Comprehensive evaluation of drought resistance of different apricot cultivars (lines) based on leaf microstructure

Abstract：【Objective】The common apricot (Armeniaca. vulgaris L.) has 2000 cultivars or genotypes, but there have been few studies on drought resistance among various cultivars. There are also few reports about the evaluation of drought resistance based on leaf anatomical structure in apricot. This study aimed to establish an efficient method for drought-resistance evaluation of apricot plants and to screen out some apricot cultivars with high drought resistance.【Methods】10 apricot cultivars (accessions) with different drought-resistance were selected as materials. Paraffin sections and scanning electron microscopy were used to study the anatomical structures of leaves. Nine structural parameters were detected, which included leaf thickness (LT), thickness of upper and lower epidermis (TUE, TLE), thicknessof palisade tissue (TP), thickness of spongy tissue (TS), ration of palisade and spongy (P/S), tightness of leaf tissue structure (TLTS), looseness of leaf tissue structure (LLTS), and SD. Based on these data, principal component analysis and subordinate function method were used to analyze the drought resistance of 10 apricot cultivars (accessions).【Results】There were great differences in leaf anatomical structure among different cultivars (lines). The smallest value of LT was 111.1 μm, and the largest LT was 222.5 μm. The TP and TS varied from 16.0 μm to 63.6 μm and from 52.8 μm to 90.0 μm, respectively. The P/S value ranged from 0.30 to 0.81. The thickness of the upper epidermal cells was higher than that of the lower epidermal cells, and the variation ranges of these two indexes were 27.7-39.7 μm and 13.4- 21.8 μm, respectively. TLTS and LLTS ranged from 14.4% to 31.3% and from 40.5% to 54.9%, respectively. Scanning electron microscopy (SEM) revealed that the stomata in apricot leaves were only distributed in the lower epidermis. Obvious difference of SD among 10 cultivars (accessions) was also observed. Luotuohuang had the highest stomata density (396 per μm² ) and G 4-25 had the lowest (102 per μm2 ), and the coefficient of variation among the 10 cultivars (lines) was 34.00%. Among the nine parameters, the coefficient of variation was in the order of TP＞SD＞P/S＞TLTS＞LT＞TS＞ TLSR＞TUE. The TP showed the largest difference among cultivars, with a coefficient of variation of 34.58%, while the TUE and LLTS showed little difference, with a coefficient of variation of 10.73% and 11.71% , respectively. The coefficient of variation for these 9 parameters ranged from 10.73% to 34.58%, which indicates that the anatomical structure change of apricot leaves sensitively responds to environment conditions. These parameters could be used to evaluate the drought resistance of different cultivars of apricot. Principal component analysis method was used to screen the crucial indicators. According to the criterium of the factor characteristic value greater than 1 and the cumulative contribution rate greater than 80%, the first two principal components were extracted out. The contribution rates of the first and second principal components were 49.61% and 30.65%, respectively, and the cumulative contribution rates of the first two principal components reached 80.26%, which could well retain most information of the nine parameters. For the first two principal components, there were difference in the load values of each parameter. The greater the load value, the greater the contribution rate to the principal component, and the more effective for evaluating drought resistance. In the first principal component, the loading values of TP, LT, P/S and TLTS, which the photosynthetic capacity and drought resistance of plant leaves, were higher than 0.8. In the second principal component, the load value of TLE, which reflects the protective characteristics of plant leaves, was the largest. By principal component analysis, TP, LT, P/S, TLTS and TLE were selected as the typical parameters to evaluate drought resistance of apricot. There is no consistent correlation between the anatomical parameters of apricot leaves and drought resistance. In order to avoid the limitation of a single parameter, the membership function analysis method was used for comprehensive evaluation. According to the results of principal component analysis, five parameters (TP, LT, P/S, TLTS and TLE) were determined as the typical indexes to evaluate the drought resistance of apricot resources. The membership function method was further used to calculate these five characteristic parameters, and the membership function values of the five parameters related to drought resistance of the 10 apricot cultivars (accessions) were accumulated and the mean values were calculated. The higher the value, the stronger the drought resistance. The drought resistance among the 10 apricot cultivars (accessions) was in the order of Qingmisha＞G 4-25＞Pinaizi＞Luotuohuang＞Chuanzhihong＞G 4- 43＞G 4- 40＞G 4- 26＞Hongjinzhen＞Dayoujia.【Conclusion】The drought resistance of 10 apricot cultivars (accessions) was comprehensively evaluated. Five parameters of anatomical structure were selected as the main indexes to evaluate the drought resistance of apricot,including leaf thickness (LT), palisade tissue thickness (TP), ration of palisade/spongy (P/S), tightness of leaf tissue structure (TLTS), and thickness of lower epidermis (TLE). With the mean value of five typical index membership functions＞0.8 as the reference threshold, we found several cultivars (accessions) such as Qingmisha, G4- 25, Pinaizi, Luotuohuang and Chuanzhihong had strong drought resistance. These results provide method for the evaluation of drought resistance and for screening and scientific utilization of apricot germplasm resources.