Metabolic Drift as Internal Defense Against Drought in Crop Plants: Evidence Unlocked from Rice Cultivars

We chose three rice (Oryza sativa L.) cultivars to understand the physiology of crop adaptation to drought. Under water stress, leaves curled up and rolled in and the water potential declined. ‘Baranideep’ maintained relatively higher water potential (–1.8 MPa) than irrigated hybrid rice cv. ‘NDRH-2’ (–2.0 MPa) and aquatic cv. ‘Jal Lahri’ (–2.6 MPa). The drought-induced stress caused variation in tissue-specific expressions of metabolic drift in sugar and starch, protein, and nitrogen, and “stay-green” traits. The genotypic differences in sugar levels varied to the extent of 130 mg g−1 fresh wt (f.w.) in ‘Jal Lahri’ >120 mg in ‘NDRH-2’ >114 mg in ‘Baranideep’, while initial starch contents were 263 mg >252 mg >230 mg in these cultivars, respectively. Sugar levels rose by 23% in cv. ‘Jal Lahri’, 16% in ‘NDRH-2’ and 17% in cv. ‘Baranideep’, whereas starch contents declined by 17%, 13% and 12.5%, respectively. The net chlorophyll concentrations were highest in ‘NDRH-2’ (up to 1,325 μg g−1 f.w.), followed by ‘Jal Lahri’ (1,050 μg) and ‘Baranideep’ (920 μg) under normal conditions, but declined during water stress; ‘NDRH-2’ maintained its superiority over the other varieties. Under water stress, chlorophyll a:b ratios increased appreciably in ‘Baranideep’; ‘Jal Lahri’ registered the largest difference between watered and stressed plants. ‘Baranideep’ accumulated the largest amount of proline (9.25 mg g−1 dry wt) during water stress. Under non-stress conditions, ‘NDRH-2’ had the highest nitrogen content in leaves (2.15%), followed by ‘Jal Lahri’ (1.85%) and ‘Baranideep’ (1.75%). The cv. ‘NDRH-2’ retained more nitrogen in leaves than other cultivars. ‘Jal Lahri’ retained the highest nitrogen in the shoot. Nevertheless, under drought stress, the shoot protein was highest in ‘NDRH-2’ (3.15%), followed by ‘Baranideep’ (2.80%) and ‘Jal Lahri’ (2.65%). These tissue-specific metabolic drifts are plant's life-support system and serve as a defense against drought.