High-Temperature Tolerance in Sorghum – What do We Know and What Are the Possibilities? P.V. Vara Prasad Department of Agronomy, 2004 Throckmorton Plant Science Center, Kansas State University, Manhattan, Kansas 66506, USA Corresponding author:
[email protected]
ABSTRACT Sorghum grown in semi-arid regions is often exposed to short episodes of high temperature stress. Studies have shown that sorghum is sensitive to high temperature (>36/26ºC, daytime maximum/nighttime minimum) stress during reproductive stages of crop development. The most sensitive phase of reproductive development to short episodes of high temperature stress (40/30ºC) is flowering and 10 d prior to it. High temperature stress during this phase caused maximum decrease in percent seed-set, seed numbers and seed yield. High temperature stress during panicle initiation inhibits or delays panicle exsertion. The knowledge on genotypic variability of sorghum for high temperature tolerance is limited and needs attention. Our preliminary research suggests presence of genetic variability in response to high temperature stress for traits such as percent pollen viability and percent seed-set. One of the major limitations for determining high temperature tolerance on large germplasm collections is lack of an easy, reliable, and high throughput screening tool. However, potential opportunities exists in terms of using pollen viability, percent seed-set, production of reactive oxygen species in vegetative and/or reproductive tissues, thylakoid membrane damage and canopy temperatures as screening tools. Better understanding of the physiological, molecular and genetic basis of reproductive failure and the available range of high temperature tolerance will help development of genotypes with adaptive traits and higher grain yield in semi-arid regions. The main challenge is to identify molecular markers that can be used for high throughput screening of germplasm collections.
INTRODUCTION Grain sorghum (Sorghum bicolor L. Moench) is an important crop in the semi-arid regions of the world. It is mostly grown under rain-fed conditions in regions that are prone to high temperatures. Although grain sorghum originated in hot tropical regions, it is currently grown in tropical and subtropical regions where high temperature and water stress limit its yield. Temperatures close to or >32/22ºC commonly occur during the crop life cycle and decreases sorghum productivity (Prasad, Boote et al. 2006). It is predicted that future climates are characterized by further increases in mean temperature and increased frequencies of short episodes of high temperature stress. If these short episodes of high temperatures occur during sensitive stages of crop st
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development (particularly reproductive stages) it could lead to significant yield losses. Therefore, high temperature tolerance will benefit sorghum producers and sorghum industry in the arid and semi-arid regions. The primary information or inputs necessary for crop improvement for high temperature stress tolerance are (a) understanding of impacts of high temperature stress and sensitive stages of crop development; (b) availability of genetic variability and diverse tolerant parent lines for hybrid development; (c) screening tool to evaluate large germplasm collections for tolerance; (d) understanding of physiological or biochemical mechanisms associated with tolerance; and (e) understanding genetic nature of tolerance. The aim of this review and research is to document current knowledge of impacts of high temperature stress on yield of grain sorghum, and potential opportunities for exploring and screening for high temperature tolerance and genetic improvement.
MATERIAL AND METHODS Several studies were conducted in outdoor temperature controlled soil plant atmospheric research (SPAR) units (Prasad, Boote et al. 2006), temperature controlled greenhouses and indoor temperature controlled growth chambers (Prasad, Pisipati et al. 2008). Experiment 1: Sorghum hybrid DK-28E was grown in SPAR units on natural soil profile of about 1 m deep from emergence to physiological maturity at daytime maximum/nighttime minimum temperature regimes of 32/22C, 36/26C, 40/30C and 44/34C (Prasad, Boote et al. 2006). Data on panicle exsertion, in-vitro pollen germination (on agar medium) and percent seed-set on tagged panicles were collected. Pollen germination was estimated as ratio of number of pollen grains germinated to the total number of pollen grains and expressed as percentage. Pollen was considered germinated if the pollen tube length was greater than the diameter of the pollen grain. Seed-set was estimated as ratio of number of filled grains to the total number of reproductive sites and expressed as percentage. Experiment 2: Sorghum hybrid DK-28E was grown in indoor growth chambers in 15L pots from sowing to panicle initiation and thereafter plants were exposed to short periods (10 d) of high temperature stress (40/30C) at 10 d intervals during the entire reproductive period (until 10 d before physiological maturity) (Prasad, Pisipati et al. 2008). Data on panicle exsertion, in-vitro pollen germination (on agar medium), percent seed-set on tagged panicles and seed yield (g plant) were collected.
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Experiment 3: Four sorghum hybrids (DK-28E, DKS-29-28, DK-5400, and Pioneer 84G62) were grown in an outdoor green house from sowing to 10 d prior to start of panicle exsertion at day/night temperature regime of 32/22C. Thereafter, a set of plants was transferred to indoor growth chambers for exposure to temperature stress, with half the plants exposed to optimum temperature (32/22) and the other half to high temperature stress (38/28C) for a period of 10 days. After the temperature treatment, all plants were returned to the greenhouse and maintained at 32/22C until maturity. Data on pollen viability and percent seed-set from the tagged panicles (exposed to temperature stress) were collected. For two hybrids (DK-28E and DK-5400) from optimum temperature control plants, pollen grains were collected, plated on agar medium and exposed to temperatures ranging from 10 to 45C in dark incubators to determine pollen response curves and cardinal temperatures: Tmin (minimum temperature below which pollen grains did not germinate), Topt (temperature range where maximum germination occurred) and Tmax (temperature above which pollen grains did not germinate). Plants in all three experiments were fully irrigated to ensure no water stress was experienced at any stage of development. Plants were healthy without any incidence of pests or diseases. There were at least five biological replications of each treatment. For pollen viability, a minimum of 250 pollen grains were plated for each replication. Data were analysed used PROC GLM procedures in SAS, and means were separated using LSD at P
