<p id="C3">To investigate the effect of low temperature at jointing stage on wheat physiological metabolism under different water conditions, a comprehensive physiological index is constructed to accurately evaluate the degree of freezing injury, which is of great significance to agricultural loss reduction, efficiency increase and macro management. Irrigation (W1) and no irrigation (W0) treatments were carried out one week before low temperature treatment using the weak-spring Yanzhan 4110 and Lankao 198, semi-winter Zhengmai 366 and Fengdecun 21 as the experimental materials. At the female and male ear differentiation stages, potted wheat was moved to the low temperature simulation room for low temperature treatment with six levels of normal (CK), -2℃ (T1), -4℃ (T2), -6℃ (T3), -8℃ (T4), and -10℃ (T5), respectively. The physiological indexes and fluorescence parameters were measured on the second day after treatment, and wheat yield was harvested at maturity stage. The results revealed that different varieties, water and low temperature stress, and their interactions had significant effects on wheat physiological indexes and fluorescence parameters at jointing stage. With the aggravation of low temperature stress, leaf water content, chlorophyll a content and fluorescence parameters <italic>q</italic><sub>p</sub>, <italic>F</italic><sub>v</sub>/<italic>F</italic><sub>m</sub>, and <italic>F</italic><sub>v</sub>/<italic>F</italic><sub>o</sub> showed the continuous downward trends, the contents of soluble protein, proline, soluble sugar, and SOD activity first increased and then decreased, but the content of MDA demonstrated the opposite trend. Irrigation treatment alleviated the influence of low temperature stress on plant physiological metabolism, and the effect of low temperature on semi-winter varieties was relatively lower. Four independent comprehensive indexes were transformed by the principal component analysis, which reflected 88.55% of the original information, and the physiological comprehensive index of freezing injury (FICEI) was constructed. The depth of color in heat map indicated the darker the color, the greater the performance degree of indicators. According to the FIPCI value, the freezing injury was divided into five levels, which was consistent with the yield loss rate. Especially under T3 treatment, the yield loss rate of each variety reduced by 30.4%-44.0% under no irrigation, reduced by 21.0%-29.2% under irrigation treatment. Under the same irrigation condition, yield loss rate of different varieties was LK198>YZ4110>ZM366>FDC21, and the yield loss rate of semi-winter varieties was lower than that of weak spring varieties. According to the results of heat map clustering and the yield of each treatment, the yield loss rate was less than 10% for CK and T1, 10%-30% for W0T2, W1T2 and W1T3, 30%-50% for W0T3 and W1T4, more than 50% for W0T4, W0T5 and W1T5. Irrigating before low temperature was conducive to alleviating the damage caused by low temperature stress and reducing the yield loss. Physiological comprehensive freezing injury index and model constructed by principal component-cluster analysis can accurately evaluate the degree of wheat late frost damage, and provide scientific basis for yield recovery and decision-making management after disaster.
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