Drought and heat tolerance tests that were developed for sorghum (Sorghum bicolor L. Moench) were adapted to and evaluated in field grown wheat (Triticum aestivum L. and T. durum Desf.) during 1977/1978 and 1978/1979.The drought tolerance test is based on the measurement of the electroconductivity of aqueous media containing leaf discs that were previously water stressed in vitro by exposure to a solution of polyethylene glycol‐6,000 (PEG). The heat tolerance test is similarly based on exposure of leaf discs to heating, in vitro, to 44 C.Drought tolerance of wheat leaves decreased with plant age. For a given plant growth stage, some variation was revealed in drought tolerance, according to leaf position. Maximal separation of wheat cultivars in drought tolerance was obtained with 40% (w/v) solution of PEG, when plants were grown under conditions of favorable moisture regime and sampled during the late jointing growth stage.Wheat was more drought tolerant than maize, sorghum or millet, based on published data.When plants were sampled during a period of water stress, they were more drought tolerant than well‐watered plants, indicating adjustment of cell membrane stability to drought stress. Wheat cultivars varied in their ability to adjust, in this respect.Unlike in sorghum, drought and heat tolerance were not correlated in wheat.The inclusion of a limited number of barley (Hordeum vulgare) and triticale (X Triticosecale Wittmark) genotypes in this study indicated that the methods discussed work equally well with these crop plants.
Epicuticular wax deposition on sorghum [(Sorghum bicolor) L. Moenchl leaf blades imparts drought resist. ante. Screening of a large number of sorghum genotypes for increased leaf epicuticular wax content, utilizing the available gravimetric method, is ! .practical. A more rapid colorimetric method was developed and evaluated against the current gra metric method. The colorimetric method is based on the color change produced due to the reaction of wax with acidic K0Cr,.O, reagent. Wax content determined by the colorimetric method was highly correlated (r = 0 .984) with that determined by the grain. metric method. Leaf epicuticular wax content in 11 fieldgrown grain sorghum cultivars did not vat) between growth stage #4 (tip of flag leaf showing) and 37 (soft dough), at which stages comparable wax readings were obtained among the four top leaf blades. A CK 60 'bloom less' (bmbm) genotype had only half of the epicuticular content of the CK 60 'normal' (BmBm) genotype. Ten cultivars, all of the BmBm genotype, significantly differed in epiculticular wax content, ranging from 1 .14 to 1 .99 (SE = 0.00(i) mg/dm of leaf .
We explored a possible association between free proline amino acid accumulation in water‐stressed leaves and the drought resistance of various grain sorghum [Sorghum bicolor (L.) Moench] cultivars.Eight sorghum cultivars were grown in a growth chamber under a drying cycle (to a leaf water potential of −18 to −21 bars) and a subsequent recovery phase. Free proline, free ammonia, dark respiration rate, and recovery rating were recorded, as well as desiccation tolerance of each cultivar when grown in the field.Free proline accumulation was initiated in leaves of all cultivars as leaf water potential was reduced to −14 to −16 bars. Free ammonia concentration in leaves of all cultivars did not change appreciably during the drying cycle. Upon rewatering, free proline content was reduced and free ammonia concentration was increased, both rather abruptly. Cultivars difered significantly in maximum free proline accumulation and post‐stress free ammonia concentration, dark respiration rate, and recovery rating.Correlations indicated that desiccation tolerance was not related to any of the attributes measured, or even to recovery rating. Free proline accumulation during water stress was correlated significantly with post‐stress recovery rating, free ammonia concentration, and dark respiration rate.It is suggested that accumulated free proline in water‐stressed sorghum leaves is related to the ability of a cultivar to recover upon the relief of stress, possibly by way of proline's role as a source of respiratory energy in the recovering plant.
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