Leaf color as a morpho-physiological index for screening heat tolerance and improved water use efficiency in rabbiteye blueberry (Vaccinium virgatum Aiton)

Abstract The rabbiteye blueberry (Vaccinium virgatum Aiton) is a promising new fruit crop in the subtropical lowland. However, mid-day temporary wilting of young shoot tips often occurs in the high summer heat and strong irradiance, warranting breeding for heat tolerance. Differences in leaf color among rabbiteye blueberry genotypes are visually distinguishable. To test the hypothesis that leaf color can be a morpho-physiological index for early screening in heat tolerance breeding with lighter leaf color preferred, leaf color of selected ‘Blueshower’ open-pollinated siblings was quantified using CIE L*a*b* color space. Diurnal leaf to air temperature differences (ΔTleaf-air), chlorophyll content (Chl), mid-day gas exchange parameters and water use efficiency (WUE) were measured. The collected data were correlated with L*a*b* information. In hot sunny days, the genotypes with dark leaf color or low L* absorbed heat quicker than those with light leaf color or high L*, resulting in greater ΔTleaf-air in the morning hours. The differences in ΔTleaf-air were similar among the genotypes after mid-day, likely due to the negative correlation between L* and transpiration (E) that facilitates heat dissipation. Maximum leaf net CO2assimilation (An) was recorded at leaf temperature around 35°C in all genotypes and was positivly corrected with b*. a* was positively correlated with leaf Chl content but not with any gas exchange parameters, suggesting that the effect of Chl content on An was overpowered by the effect of leaf lightness derived from cuticular wax layers on leaf surface. Although increasing L* reduced E and thus improved WUE, the genotype with the highest L* value had the lowest mid-day stomatal conductance and intercellular CO2 concentration, resulting in a significantly low An than that in the genotypes with moderate L*. We concluded that leaf lightness is a feasible index for heat tolerance breeding. For hot and arid climates, genotypes with high L* are recommended for their outstanding WUE but in the expense of An. For the hot humid subtropical climate, genotypes with moderate L* and high b* will be better candidates for their high An in well-watered conditions.