Influence of Temperature between Floral Initiation and Flag Leaf Emergence on Grain Number in Wheat

Transcription

1 Influence of Temperature between Floral Initiation and Flag Leaf Emergence on Grain Number in Wheat H. M. ~awson* and A. K. Bagga A Division of Plant Industry, CSIRO, P.O. Box 1600, Canberra City, A.C.T Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi , India. Abstract Plants of Kalyansona, Condor and Janak semidwarf wheats, grown at a density of 130 plants m-2, were transferred every 4 days after floral initiation of the main shoot from temperature regimes of C and 21/16"C to a regime of 15/10 C to determine if specific developmental stages of the ear are particularly important to the establishment of grain number. No stages between the appearance of double ridges and flag leaf emergence were significantly more sensitive to temperature changes than others, there being a progressive reduction in grain number per ear for every day that plants of Kalyansona and Condor remained at a higher temperature. Grain number of the main ear was closely correlated with the amount of dry matter in the stem, in the ear structure (chaff), and in the four uppermost leaves of the main shoot; the partial correlation coefficients demonstrated that leaf weight was best related to grain number. Thus large shoots with heavy ear structures had many grains and vice versa. It can be inferred from these results that the distribution of dry matter to the various plant organs before anthesis is in a strict proportionality irrespective of the availability of assimilates. Introduction Recent papers have demonstrated that temperature during the preanthesis growth of the wheat plant can have a major effect on the number of grains set per ear, aside from any effects on spikelet number (Thorne et al. 1968; Fischer and Maurer 1976; Bagga and Rawson 1977; Warrington et al. 1977). And as grain number per unit ground area is perhaps the most powerful yield determinant in wheat (Aguilar and Fischer 1975; Evans 1978), it is important to identify precisely the periods before anthesis when grain number is particularly sensitive to temperature variation. For example, are the periods of jointing and fertile floret production, nominated as critical for yield determination by Hudson (1934), as sensitive to temperature as they are to other environmental factors? Perhaps of greater importance is to establish whether differences in sensitivity exist between cultivars. Warrington et al. (1977) found that plants of Gamenya wheat grown at 25 C during the period from double ridges to anthesis had only 30 grains in the main ear, whereas plants grown for this developmental period at 15'C had approximately 70 grains. Bagga and Rawson (1977), considering the later phenological stage from terminal spikelet appearance to 7 days after anthesis, found that the same range of day temperatures resulted in grain numbers of 20 and 50 in the cultivar Condor, and a constant grain number of 50 in Kalyansona. Fischer and Maurer (1976), in experiments with heated or cooled canopies of wheat, showed that effects on grain numbers were confined to the period between the appearance of the terminal spikelet and the emergence of the flag leaf. Evans (1978), discussing responses to irradiance, concluded

2 that grain number per ear was affected most during the period from 15 days before to 5 days after anthesis, but that the period from floral initiation to 20 days later was also sensitive. Consequently, it seems that grain number can be affected throughout the period from floral initiation to anthesis. In the current study, the period considered was from floral initiation, when plants had four or five leaves emerged, to 4 weeks later. To pinpoint critical stages, we moved batches of plants every 4 days during this period from one temperature regimen to another. In the main experiment the transfers were from higher to lower temperatures so that any preanthesis effects would not be masked by high-temperature sterilization of grain sites at anthesis. The main shoot alone was considered at a maturity harvest for each of three cultivars, Kalyansona, Condor and Janak, used in cur earlier study (Bagga and Rawson i977). Materials and Methods Growing Conditions Seeds of three cultivars of wheat (Tritium aestivum), Kalyansona, Condor and Janak, were sown on 5 May 1976 in plastic pots 5 by 15 cm, containing a mixture of perlite and vermiculite. The pots, each containing one plant, were packed at a density of 130 m-2 in trays in glasshouses of the Canberra phytotron (Morse and Evans 1962) where the natural photoperiod was extended to 16 h by the use of low-intensity, incandescent lamps. Plant trays were lined with plastic sheet; this facilitated subirrigation of the pots which ensured a constant supply of water at high temperatures. A modified Hoagland's solution and deionized water were added daily to the trays during the initial growth period, but the nutrient application was reduced to twice weekly in the second month. Twice weekly the trays were flushed with deionized water to prevent any salt accumulation and algae growths. The trays were drained at 1630 h and remained empty overnight. Mean daily short-wave irradiance ( nm) receipt on the phytotron roof, reduced by 20% to provide an estimate of irradiance inside the glasshouses, was , 727t-44, and J cm-2 for May, June and July respectively (J x 0.24 = Cal). Temperature Treatments Day temperatures applied between 0830 h and 1630 h. Until the double ridge stage of the main shoot (our definition of floral initiation), plants were raised under daylnight temperature regimes of 27/22"C, 21/16"C or 15/1O0C, which had daily means of 23.7"C, 17.7"C and ll.7"c, respectively. Every 4 days from floral initiation until 24 days later, batches of 15 plants were moved from C and from 21/16"C to 15/10 C where they remained until harvested at maturity. A further treatment involved the transfer of plants every 4 days from 21/16"C to 27/22OC, but only Janak was used. In a final treatment, plants of all cultivars were moved from C to C at the emergence of the flag leaf and at the emergence of the main ear. These last two treatments will be referred to only briefly. Measurements Plants were dissected daily to ascertain the double ridge stage of the main ear in each temperature regime and thereafter dissections were done every time plants were transferred between temperatures. Observations were made of spikelet number and the stage of development of spikelets and florets. At maturity, plants were harvested and the shoot portion separated into tillers and main shoot. For the latter, weights of grain, ear structure (chaff), top four leaf blades, and stem plus sheath were determined. Grain number of main ears was counted and the distribution of grains within ears was mapped. An estimate of potentially fertile florets (grain sites) within main ears was made at maturity using an arbitrary scale based on the minimum lemma size of florets actually bearing grain at maturity. Grain number expressed as a percentage of grain sites gave percentage fertility for each ear.

3 Results and Discussion Temperature and Phasic Development Development was faster at higher temperatures [Table 1 and see Rahman (1977) and Rawson (1970) for data on other cultivars]. Within each regime, Janak was earliest and Condor latest to any stage of development but the spread across cultivars was small, ranging over less than 1 week for first anthesis. Spikelet numbers in main ears of Kalyansona and Condor were affected by the temperatures prevailing before the double ridge stage: mean spikelet numbers for Janak, Kalyansona and Condor respectively were , 13.7i-0.1 and 14.1 *0-3 for plants raised at C and , and for plants raised at C. The transfer ef plants from m e temperature tc! another after the double ridge stage also affected final spikelet number as found earlier (Fischer and Maurer 1976), but the effects were always small and reached significance only in Condor raised at C. In that cultivar, the plants transferred to 15/10 C immediately after floral initiation had 15.2f 0.4 spikelets, whereas those transferred after the appearance of the terminal spikelet had spikelets per main ear. Thus transfers after floral initiation generally affected spikelet number by less than one per ear, demonstrating that the effects of temperature on spikelet number during this period are small, much smaller than the effects of photoperiod (cf. Rawson 1970, 1971). Table 1. Time from sowing to double ridge, terminal spikelet, flag leaf emergence and anthesis in plants grown throughout in temperatures of 27/22"C, C or C DR, double ridge. TS, terminal spikelet. FE, flag leaf emergence. A, anthesis Time (days) to each stage at temperatures of: C 21/16"C 15/10 C DR TS FE A DR TS FE A DR TS FE A - Janak Kalyansona Condor Phasic Development and Grain Number The later after floral initiation that plants were transferred from higher to lower temperatures, the fewer grains that were set in main ears (Fig. 1). Moreover, the higher the temperature experienced by plants before floral initiation, the lower the grain number. Thus, according to the linear regressions of Fig. 1 (y = x, r2 =O.89 and y = x, r2 =O + 66 for the C and C transfers respectively), when plants were transferred at double ridges from 21 C, they had 58 grains per main ear, whereas when they were transferred from 27 C they had only 42 grains per main ear. This difference of 16 grains per main ear rose to 30 (12 v. 42) if plants were retained at their starting temperature for a further 24 days. Other conclusions arising from Fig. 1 are: first, that Kalyansona and Condor responded similarly in grain number to the treatments applied whereas Janak was different (it failed to set many grains in any treatment); and second, that there were no clearly critical stages in the establishment of grain number although it is noted

4 that the plants transferred immediately after the appearance of the terminal spikelet had lower grain numbers than indicated by the regressions of Fig. 1. Day of transfer after double ridge Fig. 1. Grain number per main ear i twice the s.e.m. for plants of Kalyansona (A), Condor (0) and Janak (0) transferred from C to 15/10"C (open symbols) and from 21/16"C to 15/1OoC (closed symbols) at different times after the rrppearmce sf double ridges. Times of terminal spikelet (TS) and flag leaf emergence (FE) for plants at 27/22'C and 21/16"C are denoted by arrows. The linear regressions are fitted through Kalyansona and Condor; Janak is excluded. Temperature, Light and Grain Number The longer that a plant spent at lower temperatures, the longer was the interval between sowing and anthesis (e.g. Table 1). Grain number per main ear (NG) was positively related to days to anthesis (TA) in the fashion Although the correlation was moderately good (r2 =0.58), there are weaknesses in using time alone to predict plant responses; factors such as light receipt may only marginally affect duration yet do have large effects on growth. However, we found that grain number was scarcely better related to cumulative radiation receipt between sowing and anthesis (r2 =0.61) than it was to the time interval itself. When the time interval was expressed as cumulative day degrees above a base of 7"C, a significant improvement occurred in reliability of the prediction of grain number for the transfer treatments of Kalyansona and Condor (Fig. 2, r2 =O.88, y = 151 a ~). The regression of Fig. 2a indicates that the fewer day degrees above 7 C that are accumulated, the more grains that are set in the ear. While the regression of Fig. 2a is useful for descriptive purposes, it has limited physiological meaning because heat sums consider only the processes linked with the rates at which processes proceed and with their duration. The photothermal quotient of Nix (1976), which expresses the 'light energy available per unit of development time', is a more useful tool, particularly for the simulation of crop growth. Roughly this quotient provides a ratio between the potential supply of assimilates (light receipt) and the potential requirement for assimilates by the plant (mean temperature). In our context this is the sum of total shortwave radiationlsum day degrees greater than 7 C between sowing and anthesis. Fig. 2b shows that grain number was well correlated with this quotient (r2 =0.88) and that grain number increased as the utilization of potential source availability per unit of potential source also increased.

5 Grain Number and the Weight of the Stem and Leaves If grain number is dependent on the ratio between potential source availability and potential source utilization, then it might be expected that the size and number of other organs would be similarly dependent. Using a multiple linear regression analysis of leaf weight (W,), stem weight (W,) and grain number (NG) of the main shoot of all Kalyansona and Condor transfer treatments, the following relationship was found: NG= l+O.O22 Ws WL, with a coefficient of determination, r2 =O.90. The individual linear regressions which demonstrate that the three components were closely related are as follows: An analysis of the partial correlations showed that, when stem weight is held constant the correlation between leaf weight and grain number is highly significant (P< O.001), I 8 Fig. 2. Grain number per main ear plotted against (a) day degrees greater 2 than 7 C summed from sowing to lo.s 0 anthesis and (b) against Cal ~m-~/surn % A day degrees greater than 7 C for Sum day degrees > 7'C (sowing to anthesis), Kalyansona (A) and Condor (0) transferred from C to 15/10 C /. (open symbols) and from C to e A 15/1OoC (closed symbols) at different times after the appearance of double 40 ridges. 30, 1 :\ I Cal degree day-' > 7" (sowing to anthesrs) whereas the other relationships were not significant. Therefore, it seems that the primary effect of a large ratio between source availability and source utilization is on the size of the upper four leaves of the main shoot which in turn affects the number of grains set. Within these experiments, one more grain was added to an ear for every 8 mg of leaf weight, above a threshold value, that was added to the main shoot (Fig. 3a).

6 Grain number and lamina plus sheath weight were also closely related in the study of Warrington et al. (1977, and unpublished data), in which wheat plants were exposed to a range of temperatures before anthesis. This relationship (Fig. 3b, y = $ n(x-800), r2=0.84) is logarithmic presumably because the plant puts less material into sheath than into lamina as the ratio between source availability and utilization increases. Fig. 4 g4 70 Main shoot leaf weight (mg) /* I000 I Main shoot weight excluding grain (mg) 'k'gbo loh,;00 I& I& I& I& I& I& Main shoot lamina plus sheath weight at 10 days afteranthesis (mg) 0LI - lo(l Weight main ear structure (ms) Fig. 3. Relationship between grain number per main ear and either (a) weight of the four uppermost leaves on the main shoot or (b) weight of the lamina and sheath of the four uppermost leaves. In Fig. 3a, Kalyansona (AA) and Condor (I,@) treatments are as depicted in Figs 1 and 2 but additional transfer treatments are included (+, x). The data for Fig. 36 are from Warrington et al. (1977), each point being the mean of three grain-phase treatments. Fig. 4. Relationship between the weight of the main ear structure (chaff) and (a) the non-grain parts of the main shoot, (b) the grain number of the main ear, for Kalyansona (A) and Condor (0) transferred from C to 15/10 C (open symbols) and from 21/16"C to 15/10 C (closed symbo's) at various times after the appearance of double ridges. Other transfer treatments (x, t) of these two cultivars are also included. Janak (0) appears in Fig. 4a. The three regressions in Fig. 4b refer to the data presented (solid line), to the data of Bagga and Rawson (1977, dotted line) and to the maturity harvest of Warrington et al. (1977, dashed line). Grain Number, Size of Ear Structure and Fertility of Grain Sites The conclusion that the plant organs grow bigger as assimilates become relatively more available, applies also to the ear structure (chaff weight). Thus chaff weight was greater in the heavier plants (Fig. 4a) and the largest ears had the most grains (Fig. 4b).

7 The regressions between chaff weight and grain number for earlier studies are included in Fig. 4b to illustrate the generality of this type of relationship. In the current study where y = x, r2 =O.97, 1 grain was equivalent to 6 8 mg of chaff. For the temperature studies of Bagga and Rawson (1977) and Warrington et al. (1977), 1 grain was equivalent to 7.2 mg and 7.8 mg chaff, respectively. The coefficients of determination for the 1977 studies were less good (r2 =0.79 and 0-60) than in the current study because a range of temperatures applied during the period of grain filling; chaff weight but not grain number would have been affected by the postanthesis treatments. From the close correlations between the weights of the various plant organs at maturity, it may be inferred that during the growth of the ear structure the assimilates are apportioned to the organs in a fixed pattern irrespective of ilie relative availabi!itj: of the assimilates. Consequently, within a particular genotype any competition for assimilates between organs is unaffected by relative assimilate availability [a conclusion also reached by Fischer and Stockman (unpublished data) from studies of shaded wheat plants]. We can also conclude that for a particular amount of assimilate available for the growth of the ear structure, a particular number of grains will be established: florets (grain sites, see Materials and Methods) are fertile if they are 'big enough' (Fig. 5a). An interesting observation was that the fertility of grain sites increased with the number of grain sites per ear and that Condor grain sites were more fertile than those of Kalyansona (Fig. 5b). Fig. 5. Relationship between the number of grain sites in the main ear of Kalyansona (A) and Condor (0) and (a) the weight of the structure of the main ear (chaff), (b) percentage grain site fertility. The symbols are as used in previous figures. No. of grain sites in main ear To return to Fig. 4b, it could be argued that the weight of the ear structure at maturity is a poor indicator of its weight at the time when the grains are set. If the presence of grains in the ear causes the ear to grow, then the ear structures which were

8 heaviest at maturity were so because they had set more grains: florets at maturity were 'big enough' because they were fertile. In order to test this hypothesis, we used the data of Warrington et al. (1977) to arrive at the following conclusions. Where temperature was high (25 C) during grain growth, the ear structure increased in weight by less than 20% of its weight at anthesis and there was no relation between grain number and growth of the ear structure. Where temperature was low (15 C) during grain growth, the increase in weight of the ear structure was greater if more grains were set (r2 =0.46 for the linear regression). However, the increase in weight of the ear structure was in proportion to the increase in shoot weight during grain filling (r2 =0-65). And as most grains were set by the largest plants, it is clear that the growth of the ear structure during grain filling was primarily related to the availability of assimilates in the whoie piant. Grain Number and Grain Size The temperature transfer treatments generated a wide range of grain numbers in main ears but there was no compensation in kernel weight even though all plants were retained at 15/10 C from before anthesis until maturity. Almost all grains weighed between 36 and 42 mg (Fig. 6). This is appreciably less than the potential kernel weight for Condor of greater than 70 mg (Bremner and Rawson 1978), and for Kalyansona of greater than 60 mg (Rawson et al. 1976), but realistic for field-grown plants (e.g. Rawson and Ruwali 1972). The lack of compensation, which was also Fig. 6. The poor relationship between grain number and weight per grain for the various temperature transfer treatments of Kalyansona and Condor. The symbols are as used in previous figures Weight per grain (mg) I I noted in the studies of Warrington et al. (1977), may indicate that it is temperature during the cell division phase which determines the potential kernel weight (all plants in Fig. 6 were exposed to 15/10 C at this time). Wardlaw (1970), however, observed that endosperm cell number per kernel was unaffected by temperature. His results from shading treatments suggest that substrate availability can be important in determining endosperm cell number per kernel. In the current studies, grain number per ear was least in the smallest plants and most in the largest plants, with the consequence that substrate availability per kernel would have been similar; the adjustment in kernel number maintained kernel weight. Presumably it is only when source and sink are out of balance due to severe natural events or to experimental manipulations

9 such as floret sterilization (e.g. Rawson and Evans 1970), and shading and flag leaf removal (e.g. Konovalov 1966; Fischer and HilleRisLambers 1978), that kernel size will change appreciably within a genotype. Temperature and Ear Number Hudson (1934) concluded, from experiments done in 1925 and after an extensive review of the literature, that the wheat plant has two phases which are critical to yield. The first was during jointing, when the number of tillers which reach maturity is being determined, and the second was when the number of fertile florets is established. The plants in our experiment were jointing during the latter half of the study. There were no significant differences in fertile tiller number between times of temperature transfers, the mean tiller ear number for plants moved from 27 to 15 C being 2.9i for Kalyansona and 2.44 for Condor. Ear numbers for the C transfer plants were 3.57 and 3-79 respectively. Thus while temperatures before floral initiation did affect fertile tiller number in this study (and in that of Fischer and Maurer 1976), those after did not. The result from our temperature treatments contrasts with results from the irradiance treatments of Evans (1978). He concluded that fertile tiller number is markedly affected by irradiance receipt during the first 20 days after floral initiation. This conclusion is verified in a comparison of our current winter study with one done in summer using the same cultivars and similar day temperatures (Bagga and Rawson 1977). Plants in the summer study had six to eight ears while those in the winter study had only three or four. Perhaps Hudson's (1934) conclusion about the importance of jointing did not relate to temperature. The results discussed here demonstrate that irradiance during the growth of the wheat plant has its primary effect on yield through fertile shoot numbers and that this component, while being sensitive up to anthesis (Fischer 1975), showed greater plasticity during the 3 weeks after floral initiation (Rawson 1967; Evans 1978). In contrast, temperature has small effects on fertile shoot number and these effects are evident only prior to floral initiation. The main effect of temperature is on grain number per ear, plasticity in this yield component spanning a period from some time before floral initiation (when spikelet number is affected) until at least flag leaf emergence, a rather longer period than proposed by Fischer and Maurer (1976). Comparisons between the present study and that of Bagga and Rawson (1977) indicate that grain number of different cultivars may respond differently to the balance between temperature and irradiance. Thus in Kalyansona grown under high irradiance (many fertile shoots), grain number per main ear was relatively unresponsive to temperature changes, whereas when Kalyansona was raised under low irradiance (few shoots), it responded to temperature in a similar fashion to Condor. Acknowledgments We are indebted to Ian Warrington and Bob Dunstone for comments on the manuscript and for allowing us to use their unpublished data; to Henry Nix, Tony Fischer and Greg Constable for helpful discussion of our results; and to the Colombo Plan Training Scheme for supporting the second author while in Australia. References Aguilar, I., and Fischer, R. A. (1975). Analisis de crecimiento y rendimiento de 30 genotipos de trigo bajo condiciones ambientales optimas de cultivo. Agvociencia (Mexico) 21,

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