Response to Temperature in a Stand of Pearl Millet: 10. PARTITION OF ASSIMILATE

Effects of temperature on partition of assimilate between leaves,stems and panicles of pearl millet are analysed in terms ofa duration (t_w) over which a structure increased in weight,and a partition factor (p)—the fraction of new dry matterallocated to the structure during t_w. The value of tw was, forall structures, inversely proportional to temperature abovea base of 10 °C and below an optimum of 28 to 30 °C.For stems and panicles, the value of p was, with one exception,little affected by temperature. The dry weight of these structureswas, therefore, proportional to t_w, and decreased with risein temperature. (The exception was panicles at the lowest temperature,19 °C, for which p was reduced by 40% because few grainswere set.) For leaves, however, p increased with rise in temperature,counteracting the effect on t_w, such that dry weight changedlittle with temperature. The optimum temperature for reproductiveyield was 22 °C, but the proportion of the total dry matterallocated to reproductive structures changed little between22 °C and 31 °C. Key words: Pearl millet, temperature, thermal time, partitioning     

Response to Temperature in a Stand of Pearl Millet: VI. LIGHT INTERCEPTION AND DRY MATTER PRODUCTION

Stands of pearl millet were grown in glasshouses in which meanair temperature was controlled to 19, 22, 25, 28 and 31 ?C withan amplitude of ?5 ?C. During the main growth period, leaf areaindex increased at a constant rate which was proportional tomean temperature above a base of 10 ?C. The warmest stand, therefore,intercepted more radiation before anthesis but the transmissioncoefficient was independent of temperature (K 0.3). Based ondry weight at final harvest, the efficiency of conversion forintercepted radiation ranged from about 2.1–2.4 g MJ^–1consistent with field experience. Combining this informationwith figures for the duration of growth in relation to temperaturesuggests that growth rate should be maximal at 25–27 ?Cand total dry weight at 20–22 ?C. Key words: Temperature, Pearl millet, Growth rate, Light     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): VIII. ROOT GROWTH

P J. Gregory. 1986. Response to temperature in a stand of pearlmillet (Pennisetum typhoides S. & H.). VIII. Root growth—J.exp. Bot. 37: 379–388. Two experiments were made in controlled glasshouscs to investigatethe growth of roots of pearl millet at different air and soiltemperatures. The experimental plants were grown in columnsof soil within stands of millet for 3 to 4 weeks and destructivelysampled at regular intervals to estimate the length of individualroot axes and of the root system. The length of individual rootaxes increased exponentially with time and at any particulartime the rate of extension was faster the higher the soil temperature.Clear ontogenctic effects on the rates of elongation were detected,with each succeeding axis elongating faster than its predecessor.Total root length was longer the higher the soil temperature(at a particular air temperature) and increased exponentiallywith time and with thermal time assessed from temperatures measuredat 2·0 cm depth. Whereas length at a particular timehad a 10-fold range, length at a particular thermal time hadonly a 3-fold range. Mean irradiance differed between the twoexperiments and as a means of exploring the importance of carbohydrateresources for root extension, relations between root length,leaf area and the amount of radiation intercepted were sought. Root length and leaf area were linearly related for all temperaturetreatments in both years as were root length and interceptedradiation. However, whilst the former relation was the samein both years, the latter was different. Root dry weight andintercepted radiation were also linearly related with the samerelation for both years so that the root length: weight ratiosdiffered between years because of factors not controlled inthese experiments. The results show the close relation between root and shoot growthand that thermal time together with the amount of radiationintercepted by the leaves might be used as the basis for quantifyingthe effects of temperature on root growth. Key words: Pearl millet, temperature, thermal time, root extension, root growth     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): III. ROOT DEVELOPMENT

This paper examines the use of thermal time (accumulated temperature)to analyse the effects of temperature on the development ofpearl millet. The plants were grown in columns of soil withinstands of millet growing in controlled environment glasshouses.A range of almost constant soil temperatures was maintainedat a number of air temperatures that were allowed to vary sinusoidallythroughout the day. The number of root axes and lateral rootswere counted on several occasions for young plants by destructivesampling of the columns. The results show that root axis and lateral development is relatedto the thermal time measured at the shoot meristem using a basetemperature of 12 ° C. The shoot meristem temperature consistentlyproved to be more closely related to root development than soiltemperature at a depth of 5.0 cm. The difficulty of relating root development to temperature ata particular depth is discussed, together with the problemsof selecting an appropriate base temperature. For the conceptof thermal time to provide a clearer understanding of temperatureeffects on root development, it will be necessary to take accountof possible differences in the thermal response of differentparts of the root system and of other environmental factors,particularly soil water status. Key words: Pennisetum typhotdes, Temperature, Thermal time, Root development     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): 4. EXTENSION OF INDIVIDUAL LEAVES

The leaf extension rate of millet plants was measured with auxanometersin temperature-controlled glasshouses. Temperature was the dominantenvironmental factor governing the rate of leaf extension. Theobserved linear relation between extension rate and meristemtemperature had a base temperature of 10 ?C and a less clearlydefined optimum of about 30–32 ?C. Leaf growth was expressed as extension per unit thermal time,mm (?C h)^–1, to examine the influence of saturation deficit,irradiance and ontogeny at different temperatures. Leaf extensionwas independent of saturation deficit below 3.0 kPA. Irradiance,ranging from 4–16 MJ m^–2 d^–1, had a greaterinfluence on the first five leaves than the subsequent onesbut there was a large effect of leaf position. The results arediscussed in relation to the growth of crop leaves in a tropicalclimate. Key words: Leaf extension, Millet, Temperature     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): V. DEVELOPMENT AND FATE OF TILLERS

The development of individual tillers in stands of pearl milletwas investigated in a suite of temperature-controlled glasshousesmaintained at mean air temperatures of 19, 22, 25, 28 and 31?C. The rate of leaf appearance of individual tillers was similarto that on the main culm but later tillers produced fewer leaves.Apical dissection revealed that 2–5 leaf primordia failedto emerge from some tillers and the cessation of developmentpreceded any external signs of premature senescence by 3–4weeks. The concept of thermal time is used to determine when leaf appearanceceased on individual tillers. Tiller development stopped synchronouslyat about 430 ?Cd in all treatments, indicating that it was relatedto a common physiological or environmental condition. This periodcorresponded to the start of stem elongation and closure ofcrop canopy but because temperature has a major influence onboth it was impossible to reach a firm conclusion about themechanisms responsible for the cessation of tiller development.The yield and fate of individual tillers are also presented. Key words: Tiller development, Millet, Temperature     

Response to Temperature in a Stand of Pearl Millet (Pennisetum typhoides S. & H.): VII. FINAL NUMBER OF SPIKELETS AND GRAINS

An index which incorporates rates of plant growth and of development—athermal interception rate (TIR)/is used to analyse final spikeletand grain number in pearl millet. Growth rate was assumed proportionalto solar radiation intercepted (MJ per plant per day) and developmentalrate to the accumulation of degree days above a previously determinedbase of 10 °C. Measurements were assembled from experimentsin glasshouses with precise temperature control and from a fieldstudy in the tropics. The final number of spikelets was less sensitive to TIR thanthe final number of grains per plant. The critical period forthe determination of grain number is from floral initiationto anthesis (GS2 period). Key words: Temperature, Spikelets, Grains, Pearl millet     

Response to Saturation Deficit of Leaf Extension in a Stand of Pearl Millet (Pennisetum typhoides S. & H.)I: I. INTERACTION WITH TEMPERATURE

Thermal time is used to analyse hourly differences in leaf extensionrate of pearl millet. The procedure enables the effects of theenvironment on leaf extension to be examined when temperatureis varying. The analysis was made on the results of two experimentsin which saturation vapour pressure deficit (SD) was controlledor allowed to vary with air temperature. In all treatments,temperature was the major environmental factor governing therate of leaf extension. The effect of SD was small in one experimentand negligible in the other. In the former, leaf extension wasalso limited by another factor, probably irradiance. Key words: Extension, Saturation deficit, Millet     

Response to Saturation Deficit of Leaf Extension in a Stand of Pearl Millet (Pennisetum typhoides S. & H.)II: II. DEPENDENCE ON LEAF WATER STATUS AND IRRADIANCE

This paper describes how the dominant relation between leafextension and temperature in pearl millet is modified by atmosphericsaturation vapour pressure deficit (SD) and irradiance. Standsof plants were grown at two levels of SD and soil moisture content.Leaf extension, water potential (₁) and stomatal conductancewere all reduced at high SD, ₁ was more closely related to transpirationrate than to SD itself. Leaf extension rate (R) was poorly correlatedwith ₁, even after correction for temperature differences, owingto variation in solute potential between leaves. However, Rin individual leaves was linearly related to turgor potential,except after periods of low irradiance. The thermal time conceptwas modified to incorporate turgor potential and used to showthat the ‘turgor thermal rate of extension’ decreasedsharply at low irradiances, presumably due to assimilate shortage. Key words: Extension, Saturation deficit, Millet     

The Response of Stomata of Pearl Millet (Pennisetum typhoides S. and H.) to Atmospheric Humidity

Canopy conductance of irrigated and unirrigated pearl milletplants was measured with a diffusive resistance porometer ina field experiment in Central India. When plants were growingin a drying soil, canopy conductance was related linearly tothe amount of light intercepted by the canopy, and was unaffectedby large changes in atmospheric saturation deficit When waterwas given to other plants growing in the same field, canopyconductance became strongly influenced by changes in saturationdeficit.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): I. CONSTANT TEMPERATURE

The germination of pearl millet (Pennisetum typhoides S. &H.) seeds was investigated at constant temperatures between12 ?C and 47 ?C on a thermal gradient plate. The rate of germination increased linearly with temperaturefrom a base T_b to a sharply defined optimum T_o beyond whichthe rate decreased linearly with temperature, reaching zeroat T_m. The linearity of the response both above and below T_oallowed time and temperature to be combined in a thermal timeat which a specified fraction of the seeds germinated. Withinthe population T_b and T_m were constant.     

Twins and Triplets in Pearl Millet: their Cytology and Origin

Multiple radicles and plumules were produced in high frequenciesin selfed seed of six plants in pearl millet (L.S. 326–3).Ninety-five of these survived to maturity. All were diploids,except in one twin where a trisomic (2n+1) was found. The occurrenceof albino twins in progenies of heterozygous plants, and normalhomozygotes in twins from translocation heterozygotes suggeststheir origin through sexual reproduction. The origin of theremaining twins and triplets is not known with certainty. Pennisetum americanum, twins, triplets, tertiary trisomy, translocation hybridity     

The Effect of Flowering on Stomatal Response to Water Stress in Pearl Millet (Pennisetum americanum [L.] Leeke)

Stomata on upper leaves of drought-stressed pearl millet (Pennisetumamericanum [L.] Leeke) crops were more open in flowering (F)than in pre-flowering (PF) plants. This was not due to differencesin leaf water potential (). Stomata of PF plants closed when fell to about –1.7 MPa, while on F plants stomata closedonly when approached –2.3 MPa. Osmotic adjustment did not account for these differences asrelations between turgor potential (_P) and were similar inF and PF plants. While stomata of PF plants closed as W becamezero, in F plants stomata remained open even after bulk leafturgor was lost. Differences between F and PF plants were not explained by differencesin age of leaves sampled. However, leaves of water-stressedPF plants had higher levels of abscisic acid (ABA) than leavesof F plants, despite similarities in water status. From theseresults and from relationships between g_L and stage of panicledevelopment, it is concluded that the tendency of stomata toremain open despite water stress and loss of bulk leaf _P isrelated to the presence of an emerged panicle. Hypotheses whichaccount for this effect are discussed. Key words: Pennisetum americanum [L.] Leeke, Pearl millet, Flowering, Stomata, Water stress, Abscisic acid     

Grain Yield in Pearl Millet in Relation to Source Size and Proximity to Sink

Two pearl millet [Pennisetum glaucum (L.) R. Br. emend. Stuntz] hybrids GHB-30 and MH-179 were given defoliation treatments prior to anthesis comprising zero leaf to intact control. Keeping or removing even flag leaf only significantly altered the grain yield. With increasing leaf area (leaf numbers) the grain yield also significantly increased. Test mass showed more or less a similar trend. The leaves in the upper portion (nearer to sink) showed a greater contribution to the grain yield than the lower ones (away from sink). However, the highest leaf efficiency in terms of contribution per unit leaf area and the contribution by the whole leaf to the grain yield was recorded by 4^th and 3^rd leaf, respectively. The stem (covered with petioles) contributed to the extent of around 12 %. The existing leaves compensated to some extent for the defoliated ones.     

Stigma Constriction in Pearl Millet, a Factor Influencing Reproduction and Disease

The sequence of events during pollination in pearl millet isdescribed. Four or five hours after pollination with compatiblepollen a constriction region located in the fused stylodia,consisting of a collar of parenchymatous cells, becomes apparent.The structural integrity of the constriction region is lostwithin 6 h of pollination, thereby eventually isolating theovary from other pollen and also from ovary pathogens whichnormally enter throught the stigma. The constriction developsin unpollinated gynoecia 4–5 d after stigma emergence,thereby contributing to out-crossing in the protogynous commercialhybrid commonly grown in the semi-arid tropics. The potentialsignificance of a stigma constriction character in a wider botanicalcontext is discussed, particularly as it is a hitherto unrecognizedincompatibility barrier. Pennisetum americanum (L.), pearl millet, Sorghum bicolor (L.), sorghum, pollination, stigma constriction, ergot, protogyny, incompatibility barrier     

Osmotic Adjustment to Water Stress in Pearl Millet (Pennisetum americanum (L. ) Leeke) in a Controlled Environment

A pressure-volume (P-V) and an expressed sap (cryoscopic) techniquewere compared for assessing osmotic adjustment to water stressby pearl millet (Pennisetum americanum (L. ) Leeke) plants grownin a controlled environment cabinet. For leaf water potentials( ) above the point of zero turgor, there was good agreementbetween estimates of solute potential ( _s)and turgor ( _p) obtainedby the two methods. Reductions in pre-dawn leaf to –1.8 MPa over 5–6d resulted in net solute accumulation as indicated by a fallin s at full hydration of about 0.3 MPa. The degree of osmoticadjustment increased linearly with the decrease in pre-dawn. Adjustment in cv. BJ 104 was significantly (P < 0.05) lessduring a second drought than during a first, and cv. Serere39 was significantly (P < 0.05) less able to adjust osmoticallythan BJ 104. Adjustment was greater in leaves which were undergoing extensiongrowth during the drought than in leaves already fully extendedbefore drought started. Much of the adjustment was lost within24 h following rewatering, the loss being most complete in theolder, fully extended leaves.     

Genotypic Variation in Pearl Millet (Pennisetum americanum (L.) Leeke), in the Ability to Accumulate Abscisic Acid in Response to Water Stress

Variation between genotypes in the ability to accumulate abscisicacid (ABA) in response to water stress was investigated in pearlmillet (Pennisetum americanum (L.) Leeke). Using a detachedleaf test a more than four-fold variation in accumulation capacitywas observed amongst a set of 16 genotypes grown in a controlledenvironment. Two genotypes which contrasted in their accumulationcapacity, BJ 104 and Serere 39 (the latter accumulating mostABA), maintained the difference over a range of leaf water contentsand potentials. Some of the genotypes were grown in the field in the semi-aridtropics with and without irrigation, and sampled for ABA content.In two experiments, substantial genotypic variation in ABA accumulationwas observed, which could not be attributed to differences inleaf water potential (). In a field experiment comparing threegenotypes (Serere 39, BJ 104, and B282), differences in ABAaccumulation were also shown to be largely independent of genotypicdifferences in turgor potential (_p). For a set of six of the genotypes, the amounts of ABA accumulatingin leaves of intact, droughted plants, in the field, when adjustedfor differences in , were found to be significantly (P<0.05)correlated with amounts of ABA accumulated in detached, water-stressedleaves. It is concluded that the detached leaf test adequatelyreflects the ability of pearl millet genotypes to accumulateABA under field conditions.     

The Possible Involvement of Lipoxygenase in Downy Mildew Resistance in Pearl Millet

The downy mildew disease, incited by Sclerospora graminicola,is a major biotic constraint for pearl millet production inthe semi-arid tropics. Sources of resistance to this diseasehave been identified. However, the mechanism of host resistancestill remains obscure. The enzyme lipoxygenase (LOX) is knownto play a role in disease resistance in many host-pathosystems.In the present study, LOX activity was tested in seeds of differentgenotypes of pearl millet with different susceptibility to downymildew. The LOX assay of the seeds indicated a good correlationbetween enzyme activity and their downy mildew reaction in thefield. Maximum activity was recorded in seeds of highly resistantgenotypes and minimum activity was found in the highly susceptiblegenotypes. Seeds obtained from plants recovered from the downymildew disease had more LOX activity than that of the originalparent seeds. Thus, in seeds, the LOX activity can be used asa biochemical marker for screening different genotypes of pearlmillet for downy mildew. The study, carried out in the susceptiblegenotype of pearl millet seedlings, showed that LOX activitydecreased after inoculating with S. graminicola zoospores whencompared with uninoculated controls. However, a significantincrease in the enzyme activity was observed on the second andthird days after inoculation in resistant seedlings. The possiblerole of LOX in conferring resistance to downy mildew infectionof pearl millet is discussed. Key words: Lipoxygenase, pearl millet, downy mildew     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): III. INHIBITION OF GERMINATION BY SHORT EXPOSURE TO HIGH TEMPERATURE

Seeds of pearl millet were germinated on wet filter paper attemperatures up to 50 ?C. In one experiment, the temperaturewas held at 50 ?C during imbibition and was then lowered to32 ?C or 25 ?C. Germination rate and the maximum fraction ofseeds germinating (G_m) both decreased as the time of exposureto 50 ?C increased. In contrast, exposure to 50 ?C after imbibitionfor 8 h slowed germination but did not significantly reduceG_m. When the ‘high’ temperature imposed after imbibitionwas reduced from 50 ?C to 45 ?C, there was a small reductionin the rate of germination but not in G_m. The responses haveimplications for the optimum time of sowing in the tropics whenmaximum daytime soil temperature at the depth of sowing is inthe range of 45–50 ?C. Key words: Pennisetum typhoides, Temperature, Germination     

Development of EST SSRs in Finger Millet (Eleusine coracana ssp coracana) and their Transferability to Pearl Millet (Pennisetum glaucum)

EST-SSR markers were developed using sequence information from 1740 expressed sequence tags (ESTs) of finger millet available in the public domain. A set of 31 SSR markers were synthesized based on di, tri, tetra and penta-nucleotide repeat sequences. These were used for PCR analysis of 11 elite germplasm lines of finger millet of Indian and African origin. Out of 31 SSR markers, amplification products were obtained for 17 primer pairs. Of these nine were found polymorphic with two alleles per locus. These 17 SSR primer pairs were also tested for amplification in three varieties of pearl millet (Pennisetum glaucum) and 11 could be transferred to pearl millet. The informative EST SSR markers developed, can be used in finger millet as well as pearl millet genetic improvement projects.