf area:root weight ratio (y = 98.5 - 0.178x, r 2 = 0.67, where y = arcsin(p) 1/2 relative spikelet number, and x = leaf area:root weight in cm 2 g-l). A possi- ble explanation of this association is that leaf area and root weight may be re- lated to plant capacities for transpiration and water uptake, respectively. Dif- ferences in this ratio could bring about variations in the degree of stress to which plants of the various cultivars were subjected, and in turn could dif- ferentially affect development of the uppermost spikelets, which would have still been under way at the stage where plants were stressed in this experi- ment (ToUenaar and Daynard, 1978). Some support for this hypothesis is given by the significant (P = 0.05) correlation between the differences in nightfall water potential between stressed and control plants during the stress period and the leaf area:root weight ratio (r 2 = 0.69). These associa- tions are surprising in view of the complexity of the interactions between determinants of plant-water balance and responses to stress, but they seem sufficiently strong to warrant further attention.
The correlations between the various traits measured and pairs of the re- maining ones were tested using multiple linear regression. It is of interest to note that the regression of relative spikelet number on leaf area:root weight ratio and loss of electrolytes under thermal stress had an r 2 value of 0.93, improving the above mentioned correlation on leaf area:root weight ratio alone. This is consistent with the idea that, although traits measured in seed- lings may be associated with plant responses to stress in other developmental stages, their expression may be strongly conditioned by other traits.
ACKNOWLEDGEMENTS
The work described in this paper was financed by grants from CIC (Comi- sion de Investigaciones Cientificas de la Provincia de Buenos Aires), CONI- CET (Consejo Nacional de Investigaciones Cientificas y T~cnicas and SECYT (Secretaria de Estado de Ciencia y Tecnologla). We thank the staffs of INTA
56
(Pergamino and San Pedro Agricultural Experiment Stations), Dekalb Argentina and Forestal Pergamino for providing the cultivars used in these experiments. A.J.H., F.V. and R.C. de H. are members of the CONICET, and N.T. held a post-graduate scholarship from the CIC.
REFERENCES
Blum, A., 1979. Genetic improvement of drought resistance in crop plants: a case for sorghum. In: H. Mussell and R.C. Staples (Editors), Stress Physiology in Crop Plants. John Wiley, New York, NY, pp. 430--445.
Blum, A. and Ebercon, A., 1976. Genotypic responses in sorghum to drought stress. III. Free proline accumulation and drought resistance. Crop Sci., 16: 428--431.
Blum, A. and Ebercon, A., 1981. Cell membrane stability as a measure of drought and heat tolerance in wheat. Crop Sci., 21: 43--47.
Eberhart, S.A. and Russell, W.A., 1966. Stability parameters for comparing Varieties. Crop Sci., 6 : 36--40.
Finlay, K.W. and Wilkinson, G.N., 1963. The analysis of adaptation in a plant breeding programme, Aust. J. Agric. Res., 14: 742--754.
Fischer, R.A. and Maurer, R., 1978. Drought resistance in spring wheat cultivars. I. Grain yield responses. Aust. J. Agric. Res., 29: 897--912.
Fischer, R.A. and Wood, J.T., 1979. Drought resistance in spring wheat cultivars. HI. Yield associations with morpho-physiological traits. Aust. J. Agric. Res., 30: 1001-- 1020.
Ginzo, H.D., 1969. Medici6n del ~rca foliar de malz (Zea mays) empleando el largo m~ximo de sus hojas. Rev. Fac. Agron. Vet. Buenos Aires, 17: 39--42.
Hall, A.J., Ginzo, H.D., Lemcoff, J.H. and Soriano, A., 1980. Influence of drought during pollen-shedding on flowering, growth and yield of maize. Z. Acker Pflanzenbau, 149: 287--298.
Hall, A.J., Lemcoff, J.H. and Trapani, N., 1981. Water stress before and during flowering in maize and its effects on yield, its components, and their determinants. Maydica, 26: 19--38.
Hall, A.J., Vilella, F., Trapani, N. and Chimenti, C., 1982. The effects of water stress and genotype on the dynamics of pollen-shedding and silking in maize. Field Crops Res., 5: 349--363.
Hanson, A.D. and Nelsen, C.E., 1980. Water: adaptations of crops to drought-prone en- vironments. In: P.S. Carlson (Editor), The Biology of Crop Productivity. Academic Press, New York, NY, pp. 77--152.
Hanson, A.D., Nelsen, C.E. and Everson, E.H., 1977. Evaluation of free proline accumula- tion as an index of drought resistance using two contrasting barley cultivars. Crop Sci., 17: 720-726.
Kaloyereas, S.A., 1958. A new method of determining drought resistance. Plant Physiol., 20: 232--233.
Kilen, T.C. and Andrew, R.H., 1969. Measurement of drought resistance in corn. Agron. J., 61: 669--671.
Pint,r, L., K~m~n, L. and P~lfi, G., 1978. Determination of drought resistance in differ- ent hybrids of maize by field trials and biochemical tests. Maydica, 23: 121--127.
Pint,r, L., K~-n~n, L. and P~lfi, G., 1979. Determination of drought resistance in maize (Zea mays L0) by proline test. Maydica, 24: 155--159.
Rebeila, C.M., Frutos, E., Puig, R. and P~rez, O., 1976. Influencia del stress hidrico sobre los rendimientos del malz en la localidad de Pergamino. IDIA, Suppl. 32: 1 6 0 - 167.
57
Richards, R.A., 1978. Variation within and between species of rapeseed (Brassica campe- str/s and B. napus) in response to drought stress. III. Physiological and physicochem- ical characters. Aust. J. Agric. Res., 29: 491--501.
S~Jachez, R.A., Hall, A.J., Trapani, N. and Cohen de Hunau, R., 1983. Effects of water stress on the chlorophyll content, nitrogen level and photosynthesis of two maize genotypes. Photosynthesis Res., 4: 35--47.
Setter, T.L. and Greenway, H., 1979. Growth and osmoregulation of Chlorella emersonii in NaC1 and neutral oamotica. Aust. J. Plant Physiol., 6: 47--60.
Singh, T.N., Aspinall, D. and Paleg, L.P., 1972. Proline accumulation and varietal adapt- ability to drought in barley: a potential metabolic measure of drought resistance. Nature New Biol., 236: 189--190.
Sokal, R.R. and Rohlf, F.J., 1969. Biometry. W.H. Freeman and Company, San Fran- cisco, CA, 253 pp.
Tollenaar, M. and Daynard, T.B., 1978. Kernel growth and development at two positions on the ear of maize (Zea mays). Can. J. Plant Sci., 58: 189--197.
