IMPROVING WHEAT GRAIN FILLING UNDER STRESS BY STEM RESERVE UTILIZATION

(Reference: Blum, A. (1998). Improving wheat grain filling under stress by stem reserve mobilization. Euphytica 100: 77–83.)

In most wheat growing regions and especially in the Mediterranean climate grain filling is subjected to several physical and biotic stresses. Grain filling often occurs when temperatures are increasing and moisture supply is decreasing. Foliar disease of wheat also tend to spread and intensify towards and after flowering. Leaf rust, stripe rust and Septoria leaf blotch can result in total leaf destruction at grain filling. The common end-result of all these stresses is kernel shriveling, reduced test weight and loss in yield.

Current assimilation as a source of carbon for grain filling depends on the light intercepting viable green surfaces of the plant after anthesis. This source is normally diminishing due to natural senescence and the effect of various stresses. At the same time the demand by the growing kernel is increasing, in addition to the demand posed by maintenance respiration of the live plant biomass.

Hence,  an important source of carbon for grain filling is stem reserve. Even under mild conditions, current assimilates may be limited for normal grain filling. In a three years study in Connecticut it was estimated (Gent, 1994) that canopy respiration and grain dry matter accumulation were approximately equal sinks for photosynthate and, together, were greater than canopy photosynthesis late in grain filling. Thus stem reserves were essential for completed grain filling.

While root storage is important in some legumes and other species, there is no evidence that root or leaves are as important as stems for reserve storage in the small grains. In most studies of stem reserves in the small grains stems include also the leaf sheaths, which in themselves contain reserves. Small grains stems store carbohydrates in the form of glucose, fructose, sucrose and starch, but the main reserve is fructan (e.g. Lopatecki et al., 1962; Dubois et al., 1990; Wardlaw and Willenbrink, 1994). Storage is commonly analyzed as total non-structural carbohydrates (TNC) or water-soluble carbohydrates (WSC).

The first step, and probably the most important one in fructan synthesis is catalyzed by the enzyme Sucrose:sucrose fructosyltransferase (SST). SST activity seems to be related to substrate (sucrose) concentration, which in itself may be affected by sucrose synthase activity in the stem (Wardlaw and Willenbrink, 1994). Fructan accumulation was greater when sucrose was high in the penultimate internode of wheat (Dubois et al., 1990). Wardlaw and Willenbrink (1994) found that during the accumulation of WSC in wheat stems, the mass of fructan with a DP (degree of polymerization) greater than 5 continued to increase whilst the mass of fructans of DP 3 to 5 reached a maximum and then remained constant. Fructan accumulated in internodes while they were extending although most of the fructan in an internode accumulated after it was fully extended. When WSC were mobilized from the stem, the mass of glucose, sucrose and fructan decreased but the mass of fructose first increased then decreased, indicating that fructan was hydrolyzed at a faster rate than its product (Bonnett and Incoll, 1992b)

Starch is found in small amounts in wheat stems but it is not mobilized, as evidenced from shading experiments (Kiniry, 1993).

RESERVE ACCUMULATION

Reserve accumulation in the stem and the size of the storage strongly depend on the growing conditions before anthesis. Total stem TNC at anthesis was shown to vary from 50 to 350 g kg-1 dry mass in different experiments (see review by Kiniry 1993).

Under optimal growing conditions with regard to temperature, water regime (Davidson and Chevalier, 1992) and mineral nutrition (Papakosta Gagianas, 1991), carbon assimilation rates are high and a proportion of the assimilates is allocated to storage. When carbon assimilation during stem elongation is reduced by stress, storage in stems is reduced. For example, remobilized WSC were 641 mg and 1047 mg in water-stressed and irrigated wheat, respectively because the former had less storage than the latter (Davidson and Chevalier, 1992). Under dryland field conditions only half the amount of water soluble carbohydrates was available for remobilization during grain filling, as compared with irrigated conditions. When ambient CO₂ concentration was raised to increase assimilation, more carbon was stored in the stems (Winzeler et al. 1989).

Developmentally, potential stem storage as a sink will be determined by stem length and stem weight density. Stem weight density is stem dry weight per unit stem length. Storage and its availability for remobilization may vary along the stem.  In winter barley the basal internodes were found to be contributing the most to grain filling (Bonnett and Incoll, 1992a). However, other studies with barley showed that the peduncle and penultimate internode (and leaf sheath) had the most storage (Daniels and Alcock, 1982). Work with wheat indeed confirmed that the peduncle and the penultimate internode had the most storage (Wardlaw and Willenbrink, 1994) and that variations in storage and remobilization under different experimental conditions were larger in the penultimate than in the fourth stem internode (Bonnett and Incoll, 1992a). Various aspects of stem anatomy with respect to storage were not thoroughly investigated except for the finding that there seems to be no consistent difference in total stem reserves accumulation between solid and hollow-stemmed wheats (Lopatecki et al., 1962).

Stem length, as affected by the height genes is important in affecting stem reserve storage. The Rht₁ and Rht₂ dwarfing genes of wheat were found to reduce reserve storage by 35% and 39%, respectively, as a consequence of a 21% reduction in stem length (Borrell et al., 1993). However, under the favorable conditions the advantage of the tall (rht) genotype in reserve storage was not expressed in greater mobilization to the ear. Under these favorable grain filling conditions only about 20% of grain yield was contributed by stem reserves in all genotypes. The contribution of stem reserves to grain yield was greater in a tall than in a short barley cultivar, but absolute yield was the same in both, indicating that the taller cultivar lacked in current assimilation as compared with the shorter one (Daniels and Alcock, 1982).

RESERVE UTILIZATION

Stem reserve mobilization or the percentage of stem reserves in total grain mass is affected by sink size, by the environment and by cultivar. The demand by the grain yield sink is a primary factor in determining stem reserve mobilization. When sink size was reduced by degraining, more reserves were stored in the stem. as compared with intact ears (Kuhbauch and Thome, 1989). The interaction between ear size and the demand for stem storage seems to depend on the environment (Bonnett and Incoll, 1992a), either that before or that during grain filling.

Environmental conditions that decrease current assimilation during grain filling cause a greater demand for stem reserves for grain filling. Thus, shading of barley plants after anthesis promoted the use of stem reserves for grain filling (Bonnett and Incoll, 1992a). When wheat plants were shaded during grain filling, up to 0.93g of grain was produced per gram of assimilates exported from the stem (Kiniry, 1993).

Stem reserve mobilization is affected by water deficit during grain filling. Even the rate of development of water deficit may affect mobilization,. Hence, Palta et al. (1994) found that total grain carbon with fast development of water deficit was reduced by 24% relative to slow rate, whereas postanthesis assimilation was reduced by 57% while remobilization of reserves was increased by 36%. Interestingly, water deficit during grain filling induced also carbon mobilization from tillers to the main stem ear.

It is therefore to be expected that estimates of the relative contributions of stem reserves to total grain mass per ear or to grain yield would vary among the different reports, according to the experimental conditions and cultivars used. These contributions were estimated to be anywhere between 6% and 100% (Austin et al., 1980; Papakosta and Gagianas, 1991; Pheloung and Siddique, 1991; Davidson and Chevalier, 1992; Borrell et al., 1993; Blum et al., 1994 ; Gent, 1994; Palta et al., 1994).

It may be conclude that the reduction in current assimilation during grain filling, under different stresses, will induce greater stem reserve mobilization to and utilization by the grain. What seemes to be important is the reduction in assimilation and not the nature of stress causing the reduction. Thus, stem reserve mobilization is a solid source of carbon for grain filling under any stress which would inhibit current photosynthesis, including biotic stresses such as late developed leaf diseases. Tolerance to Septoria leaf blotch in wheat is expressed in sustained grain filling under sever epiphytotics. It has been demonstrated that mobilized stem reserve is a major component of Septoria tolerance in wheat (Zilberstein et al., 1985).

Drought conditions during grain filling often involves also heat stress, which reduces the duration of grain filling. There is normally an increase in the rate of grain dry matter accumulation under high temperatures, but it is not sufficient to compensate for the decrease in duration. When grain filling under such stress depends on remobilized stem reserves, the rate at which these reserves are metabolized and transported to the grain becomes crucial. It seems that this rate is not sufficiently high to compensate for the reduction in grain filling duration at very high temperatures. Thus, a genetically longer grain filling duration seems to be an advantage in this respect (Blum et al., 1994). Short grain filling duration may allow some avoidance of terminal stress while longer duration may allow greater utilization of stem reserves for grain filling under stress.

IMPROVING STEM RESERVE UTILIZATION FOR GRAIN FILLING

Improving the capacity for supporting grain filling by stem reserves is an important breeding target in cereals subjected to environmental and biotic stresses during grain filling. Genotypic variations exists for various aspects of grain filling from stem reserves. The effect of height has already been mentioned above. With very few exceptions it seems that taller cultivars have greater capacity to support grain filling from stem reserves because of their greater storage.

The capacity for maintaining large storage in stems appears to be a genetically controlled constitutive trait (e.g. Blum et al., 1994; Hunt, 1979) which may be linked to assimilate partitioning during stem elongation and the developmental characteristics of the stem. If greater partitioning to the stem is at the basis of high reserve storage then it might perhaps be at the expense of grain yield potential. Indeed, Pheloung and Siddique (1991) in Australia found that the higher yielding cultivars Gutha and Kulin had less reserve storage and suffered greater reductions in grain yield under drought stress during grain filling, as compared with the potentially lower yielding cultivar Gamenya. This trend was also noted for winter wheat in Kansas, where newer cultivar releases were less capable in grain filling from stem reserves than older cultivars (Hossain et al., 1990). Landraces of wheat were better at grain filling from stem reserves than a modern high yielding cultivar (Blum et al., 1989), but the advantage of landraces in this respect could be ascribed also to their taller stature.

On the other hand some studies with wheat did not indicate a strong negative relationship between yield potential and reserve utilization for grain filling (Blum et al., 1994; Davidson and Birch, 1992). Indeed, exceptions were noted also by Hossain et al. (1990), such as the winter wheat cultivar Bounty-310 which had a fairly high yield potential and also good grain filling from stem reserves. Still, it remains that cultivars designed for tolerance to stress during grain filling must have the capacity for high stem reserve storage, if necessary even at the expense of a certain reduction in yield potential.

Delayed monocarpic leaf senescence (syn. ‘non-senescence’ or ‘stay-green’) has long been considered as a desirable trait in cereal breeding (e.g. Thomas and Smart, 1993). It is to be expected that longer leaf area duration would contribute to grain filling and yield. However, in two repeated cases for wheat (Blum et al., 1994; Fokar et al., 1996), cultivars of high capacity for stem reserve utilization for grain filling had accelerated leaf senescence under both stress and non-stress conditions. Inherently accelerated leaf senescence in such cultivars would indicate that stem reserve mobilization to the grain is a constitutive trait. This seems to be linked with accelerated export of nitrogen from leaves (e.g. Pell and Dann, 1991). It may therefore be suspected that non-senescence as a sustained source of current assimilation on one hand and stem reserve utilization on the other may be mutually exclusive. While inherently delayed senescence may be advantageous for yield under optimal growing conditions, it may be of no consequence under post-anthesis stress because then the overriding stress factor will impose accelerated senescence or leaf killing. Probably, selection for non-senescence under non-stress conditions may even prefer genotypes which do not use stem reserves for grain filling.

It seems that large TNC storage is the primary factor for sustaining kernel growth from stem reserves. For example, use of storage for grain filling was found to be proportional to the size of storage across 20 winter wheat cultivars (Hunt, 1979). Stem size (and length) seems to be important in sorghum storage and the capacity for mobilizing storage to the grain (Blum et al., 1997). Also, better grain filling under stress was proportional to stem sugar concentration at flowering across different wheat cultivars (Nicolas and Turner, 1993).  It was however noted that some wheat cultivars (e.g. cv. TAM-101) had sufficient storage but were lacking in the extent of remobilization into the grain (Hossain et al., 1990). The remobilization and utilization of the stored carbon depends also on demand. There may be genetic differences among cultivars in enzymatic activity involved with remobilization, but such activity may also be a function of demand and substrate concentration (Dubois et al., 1990).

Another source of imbalance between storage size and it remobilization is the capacity to deposit starch in the kernel endosperm under heat stress. Soluble starch synthase is a key enzyme in endosperm starch biosynthesis. Compared with all the other endosperm starch synthesis pathway enzymes, it is highly thermosensitive, especially at temperatures above 34⁰C (Keeling et al., 1993). A more thermostable form (or a thermoprotected form) of this enzyme has been identified in a wheat cultivar (Kumar et al., 1996). With this form of heat tolerance in the endosperm, stem reserves were well utilized for kernel growth at temperature reaching 38⁰ to 40⁰C. (Blum et al., 1994). Thus, while the size of the storage is preeminent, the size of the sink and its capacity to utilize the imported carbon is also important for allowing grain filling from stem reserves.

METHODODLOGY AND SELECTION

Clarke et al. (1984) demonstrated very well that simple relationships between stem reserve storage or remobilization and varietal drought resistance in terms of yield (such as by the “stress susceptibility index”) are not to be expected. The impact of stem reserves should be evaluated only under stress conditions which equally inhibit crop assimilation during grain filling in all materials tested.

Selection for better reserve supported grain filling under stress may be performed by subjecting the population to the actual stress conditions in the field, may it be drought, heat or disease epiphytotics. It has been repeatedly argued that a standard level of biotic or abiotic stress is difficult to achieve during grain filling in diverse genetic materials. The first difficulty is in the techniques for imposing stress in large breeding populations in the field. The second difficulty is in the variable phenology of breeding materials, which would not allow to affect the same timing and degree of stress after flowering in all materials (Blum et al., 1983b; Mahalakshmi et al., 1994; Clarke et al., 1984).

Blum et al. (1983a, 1983b) proposed the use of chemical desiccation of the canopy after flowering as means for inhibiting plant photosynthesis and thus revealing the capacity for grain filling by stem reserves. The treatment (see Appendix) does not simulate drought stress. It simulates the effect of stress by inhibiting current assimilation. With this method they applied a chemical desiccant (magnesium chlorate or sodium chlorate; 0.4% w/v) as a spray to the canopy, including the ears. The treatment was applied to each genotype at 14 days after anthesis, when kernel growth entered its linear phase. At maturity, kernel weight was compared between treated and non-treated (control) plants, calculating the rate of reduction in kernel weight caused by the treatment. The rate of reduction was typically between 5 and 50% in different wheat materials. An important component of this test is that it must be free of any biotic or abiotic stress, simply because if it is stressed then grain filling would also be reduced in the controls, as noted also by others (Regan et al., 1993).

Nicolas and Turner (1993) confirmed the utility of chemical desiccation as means for revealing genetic variation in grain filling from stem reserves and proposed the use of a leaf spray of potassium iodide (0.4% w/v) in wheat as a milder treatment which mainly destroy chlorophyll. Potassium iodide was working well also for millet (Mahalakshmi et al., 1994). Royo and Blanco (1998) found that the method mimicked the effect of drought stress on grain filling of various triticale lines.

The correlation across diverse genetic materials between the rate of reduction in kernel weight by chemical desiccation and the rate of reduction by drought stress was found to be significant and reasonably high. It was r=0.81*** and r=0.79** over two years in Blum (1983b); it was r=0.48** and r=0.81** over two years in Nicolas and Turner (1993). The relationship also held well for several millet genotypes treated with KI (Mahalakshmi et al., 1984).  Hossain et al. (1990) noted that winter wheat cultivars of stable kernel weight over years and locations sustained relatively less reduction under sodium chlorate desiccation of the canopy.  Blum et al. (1994) found a correlation of r=0.94** across five wheat cultivars between the reduction in kernel weight by chemical desiccation and the reduction in kernel weight by heat stress (35⁰/25⁰C day/night temperatures) during grain filling. The rate of reduction in kernel weight under harsher heat stress conditions was well correlated across different wheat cultivars (r=0.74**) with the reduction in kernel weight caused by post-anthesis defoliation and shading of plants under optimal temperatures (Fokar et al., 1996). Finally, the reduction in kernel weight by chemical desiccation was significantly correlated across different wheat cultivars (r=0.48*) with reduction in kernel weight caused by late epiphytotics of Septoria leaf blotch disease (Zilberstein et al., 1985).

Chemical desiccation can be incorporated into breeding programs in two ways. Firstly, it can be used to assess responses of individual advanced lines or families, always compared with non-treated controls under non-stress conditions. This can be easily performed with nursery ear-rows. Secondly, it can be used in mass selection. Blum et al. (1991) performed mass selection, where six spring wheat F₂ bulks were chemically desiccated with magnesium chlorate after which grain were divergently selected for kernel weight by mechanical sieving. After two or three cycles of selection, random lines were selected and tested for their response to chemical desiccation stress. Mass selection for large kernels under chemical desiccation significantly improved kernel weight and grain yield under chemical desiccation stress, as compared with controls where selection for kernel weight was performed without chemical desiccation. There was no shift in phenology or plant height under chemical desiccation selection, probably because the variation in these traits within the populations used was small.

Haley and Quick (1993) performed a similar selection program under chemical desiccation with sodium chlorate in winter wheat. Two cycles of selection produced F₄ bulks that were indeed more resistant to chemical desiccation stress.

It is concluded that stem reserves offer a powerful resource for grain filling under any type of stress which inhibits current assimilation. In 1983 chemical desiccation has been proposed as a method of selection for improved grain filling from stem reserves. Since then the method has been confirmed by several independent studies to be useful and effective.

REFERENCES

Austin, R.B., Morgan, R.B., Ford, M.A., and Blackwell, R.D. 1980 Contributions to grain yield from pre-anthesis assimilation in tall and dwarf barley genotypes in two contrasting seasons. Ann. Bot. 45:309-316.

Blum, A., Golan, G., Mayer, J., Sinmena, B., and Burra, J. 1989. The drought response of landraces of wheat from the Northern Negev desert in Israel. Euphytica 43:87-96.

Blum, A., Poyarkova, H., Golan, G., and Mayer, J. 1983a. Chemical desiccation of wheat plants as a simulator of post-anthesis stress. I.Effects on translocation and kernel growth. Field Crops Res. 6:51-58.

Blum, A., Mayer, J., and Golan, G. 1983b. Chemical desiccation of wheat plants as a simulator of post-anthesis stress. II. Relations to drought stress. Field Crops Res. 6:149-155.

Blum, A., Shpiler, L., Golan, G., Mayer, J., and Sinmena, B. 1991. Mass selection of wheat for grain filling without transient photosynthesis. Euphytica 54:111-116.

Blum, A., Sinmena, B., Mayer, J., Golan, G., and Shpiler, L. 1994. Stem reserve mobilisation supports wheat grain filling under heat stress. Aust. J. Plant Physiol. 21:771-781.

Blum A., Golan G., Mayer J. and Sinmena B. 1997. The effect of dwarfing genes on sorghum grain filling from remobilized stem reserves, under stress. Field Crops Res. 52:43-54.

Bonnett, G.D., and Incoll, L.D. 1992a. Effects on the stem of winter barley of manipulating the source and sink during grain-filling 1. Changes in accumulation and loss of mass from internodes. J. Exp. Bot. 44:75-82.

Bonnett, G.D., and Incoll, L.D. 1992b. Effects on the stem of winter barley of manipulating the source and sink during grain-filling 2. Changes in the composition of Water-Soluble carbohydrates of internodes J. Exp. Bot. 44:83-91.

Borrell , A.K., Incoll, L.D. and Dalling, M.J. 1993. The influence of the rht1 and rht2 alleles on the deposition and use of stem reserves in wheat. Ann.Bot.71:317-326.

Clarke, J.M., Townley-Smith, T.F., McCaig, T.N., and Green, D.G. 1984. Growth analysis of spring wheat cultivars of varying drought resistance. Crop Sci. 24:537-541.

Daniels, R.W., and Alcock, M.B. 1982. A reappraisal of stem reserve contribution to grain yield in spring barley (Hordeum vulgare L.) J. Agric. Sci. 98:347-355.

Davidson, D.J., and Chevalier, P.M. 1992. Storage and remobilization of water-soluble carbohydrates in stems of spring wheat. Crop Sci. 32:186-190.

Dubois, D., Winzeler, M., and Nosberger, J. 1990. Fructan accumulation and sucrose:sucrose fructosyltransferase activity in stems of spring wheat genotypes. Crop Sci. 30:315-319.

Evans, L.T., and Rawson, H.M. 1970. Photosynthesis and respiration by the flag leaf and components of the ear during grain development in wheat. Aust. J. Biol. Sci. 23:245-254.

Fokar, M., Blum, A., and Nguyen H.T. 1998. Heat tolerance in spring wheat. II. Grain filling. Euphytica 104:9-15.

Gent, M.P.N. 1994. Photosynthate reserves during grain filling in winter wheat. Agron. J. 86:159-167.

Haley, S.D., and Quick, J.S. 1993. Early-Generation selection for chemical desiccation tolerance in winter wheat. Crop Sci. 33:1217-1223.

Hossain, A.B.S., Sears, R.G., Cox, T.S., and Paulsen, G.M. 1990. Desiccation tolerance and its relationship to assimilate partitioning in winter wheat. Crop Sci. 30:622-627.

Hunt, L.A.  1979. Stem weight changes during grain filling in wheat from diverse sources. Proc. 5th Inter. Wheat Genet. Symp. New Delhi pp.923-927.

Keeling, P.L., Bacon, P.J., and Holt, D.C. 1993. Elevated temperature reduces starch deposition in wheat endosperm by reducing the activity of soluble starch synthase. Planta  191:342-348.

Kuhbauch, W., and Thome, U. 1989. Nonstructural carbohydrates of wheat stems as influenced by sink-source manipulations. J. Plant Physiol. 134:243-250.

Kumar, Sanjay, Blum, A. and Nguyen, H.T. 1996. Genotypic variation in wheat for endosperm soluble starch synthase activity under heat stress. (Submitted to Crop Science.)

Lopatecki, L.E., Longair, E.I., and Kasting, R. 1962. Quantitative changes of soluble carbohydrates in stems of solid- and hollow- stemmed wheats during growth. Can. J. Bot. 40:1223-1228.

Mahalakshmi, V., Bidinger, F.R., Rao, K.P., and Wani, S.P. 1994. Use of the senescing agent potassium iodide to simulate water deficit during flowering and grainfilling in pearl millet. Field Crop. Res. 36:103-111.

Nicolas, M.E., and Turner, N.C. 1993. Use of chemical desiccants and senescing agents to select wheat lines maintaining stable grain size during Post-Anthesis drought. Field Crop Res. 31:155-171.

Palta, J.A., Kobata, T., Turner, N.C., and Fillery, I.R. 1994. Remobilization of carbon and nitrogen in wheat as influenced by postanthesis water deficits. Crop Sci. 34:118-124.

Papakosta, D.K., and Gagianas, A.A. 1991. Nitrogen and dry matter accumulation, remobilization, and losses for Mediterranean wheat during grain filling. Agron. J. 83:864-870

Pell, E.J., and Dann, M.S. 1991. Multiple stress-induced foliar senescence and implications for whole-plant longevity. In H.A. Mooney, W.E. Winner and E.J. Pell (eds) Response of Plants to Multiple Stresses. Academic Press, San Diego, pp.189-204.

Pheloung, P.C., and Siddique, K.H.M. 1991. Contribution of stem dry matter to grain yield in wheat cultivars. Aust. J.  Plant Physiol. 18:53-64.

Regan, K.L., Whan, B.R., and Turner, N.C. 1993. Evaluation of chemical desiccation as a selection technique for drought resistance in a dryland wheat breeding program. Aust. J. Agr. Res. 44:1683-1691.

Royo C. and Blanco R., 1998. Use of potassium iodide to mimic drought stress in triticale. Field Crop.Res.59:201-212.

Thomas, H., and Smart, C.M. 1993. Crops that stay green. Ann. Appl. Biol. 123:193-219.

Wardlaw, I.F., and Willenbrink, J. 1994. Carbohydrate storage and mobilization by the culm of wheat between heading and grain maturity: the relation to sucrose synthase and sucrose-phosphate synthase. Aust. J Plant. Physiol. 21:255-271.

Winzeler, M., Monteil, Ph., and Nosberger, J. 1989. Grain growth of tall and short spring wheat genotypes at different assimilate supplies. Crop Sci. 29:1487-1491.

Zilberstein, M., Blum, A., and  Eyal Z. 1985. Chemical desiccation of wheat plants as a simulator of postanthesis speckled leaf blotch stress. Phytopathology 75:226-230.

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ADDED APPENDIX:  Technical details of chemical desiccation treatment of wheat breeding materials.

Wheat sprayed with 0.4% KI (left) and non-sprayed control (right).

Wheat F₃ mass selection under chemical desiccation with magnesium chlorate (left) as compared with non-treated control (right); see Blum et al. 1991.

The treatment can be used to evaluate advanced lines (>F3) for grain filling from stem reserves. It can be used, together with grain sieving, to affect mass selection for grain filling from stem reserves in early generations.

1. Planting of materials for chemical desiccation should be performed in rows spaced at least 30 cm apart (in mass selection work) or in nursery rows (for evaluating advanced lines). Such spacing would allow the spray to reach the lower parts of the canopy. Tests of advanced lines are planted under two treatments: desiccation and non-treated controls. The test is planted under non-stress conditions to avoid any reductions in kernel weight in the controls. This is achieved by assuring sufficient moisture and by controlling leaf diseases during grain filling. The test is performed by comparing the reduction in kernel weight from controls to the treated plots. The reduction in grain yield may also be used.

Work in mass selection should be confined to genetic materials that do not segregate extensively for heading date, otherwise the treatment will simply shift the population towards earliness, as an avoidance mechanism. This, however, may at times be a desirable goal of selection.

2. Spray treatment is generally applied to each genotype at 15 days after heading, or at any other common time which coincides with the onset of the exponential phase of grain filling at the specific test site. Later flowering genotypes (>1 week) in which grain filling is generally subjected to higher temperatures, should be sprayed at a comparatively earlier time, e.g. 13 days after heading.

The spray may consist of solutions of magnesium chlorate, sodium chlorate or potassium iodide, all at 0.4% active ingredient. Magnesium chlorate may be more difficult to purchase. The chlorates are more aggressive treatments and leaf desiccation can be seen 1 to 2 days after spraying. Potassium iodide is milder and the effect can be seen 3 days after treatment. With chlorates the leaves are desiccated and bleached while with potassium iodide the leaves turn yellow.

The spray is applied manually (usually with a back-sprayer) to the whole plant to full wetting, including the ears. In nursery rows it is possible to avoid spraying the ears, if considered desirable.

 3. “Percent reduction in kernel weight” by chemical desiccation is obtained by comparing mean kernel weight under desiccation with mean kernel weight in the controls, for each tested genotype.

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