###### Abstract  
In the context of the continuous change in global climate, the frequency and intensity of drought and heatwaves are increasing. This study took the extreme drought event in southwest China in 2009/2010 as a case study. Based on the sunlight-induced chlorophyll fluorescence (SIF), we explored the effects of high-temperature weather on the photosynthetic efficiency, the vegetation responses to drought in two ecosystems, and the differences in influencing factors. The results showed a disproportionate change between the vegetation productivity represented by SIF and the greenness symbolized by the leaf area index (LAI). The response of photosynthetic efficiency to drought was significantly inequitable between the grassland and cropland. The geodetector showed that grassland ecosystems with more superficial canopy structures were more susceptible to high temperature. The correlation between the Photosynthesis efficiency index (PEI) and temperature (T) and vapor pressure deficit (VPD) of the grassland ecosystem was above 0.6. This study suggests that drought exacerbates the disproportionate change between vegetation productivity and greenness, and grasslands are more vulnerable to drought. The result is helpful for ecosystem management. sun-induced chlorophyll fluorescence (SIF); leaf area index (LAI); photosynthetic efficiency index (PEI); geodetector; Southwest China 2023150951095  
past few years [18; 19]. However, it remains unclear whether the positive correlation between green vegetation greening and productivity changes in the contemporary context of frequent extreme drought events. The traditional vegetation index based on the green-degree can detect potential photosynthesis and understand vegetation growth [19; 20]. The normalized vegetation index (NDVI) and the enhanced vegetation index (EVI) were widely used for vegetation monitoring. In recent years, with the continuous development of remote sensing technology, the emerging sun-induced chlorophyll fluorescence (SIF) index provided great convenience in monitoring the vegetation at the regional and global scale [12; 21]. Sunlight-induced chlorophyll fluorescence can serve as a direct proxy for photosynthesis to characterize the actual physiological conditions of vegetation [22; 23; 24]. Recently, that the relationship between vegetation greenness and productivity is not positive was pointed out by some researchers, especially on a short time scale, where changes are inconsistent, and using traditional vegetation green-based indices to represent vegetation productivity is not always effective [25; 26]. For instance, Southern China experienced a heat wave in 2013; [PERSON] found that EVI was slow to respond to environmental pressure and did not capture changes in vegetation's physiological state in time [27]. In southwestern North America, EVI failed to capture seasonal and interannual gross primary productivity (GPP) dynamics in dryland ecosystems [25]. [PERSON] found a significant association between SIF and GPP, while the NDVI did not respond very well to changes in system productivity during the severe drought that occurred during the growth period of the North China Plain [7]. Studies show that the sensitivity of vegetation productivity to environmental change is higher than that of greenness [7; 21; 27; 28]. Our understanding of the relationship between photosynthesis and plant greenness is not comprehensive. Under the conditions of constant environmental changes, especially drought, the impact of the relationship between actual vegetation productivity and canopy greenness, and whether there are differences in this relationship under different ecosystems, is still unknown. The Leaf Area Index (LAI) refers to the multiple that the total area of plant leaves occupies the land area on the unit land area [29]. It is used to characterize the leaf density and canopy structure and reflect the structural parameters of vegetation. The Leaf Area Index is a good indicator of vegetation greenness or leaf biomass [30].