Читать книгу Quinoa - Atul Bhargava - Страница 14

Quinoa exhibits high levels of resistance to several of the predominant adverse factors such as soil salinity, drought (Jensen et al., 2000; González et al., 2009, 2011; Jacobsen et al., 2009; Fuentes and Bhargava, 2011), frost (Jacobsen et al., 2005, 2007), diseases and pests (Jacobsen et al., 2003a; Bhargava et al., 2003). Due to its durability under adverse climate conditions, quinoa may be one of the options for food production under various adverse abiotic constraints (FAO, 1998).

Quinoa is a halophytic species that is regarded as having an unusually high tolerance to salinity. Some varieties of the crop show remarkable resistance to salt during germination. Many varieties of this crop can grow in salt concentrations as high as those found in seawater (40 mS/cm) (Jacobsen et al., 2001, 2003a; Wilson et al., 2002; Jacobsen, 2007; Delatorre-Herrera and Pinto, 2009; Adolf et al., 2012). These characteristics make it an attractive crop for regions where salinity has been recognized as a major agricultural problem (Prado et al., 2000). Quinoa has several mechanisms that aid in successful acclimatization of the plant to saline environments. In the cotyledonous stage, high adaptability to soil salinity is probably due to improved metabolic control based on ion absorption, osmolyte accumulation and osmotic adjustment (Ruffino et al., 2010). Quinoa also accumulates salt ions in its tissues and thereby adjusts leaf water potential, enabling the plant to maintain cell turgor and to limit transpiration under saline conditions (Jacobsen et al., 2001; Hariadi et al., 2011). In addition, quinoa is able to maintain K⁺/Na⁺ and Ca²⁺/Na⁺ selectivity under saline conditions (Rosa et al., 2009).

The drought resistance of quinoa is attributed to morphological characters such as a deep, extensively ramified root system, reduction of leaf area through leaf dropping, small and thick walled cells adapted to large losses of water, and the presence of vesicles containing calcium oxalate that are hygroscopic in nature and reduce transpiration (Canahua, 1977; Jensen et al., 2000; Jacobsen et al., 2003a). Physiological characteristics indicating drought resistance include low osmotic potential, low turgid weight/dry weight ratio, low elasticity and an ability to maintain positive turgor even at low leaf water potentials (Andersen et al., 1996). It has been observed that the stomatal conductance of quinoa remains relatively stable with low but ongoing gas exchange under very dry conditions and low leaf water potentials (Vacher, 1998). Quinoa maintains high leaf water use efficiency to compensate for the decrease in stomatal conductance and thus optimizes carbon gain with a minimization of water losses. Jensen et al. (2000) studied the effects of soil drying on leaf water relations and gas exchange in quinoa. The study showed that high net photosynthesis and specific leaf area (SLA) values during early vegetative growth resulted in early vigour of the plant, supporting early water uptake and tolerance to a following drought. The leaf water relations were characterized by low osmotic potentials and low turgid weight/dry weight ratios during later growth stages sustaining a potential gradient for water uptake and turgor maintenance under high evaporation demands. Garcia et al. (2003) calculated the seasonal yield response factor (Ky) for quinoa and observed that it was lower than that of groundnut and cotton. This low Ky value for quinoa indicated that a minor drought stress does not result in a large yield decrease.

The frost resistance of quinoa has been recognized for many years (Rea et al., 1979). The species exhibits 100% germination even at 2°C and no serious effect on the plant at temperatures close to −3°C (Bois et al., 2006). The main mechanism for the frost resistance of quinoa seems to be that it tolerates ice formation in the cell walls and the subsequent dehydration of the cells, without suffering irreversible damage (Jacobsen et al., 2003a). The presence of soluble sugars, such as fructans, sucrose and dehydrins, may be good indicators of frost tolerance in quinoa (Jacobsen et al., 2003a, 2005). Results have shown that quinoa seeds germinate rapidly even at low temperatures, with the base temperature for germination lower than 0°C for 9 cultivars out of 10 (Bois et al., 2006).

Hail and snow are sporadic and localized in the Andean region and sometimes causes irreversible damage, especially when the crop is near to maturity (Jacobsen et al., 2003a). Cultivars of quinoa exist with good tolerance to hail, mainly due to a minor leaf angle and greater thickness and resistance of leaves and stem. Flooding occasionally occurs in rainy years on flat areas and produces root rot, greatly reducing yield (Jacobsen et al., 2003a). Wind affects crop productivity by causing plants to fall, especially in the arid region of the altiplano and in some inter-Andean valleys. Wind is also responsible for erosion and drying of soil and plants. When quinoa is cultivated in deserts and hot areas, high temperatures can cause flowers to abort and the death of pollen (Jacobsen et al., 2003a). Fortunately, the genetic variability of quinoa makes it possible to select cultivars with greater tolerance to each of these environmental factors.