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a Institute of Desert Meteorology, China Meteorological Administration, 46 Jianguo Rd., Urumqi, 830002, P.R. China
b Semiarid Prairie Agric. Res. Centre, Agric. and Agri-Food Canada, Swift Current, SK, S9H 3X2, Canada
* Corresponding author (gan{at}agr.gc.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DW, shoot dry weight • EF, early flowering stage • PD, pod development stage • RH, relative humidity • DM, dry matter • SED, Standard Errors of the Differences
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In arid to semiarid environments, annual crops are often grown under high temperature stress during their reproductive growth (Kumar and Abbo, 2001; Machado and Paulsen, 2001; Siddique et al., 2002). High temperature stress results in crop yield losses (Campbell et al., 1992) because of damage to reproductive organs (Paulsen, 1994; Hall, 2004), acceleration in the rate of plant development (Savin and Nicolas, 1996; Gan et al., 2004), and shortened period of growth of reproductive organs (Entz and Fowler, 1991; Angadi et al., 2000). Additionally, high temperatures during reproductive development often negatively impact pollen viability and fertilization (Hall, 2004), floral bud development (Prasad et al., 2002), seed filling (Boote et al., 2005), and seed composition (Thomas et al., 2003). Atmospheric temperatures are expected to increase in the future due to potential climatic changes (Cutforth, 2000). This may increase the frequency of temperature stress for annual crops including chickpea.
Temperature and drought stresses may occur simultaneously (Machado and Paulsen, 2001), but temperature stress is far more detrimental to reproductive development in canola (Brassica napus L.) and mustard (Brassica juncea L.) (Angadi et al., 2000; Gan et al., 2004). Limited information exists regarding the effect of temperature stress during reproductive development on pod fertility and seed filling in chickpea. Little is known about the ability of chickpea plants to recover from a short period of stress when the stress is removed. Therefore, the primary objective of this study was to determine the effect of high temperature stress imposed during early flowering and pod development on pod fertility, seed set, and seed yield of chickpea. We hypothesized that high temperature stress imposed during early flowering would have a similar effect as the stress imposed during pod development, and that plants might recover, to a certain degree, from a short-period (10 d) high temperature stress once the stress was removed.
There are two classes of chickpea, namely desi and kabuli, that are grown in the major chickpea production regions of the world (Kumar and Abbo, 2001; Nleya et al., 2002; Siddique et al., 2002). The desi chickpea has a mean seed weight of 170 to 250 mg seed–1, whereas the seed weight of kabuli chickpea is between 270 and 550 mg seed–1 (Anonymous, 2006, p. 16; Nleya et al., 2002). For a specific cultivar, the seed size of desi chickpea is relatively small but uniform, whereas the seed size of kabuli chickpea varies greatly (Liu et al., 2003). Because of its large seed size, seed costs for commercial production of kabuli chickpea are one of the major input expenses. For example, seed was priced at US$1.1 to $1.7 kg–1 during the 2000–2004 periods in western Canada, which translates into US$215 to $230 ha–1 (based on the authors' investigation, no reference available). Some producers buy certified, new seed for sowing every year, while others use their own ‘farm-saved’ seed to reduce production costs. Therefore, a secondary objective of this study was to assess differences between ‘Myles’ desi and ‘Xena’ kabuli cultivars, and between certified and ‘farm-saved’ seed, in response to high temperature stress. We hypothesized that all chickpea plants would respond similarly to the high temperature stress imposed during reproductive development regardless of seed lots from which the plants derived.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Soil (Aridic Haploboroll, Orthic Brown Chernozem, silt loam) collected from the SPARC research farm was placed in 2-L milk cartons and commercial-grade peat moss was placed on the soil surface in a layer 25-mm thick as well as in the bottom 10-mm of the cartons. Seed was treated with metalaxyl, carbathiin, and thiabendazole, as described by Hwang et al. (2000, p. 24), to minimize seed- and soil-borne diseases. On the day of planting, the seed was inoculated with Nitragin Nitrastick GC (Nitragin Inc., Brookfield, WI) at a rate of 4 g per kg of seed. Three seeds were planted in each pot at a depth of 30 mm. The plants were grown in controlled environment growth chambers (Model GR96, CONVIRON, Control Environment Ltd., Winnipeg, Canada) at 20/16°C day/night temperatures, with day/night cycles for temperature and photoperiod being 16 h day and 8 h night. Constant temperatures were maintained during the day and the night. The growth room had a growth area of 8.9 m2 with interior dimensions of 3.35 x 3.96 m. Requirements for control of temperature, light, relative humidity (RH), and airflow, as well as environmental monitoring and data acquisition were accomplished with a built-in CMP4030 computerized control system. The system has a graphical user interface to allow operators to perform all controlling functions. Temperature was measured using thermistor sensors located in a self-contained, portable aspirator positioned near the canopy level (70 cm below the light bench), with the accuracy of temperature measurements of ±0.5°C. Relative humidity was set at 80%, which was achieved through airflow introduced into the growth area through refrigerated coils. Built-in air handling plenums ensured a consistent flow of air providing stable and uniform temperatures and RH in the chamber. Photosynthetic photon flux density (PPFD) was 494 ± 12 µmol m–2 sec–1 (the mean of 10 readings made across the chamber bench where plants were grown). The PPFD readings were done near the canopy level (approximately 70 cm below the light bench) using an OL-2000Q quantum sensor (Optisk Laboratorium ATV, Lyngby, Denmark). The CO2 in the chamber was not controlled or measured. The set up of the growth chamber conditions was similar to those described by Angadi et al. (2000) and Gan et al. (2004).
Two weeks after initial emergence, plants were thinned to one per pot. The pots were watered at 0900 h daily by weighing the individual pots and supplying each pot a given amount of water that had been pre-calculated based on the water content of dried soil and its water holding capacity. The daily watering brought the water content of the pots to 95% of the field capacity, allowing the plants to grow under conditions free from water stress. The watering was stopped when 80% of the pods turned brown in color (considered as physiological maturity). During the early vegetative growth period (40 d after seeding), each pot received 100 mL of nutrient solution (5 g of 20N-20P-20K of TuneUp, a product made by Professional Gardener Inc., Calgary, Canada) dissolved in 1 L water.
Experiment Design and Stress Treatments
The experiment was a split-plot design with three air temperatures being the main plots, and five seedlots and two growth stages being the subplots in a factorial combination. The experiment was replicated twice over time with the growth chambers being re-randomized between the two replicates. The two growth stages were (i) early flowering (EF) (55 d after seeding) and (ii) pod development (PD) (85 d after seeding). In each of the two replicates, all plants (150 pots) were grown in Chamber 1 during the vegetative growth period under the environmental conditions described above. Two seeding dates (12 d apart, based on previous observations) were used to produce two sets of plants; one set (75 pots) was flowering while the other set (75 pots) was at podding stage. At the desired growth stages, 50 pots (25 pots of flowering plants and 25 pots of podding plants) were moved into each of Chamber 2 at 28/16°C (moderate temperature stress) and Chamber 3 at 35/16°C day/night temperature (high temperature stress). The remaining 50 pots were maintained at 20/16°C (no-stress control, in Chamber 1). The levels of high and moderate temperatures for the stress treatments were based on typical field conditions in the northern Great Plains where the maximum air temperatures at 28 to 35°C occur during the growing season (McCaig, 1997; Padbury et al., 2002). After 10 d of the temperature treatments, all plants in Chambers 2 and 3 were returned to Chamber 1 and remained under conditions that were consistent with those before the stress was applied. The three growth chambers had been set up and operated identically, except temperatures in Chamber 2 and 3 in which moderate and high temperatures were, respectively, set for the 10-d stress treatment periods. In each of the Chambers, plants were spaced sufficiently apart to preclude competition effects among treatments. The two factors at the subplot level (i.e., two growth stages and five seedlots) were replicated five times in randomized blocks. The growth chamber bench was divided into five sections, one for each of the five replications. Each replication contained 10 pots (i.e., 2 stages x 5 seed lots combinations) which were randomly placed within the section on the bench. The five replications were to account for any variability in growing conditions along the bench, and also to increase the degree of freedom for the error term. Therefore, the whole experiment consisted of 300 (150 x 2) pots (i.e., 3 temperatures x 2 stages x 5 seed lots x 5 replications at the subplot level x 2 replications over time).
Data Collection and Analysis
The opening of flowers during stress periods was monitored on all plants by marking (with a jiffy marker pen) the distance on the main stem between where the last flower opened before stress imposition and where the last flower opened when the plants were removed from the stress treatment chambers. The pods produced between the marked distances were tagged, and the number of seeds in those pods determined at harvest. This marking system allowed an assessment of pod fertility during the stress and later in the recovery period. At harvest, the following variables were also determined: the total number of fertile pods (i.e., the pod with at least one seed) on the main stem and on the branches, the number of branches bearing at least one pod, the number of pods initiated during stress imposition and filled during post-stress recovery periods, the proportion of pods with 0, 1, or 2 seeds per pod, seed yield per plant, weight per seed, and shoot dry matter (DM). Harvest index was calculated as seed weight divided by total dry weight (excluding root). After above-ground tissues were harvested, the entire root-soil matrix was soaked in water for 60 min, roots rinsed with running water, oven-dried, and weighed for root dry weight assessment.
Data were analyzed using the linear model of the MIXED procedure of SAS (SAS Institute Inc., 2003). The fixed effects were temperature, growth stage, seedlots, and all combinations of their interactions, and the random effects were replicate and replicate x temperature. A priori difference in treatment means, t tests (difference is equal to zero), and the appropriate Standard Errors of the Differences (SED) were obtained from the PDIFF option in the LSMEAN statement of the MIXED model (Lynch and Walsh, 1998, p. 980). ESTIMATE statements were used to determine significances in differences between temperatures at a given stage for each of the seedlots, and between the desi and the mean of the four kabuli seedlots under the same temperature at a given stage. The MIXED model also took into account the five sub-plot replications in the degree of freedom for the denominator. Two- or three-way interactions were often significant (Table 1), thus, the effect of temperature was determined for each individual seedlot (or desi vs. kabuli) at each growth stage.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Pod Production
The EF high temperature (35/16°C) stress decreased pod production by 34% (P < 0.01) for desi chickpea and 22% for kabuli chickpea compared to the control which remained at 20/16°C (Table 2). The number of pods on the main stem decreased by 53% for the desi and 30% for the kabuli (P < 0.01), whereas the pods on the branches decreased by 22% for the desi and 25% for the kabuli (P < 0.01) (data not shown). Similarly, the PD high temperature stress decreased pod production by 22% for the desi and 11% for the kabuli (P < 0.01). In contrast, moderate temperature (28/16°C) during EF did not affect pod production for either desi or kabuli chickpea, whereas the moderate temperature during PD decreased pod production by 9% for the kabuli cultivar (P < 0.05) but it had no effect on the desi.
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Seed Yield
The EF high temperature stress decreased seed yield per plant by 39% for desi chickpea and 42% for kabuli chickpea (P < 0.01) (averaged over the four kabuli seedlots) (Table 4). The EF moderate temperature (28/16°C) decreased the seed yield of kabuli chickpea by 7% (P < 0.05), but had no effect on the desi. When delayed until pod development, the high temperature stress caused a greater degree of yield losses than when it was applied during early flowering. In desi chickpea the PD high temperature stress decreased the seed yield by 59%, whereas the EF high temperature reduced seed yield by 39%. Similarly, in kabuli chickpea, the PD high temperature decreased seed yield by 53%, and the EF high temperature stress caused a yield loss of 42%, from the control.
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Shoot Dry Weight (DW) and Harvest Index
The EF high temperature stress decreased DW by 22% for the desi and by 26% for the kabuli (P < 0.05) compared to the control (Table 3). The PD high temperature stress decreased DW by 34% for the desi and by 15% for the kabuli (P < 0.05). With high temperature stress, the plants produced 8% fewer (P < 0.05) branches in the desi and 15% fewer (P < 0.05) in the kabuli (data not shown). Kabuli plants grown from large seed produced 17% greater DW than the plants from small seed when elevated temperature was applied during EF. No differences in DW were observed among seed size treatments when the temperature stress was applied during PD. Either the EF or PD high temperature stress had no effect on root dry weight in this study (data not shown).
The effect of temperature stress on harvest index (Table 3) followed a similar pattern as the effect on seed yield (Table 4). High temperature stress decreased harvest index significantly for both desi (by 29%) and kabuli (by 34%) chickpea compared to the control (P < 0.05), and the reduction of harvest index was greater with the PD high temperature stress compared to the EF high temperature stress. The desi cultivar had a greater harvest index than the kabuli (0.45 for the desi vs. 0.29 for the kabuli) under the same growing conditions.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Pollen development, fertilization, and asynchrony of stamen and gynoecium development are sensitive to temperatures during flowering (Prasad et al., 1999; Croser et al., 2003; Boote et al., 2005). The loss of pollen or stigma viability under high temperature stress might be the primary reason for the lowered number of seeds produced in the legume (Srinivasan et al., 1998; Davies et al., 1999; Hall, 2004). Flower abortion also has been attributable to the decreased seeds per plant and seed yield in other crops such as Brassica napus (Angadi et al., 2000), B. rapa (Morrison and Stewart, 2002) and B. juncea (Gan et al., 2004).
High temperature stress decreased both biomass and seed yield in this study, but the effect on seed yield was more pronounced than the effect on biomass. This was reflected by the decreased harvest index for both desi and kabuli chickpea under high temperature stress. Our results suggest that high temperature stress during reproductive development affects sink size more than assimilate source in chickpea. A large portion of the carbohydrates utilized in filling the grains may have been synthesized before the imposition of the temperature stress. It is also possible that the stress during reproductive development may have indirectly affected the remobilization of the photosynthates to the grain (reflected in fewer seeds per plant and lower weight per seed). Phenologically, the seeds which formed during post-stress periods did not have enough time to develop fully before maturity. Physiologically, the high temperature stress during reproductive development may have affected flower abortion, subsequent sink site, and later pod abscission, resulting in a decreased number of seeds per plant (Duthion and Pigeaire, 1991). Also, high temperature stress during reproductive development may have negatively affected cell expansion, cotyledon cell number and thus seed filling rate, resulting in the lowered weight per seed (Munier-Jolain and Ney, 1998). Seeds have a highly regulated capacity to achieve the same seed size, but the high temperature stress imposed during the mid-reproductive stage seems to act by preventing the seeds from filling to their full potential size in chickpea.
Recovery from Stress
Chickpea plants tended to recover from short (10-d) periods of temperature stress once the stress was removed; this was reflected by the addition of filled pods during the post-stress recovery period observed in the present study. Chickpea has a strong indeterminate growth habit and great plasticity (Liu et al., 2003); this may serve as the mechanism of the recovery. However, the magnitude of the recovery was limited. In soybean, Ferris et al. (1998) also found that photosynthesis and seed filling activity resumed and recovered to a certain degree after environmental stresses were removed during seed filling. The plasticity of crop plants allows plants to compensate for part of the lost yield due to high temperature and drought stresses by adjusting yield components (Angadi et al., 2003; Clarke and Siddique, 2004). Our study indicates that yield compensation in chickpea can come from increased number of seeds as well the weight per seed, which is in agreement with findings in other crops (Srinivasan et al., 1998; Angadi et al., 2003).
Cultivar Differences
The Myles desi cultivar produced twice as many pods per plant as the Xena kabuli cultivar regardless of the developmental stages during which the temperature stress was applied. The greater ability of Myles to produce more pods on branches was responsible for the greater number of pods per plant. On average, 4.5 branches on a single Myles plant bore fertile pods, while only 3.4 branches on a Xena plant bore fertile pods (data not shown). Under the no-stress control conditions, the Xena cultivar filled 85% of the pods formed, whereas the Myles cultivar filled more than 97% of pods formed (data not shown). Similar results have been reported by Liu et al. (2003) who found in a field experiment that the Myles desi plants produced significantly more seeds per pod than a kabuli crop. These results suggest that the Myles cultivar has a greater ability than the Xena cultivar in utilization of photosynthates for pod filling.
Myles and Xena are the representative cultivars for desi and kabuli classes of chickpea, respectively. Previous research has shown that differences in response to environmental conditions between desi and kabuli chickpea were far greater than the difference existing among genotypes within a chickpea class (Davies et al., 1999; Gan et al., 2003a, 2003b). Determination of genetic variability within a chickpea class is beyond the objective of the present study, while the results of this study may have application in studies where discrimination amongst genotypes is desired.
No difference was found in pod production, seed set, or seed yield between the large- and the small-sized Xena kabuli seeds regardless of temperature stress. Although the large-seeded Xena kabuli produced greater shoot dry weight than the Xena plants grown from the small seed, this did not translate into an increased seed yield. These results suggest that physical size of the Xena seed may not alter the source-sink relationship, potential sink size, or their susceptibility to high temperature stress studied in the resulting plants.
The Xena plants grown from certified seed produced more (P < 0.01) pods per plant (Table 2) and greater (P < 0.01) seed yield (Table 4) than those from ‘farm-saved’ seed under the same growing conditions. These results suggest that use of certified seed in chickpea production may increase seed yield potential as well reduce production risks associated with high temperature stress. ‘Farm-saved’ seed used in the present study was from the same source as the certified seed; both having similar characteristics in terms of the size of seed, germination, and post-harvest processing. The mechanisms responsible for the yield advantage of certified over ‘farm-saved’ seed were unknown. More detailed studies involving multiple cultivars are needed to elucidate this effect.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication February 11, 2006.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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