Response Of Tuber Crops To Abiotic Stress

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Abiotic stress is defined as environmental factors that affect plants and reduce growth and yield below optimum levels. Plant abiotic stress factors include extremes in temperature, water, gasses, radiation, nutrients, wind, and other environmental conditions. CO2 and methane emissions are likely to increase which may cause an impact in terms of more demand for water, increased temperature, and increased biotic and abiotic stresses. Many researchers around the world are giving evidence about the positive responses of many crops, mainly C3 crops to growing atmospheric CO2 concentrations under stress-free conditions (Long et al., 2004). But this direct advantageous effect of eCO2 can be hindered by other players of climate change such as elevated temperature, changes in the patterns of precipitation, and higher tropospheric ozone concentrations (Easterling et al., 2007).

Being a C3 plant Tuber crops have advantages from elevated atmospheric CO2. Tuber crops are tolerant to mid-season drought and high temperatures. Under rain-fed conditions, its spread is limited by the length of the rainy season as being long duration crop. An increase in temperature due to global warming may shorten the duration of tuber crops. Hence it may be grown in short rainy season areas also. Further being tropical crop, an increase of temperature and CO2 will enhance its productivity when water is not limited (Jata et al., 2010). Elevated CO2 generally increases biomass, length of roots, and volume as well as increasing biomass allocation to roots (increased root-shoot ratio). Root and tuber crops tend to have a greater yield (Allen and Prasad, 2004).

The photosynthetic rate of sweet potato increases with an increase in CO2 concentration from 250 to 560 ppm under controlled conditions and it is due to the increase in intercellular CO2 concentration and the response is highly temperature-dependent (Cen and Sage, 2005). An increase in the storage root yield of sweet potato is observed up to a CO2 concentration of 750ppm (Mortley et al., 1996). Both above ground and below ground biomass of sweet potato is increased at a higher CO2 concentration of 1520 ppm (Czek et al., 2012). The increase in above-ground dry biomass was 43% for the organic source of nutrients, whereas the increase was 31% for the inorganic source of nutrients at eCO2. The below-ground biomass increased by 61% in organic treatment and 101% increase in inorganic treatment. The increase in below-ground biomass is considerably higher than the increase in above-ground biomass for both organic and inorganic treatments under higher CO2 concentrations (Czek et al., 2012). It attributes the importance of root and tuber crops under a high CO2 environment.

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Previous studies indicated that the net photosynthetic rate (Pn) of field-grown six sweet potato genotypes varied between 21.4 and 23.7 µmol CO2/m2/s (Ravi and Saravanan, 2001) and the rates saturated at 600 µmol/m2/s photon flux density (PPFD) at ambient CO2 concentration (~330 ppm) (Mortley et al., 1996, Ravi and Saravanan, 2001). The Pn increased at the rate of 0.056 µmol CO2/m2/s per 1 ppm rise in CO2 at 34oC and the rate of 0.048 µmol CO2/m2/s per 1 ppm rise in CO2 at 38oC. Sweet potato tuber yield per plant increased significantly (41.3 g/plant) due to a rise in CO2 concentration from 363 ppm to 514ppm but a further rise in CO2 did not significantly increase tuber yield. Under high photon flux density (PPFD), leaf carboxylation rate is linked with the availability of CO2, and increasing CO2 concentration positively influences the Pn and yield. Sweet potato responds positively to an increase in atmospheric CO2 concentration. Ravi et al. (2017) reported an average Pn of 26.30, 33.41, 38.02, and 40.32 µmol/m2/s at 400, 600, 800, and 1000 ppm CO2 respectively. The percent of increment in Pn at eCO2 significantly reduces (average 5.98%) at CO2 concentrations above 800 ppm. However, the beneficial effects are persisting.

Cassava plants grown under a higher CO2 concentration of 750 ppm produced more dry mass than plants grown under a CO2 concentration of 390 ppm (Imai et al. 1984; Fernandez et al., 2002; Rosenthal et al., 2012). Elevated CO2 concentration delays the occurrence of water-deficit stress in cassava by reducing stomatal conductance and minimizing transpiration rates. Cruz et al., (2016) reported that even underwater deficit stress under elevated CO2 conditions biomass production is greater in cassava due to an increase in photosynthetic rate and instantaneous transpiration efficiency (ITE). Cassava cultivation usually does not require any chemical input (Asher et al., 1980). Under drought conditions, cassava maintains nearly 50% of the photosynthetic rate (Ravi and Saravanan, 2001). Cassava also shows significant growth under high temperature and CO2 conditions (Ravi et al., 2011). These features make cassava a future food security crop. Although cassava genotypes show tolerance (survival) under drought conditions, genotypes show a significant reduction in tuber yield, and a wide variability was reported in tuber yield under drought conditions (Ramanujam 1990; Ravi and James, 2003).

Tuber yield increased in potatoes up to 1000 ppm (Wheeler et al., 1994). Chinese yam grown under elevated CO2 concentrations exhibit an increase in the vine length, leaf area, leaf dry weight (DW), number of leaves, vine DW, and root DW, and total plant dry weight than that is grown under ambient CO2 levels under both low and high-temperature regimes (Thinh et al., 2017).

In studies conducted at CTCRI, Thiruvananthapuram, Kerala, the photosynthetic rate of elephant foot yam steadily increased with an increase in CO2 concentration. 56.71 to 82.51% hike in Pn at eCO2 (1000 ppm) compared to ambient CO2 (400 ppm) was reported by Ravi et al. (2018). Elephant foot yam responds well to an increase in photosynthetic photon flux density. Pn steadily increases with an increase in PPFD and reaches a maximum of 1500 µmol/m2/s PPFD (Ravi et al., 2018). However increase in Pn at PPFDs above 1000 µmol/m2/s is reported as insignificant (Ravi et al., 2017). Elephant foot yam, tannia, and arrowroot are tolerant to shade conditions. Recent studies reveal that taro exhibit a 61.80 to 113.30% increase in the photosynthetic rate at eCO2 (1000 ppm) compared to the ambient atmospheric concentration of CO2, 400 ppm (Ravi et al., 2018). Taro can benefit from increasing PPFD. Pn steadily increases with an increase in PPFD and reaches a maximum of 1500 µmol/m2/s PPFD. An increase in Pn up to 600 µmol/m2/s PPFD is reported as statistically significant (Ravi et al., 2018).

Roots and tubers are highly important food resources in developing countries. Increasing demand for food under climate change scenarios points out the importance of tropical root and tuber crops as a staple food commodity in the diet, especially at lower-income groups in developing nations. They are also used for animal feed and various industrial applications. An important advantage of tropical root and tuber crops is that tuber and shoots which are economically important grow simultaneously under normal as well as unfavorable conditions. They cease tuber development and vegetative growth and become dormant during stress conditions such as drought, flood, and heat stress and resume growth when conditions become favorable. So an instance of crop failure is very less for tropical tuber crops (Lal et al, 2014), whereas other staple food crops such as cereals are highly sensitive to environmental stress conditions. Tuber crops need less input and maintenance but provide high yields and provide food security for poor and marginal farmers.

Tropical root and tuber crops can provide food security for the future environment under climate change. Because being C3 plants they have a positive effect from the eCO2 concentrations. They can also adapt to higher temperatures and water deficit conditions. However, studies regarding the response of tropical root and tuber crops under future changing climate scenarios are very few compared to other crops such as rice, wheat, cotton, and soybean. The major objective of this study is to determine the interactive effect of elevated CO2, higher temperature, and drought stress on various photosynthetic parameters of cassava and sweet potato varieties. 

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