Comparative analysis of photosynthetic heat tolerance mechanisms in select Rubus subgenus Idaeobataus species using a numerical modeling and partitioning approach

Abstract

The cultivated red raspberry (Rubus idaeus) in the genus Rubus subg. Idaeobatus is an economically valued berry crop but its cultivation has been limited by heat in the warm climate regions. Many species in the same subgenus are native to the subtropics and have been considered as potential germplasms for improving heat tolerance of raspberry cultivars. However, their photosynthetic responses to high temperatures and adaptation mechanisms have not been comprehensively exploited. In this study, leaf gas exchange characters of raspberry ‘Summer Festival’ and three subtropical native species, R. fraxinifolius, R. rosifolius, and R. croceacanthus, were mathematically analyzed. Net photosynthetic assimilation rate (A) and chlorophyll fluorescence against photosynthetic photon flux density (PPFD) and internal carbohydrate concentration (C_i) were measured at 25, 30, and 35℃. Data from the measurements were fitted to modified Farquar, von Cammerer and Berry (FvCB) models and key photosynthetic variables were extracted. Inter- and intra- species differences in A (% ΔA) were quantified by partitioning the total difference into individual and categorized variables using a numerical integration method. The results showed that light saturated A and transpiration (E) of R. idaeus were suppressed at 35℃ while both in the subtropical native species sustained or elevated at the high temperature of 30 and 35℃. The two key photosynthetic biochemical variables, maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max), were both increased at high temperatures in all studied taxa. Nevertheless, the superior A of the subtropical species was mainly contributed by the improved diffusional factors, i.e., stomatal conductance (g_sc) and mesophyll conductance (g_m), in response to elevated temperatures. This study demonstrated that the select subtropical native species better adapt to heat than the cultivated raspberry by maintaining leaf temperature low and photosynthetic efficiency high through consistent g_sc and improved g_m. The information suggested that for breeding programs targeting on heat tolerance, screening candidate genotypes with photosynthetic diffusional factors should be more efficient than evaluating biochemical factors. The results also provide useful references for raspberry industry to develop cultivation strategies to alleviate the impact from intensifying climate change situations.