Seed Protein and Starch Qualities of Drought Tolerant Pigeonpea and Native Tepary Beans

Tepary bean and pigeonpea were highly drought tolerant food legumes and were profitable in arid regions with sustainable productivity. Protein and starch, major constituents of legume grain, were used in various forms for human and animal consumption. Therefore, present investigation was carried out with an objective to evaluate protein content and various factors influencing qualities of starch in pigeonpea and tepary bean. Significant differences (p < 0.05) were observed for protein and starch qualities among the two crops and between the drought responsive cultivars of these crops. The protein content of pigeon pea (22.5-26.2 mg/100 mg) was high compared to tepary bean (17 to 26 mg/100 mg) seed. The resistant starch content of pure starch extracted from drought responsive cultivars varied from 1.09 (TB #7) to 8.75 (TB #31) in teparybean and 14.68% (G2) - 21.35% (W1) in pigeonpea. Amylose content of pigeonpea varied from 7.76 to 12.31% and the degree of crystallinity were 13.42 - 19.30% with X-ray diffraction pattern of the C-type. The pigeonpea starch granules were large, oval to ellipsoidal shape with a smooth surface and mean granule size of 47.6 µm in length X 36.4 µm in width. The transition temperatures and enthalpy of gelatinization ranged from 67.25-68.29 oC (To); 71.4-72.49 oC (Tp) and 77.04-78.51 oC (Tc) and 4.80-6.43 J/g respectively. Amylose content of tepary starch varied from 8.08 to 12.38% and the degree of crystallinity were 13.52-18.29% with X-ray diffraction pattern of the C-type. The tepary bean starch granules were round to oval shape with a smooth surface and mean granule size of 39.26 µm in length X30.36 µm in width. The transition temperatures and enthalpy of gelatinization ranged from 73.12-74.65 oC (To); 77.71-78.85 oC (Tp); 82.65-84.29 oC (Tc) and 2.21-5.27 J/g respectively. Pigeon pea starches were with low molecular weights of amylopectin (5.26 x 108 Da) and high molecular weights of amylose (4.63 × 105 Da) compared to those in tepary bean (13.6 × 108; 1.35 × 105) on an average. In both crops, the gelatinization showed significant correlation with amylose content, crystallinity and granule size. Pigeonpea grain was superior with quality starch and protein with adequate amounts of digestible fiber without any reduction in quality during extraction of starch compared to tepary bean. Black seed coated tepary had lowest gelatinization energy compared to brown and white. The drought tolerant cultivars identified in teparybean (TB #24) and Pigeonpea (W1) with high resistant starch and protein content will be useful in selection for crop production, developing homozygous line for industrial use in southern USA.