**5. Conclusions**

Our results on pea demonstrated several novel findings. Firstly, heat stress reduced chlorophyll a, chlorophyll b, and carotenoid concentrations, but increased wax and anthocyanin concentrations, and chlorophyll a/b ratio in leaves. Generally, leaf pigments (chlorophyll, carotenoid, and anthocyanin) both from petiole and lamina were positively correlated with heat tolerance index and contributed to lower canopy temperature. Secondly, surface wax contributed to heat resistance presumably by reflecting excess radiation and heat from the plant canopy; and by minimizing water loss through reduced stomatal and residual (i.e., non-stomatal) transpiration. Thirdly, cultivars with the semi-leafless leaf type, upright habit, and dark-green canopies were associated with high (>1) HTI under the heat stress environments, inferring that these traits conferred heat resistance and upright, dark green canopies result in more stress resistant crops. Finally, vegetation indices including GNDVI, PRI, NPCI, and WBI measured from stipules showed consistent relationships with pigment and wax concentrations and other heat tolerance traits, suggesting these indices can be useful proxies in future heat stress studies, and for high throughput phenotyping for heat stress resistance.

**Author Contributions:** Conceptualization, R.B. and T.D.W.; methodology, E.G.T. and R.B.; software, E.G.T. and R.B.; validation, R.B., T.D.W., S.S. and S.N.; formal analysis, E.G.T.; investigation, E.G.T. and R.B.; resources, R.B. and T.D.W.; data curation, E.G.T.; writing—original draft preparation, E.G.T.; writing—review and editing, E.G.T., R.B., T.D.W., S.S. and S.N.; visualization, E.G.T., T.D.W. and R.B.; supervision, R.B. and T.D.W.; project administration, R.B.; funding acquisition, R.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Saskatchewan Agriculture Development Fund, Saskatchewan Pulse Crop Development Board, and Western Grains Research Foundation.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This manuscript includes the essential data used either as tables or figures in the Section 3.

**Acknowledgments:** The authors thank B. Louie, J. Denis, Z. Wang, S. Ryu, and R. Xiang for their assistance in field measurements, and the Crop Development Centre, University of Saskatchewan Pulse Crop Breeding staff for seeding the trials and plot management.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.

#### **References**

