Heat stress resistance in pea (Pisum sativum L.) based on canopy and leaf traits

Pea (Pisum sativum L.) is a grain legume widely grown in semi-arid conditions in western Canada, where summer daily temperatures may rise above 28 °C for several days, hampering plant growth and yield. Heat stress leads to impaired photosynthesis, shortened life cycle, abortion of flowers, pods and ovules, and thus yield loss. To minimize yield loss and stabilize production, heat resistance is a desirable trait. Pea has genetically diverse leaf and canopy characteristics including plant growth habit, leaf type, determinacy, canopy color, flower color, leaf wax and more that may be used in searching for traits to mitigate heat stress. The goals of this study were to identify leaf, canopy and biochemical traits involved in pea heat resistance, and determine effects of individual or combined heat and drought stresses on physiological, growth and yield performance. Results from field trials across six environments in western Canada using 24 cultivars demonstrated that heat stress increased canopy temperature (CT), leaf chlorophyll a/b ratio, leaf wax and leaf anthocyanin concentrations, but reduced leaf chlorophyll a, chlorophyll b, and carotenoid concentrations, plant height, reproductive stem length, internode length, flowering duration, pod number, pod set ratio, and seed yield. The pea cultivars had a differential response to the various growth and yield traits. Under heat stress environments, cultivars with the semileafless leaf type and upright canopy habit had a significantly lower CT (up to 2 °C), and greater heat tolerance index (HTI) than cultivars with the vining habit and normal leaf type, likely due to less ground contact, high light reflection in the visible and near infrared wavelength and enhanced aeration through the canopy. Lodging contributed to high CT and exacerbated heat susceptibility. In contrast, under non-heat stress conditions, cultivars with the normal leaf type, indeterminate and vining canopy habit had greater yield potential. Generally, greater leaf pigment and wax concentrations were associated with high HTI and contributed to a lower CT in the field. Vegetative indices including photochemical reflectance index (PRI), green normalized vegetation index (GNDVI), normalized pigments and chlorophyll index (NPCI), and the water band index (WBI) showed a consistent relationship with heat tolerance traits. Exogenous wax application of 100 µg stipule-1 on selected cultivars under the field condition led to 40 and 14% more radiation reflected in the ultraviolet (UV) and near infrared (NIR) regions, respectively compared to an untreated control. Enhanced reflection in the UV and NIR regions was associated with excess heat avoidance. Shading resulted in a significant chlorophyll and carotenoid loss, which led to enhanced spectral reflectance in the visible spectral regions. Limited light absorption and thus photosynthesis efficiency was demonstrated by a low (63% less) PRI and a high (over 700%) NPCI compared to the control. As a heat avoidance trait, leaf surface wax concentration reduced organ heat absorption by enhanced reflection both in the high energy UV and in the NIR regions. A controlled growth chamber experiment to examine individual and combined effects of heat and drought demonstrated that stomatal conductance and cumulative evapotranspiration decreased due to drought and the combined stresses of heat and drought, but not by heat stress alone. Under the heat treatment, optimal water supply reduced leaf temperature by 2.2 °C. Pea growth and seed yield traits decreased due to heat or drought, and their combined occurrence exacerbated their individual impacts. Drought and combined stress effects had a similar pattern although the combined stress was most detrimental onto overall pea performance. Pea cultivars differed in sensitivity to drought and heat stresses. The second controlled growth chamber experiment had a 5 to 7 °C greater threshold temperature compared to the field for comparable growth and yield damage. Growth related parameters including plant height, internode length, and node numbers, had high thresholds (≥ 34 °C) for significant growth reduction whereas yield related traits including pod numbers and seed yield had relatively low (≤ 31 °C) thresholds for yield loss. Expanding leaves were more sensitive to heat stress and had a lower threshold temperature than mature and senescing leaves. Overall, cultivars with the semileafless leaf type and upright nature were better adapted to heat and drought stressed environments than cultivars with the normal leaf and vining habit, and maintained a cooler CT and overall greater yield. Leaf spectral reflectance was dependent on pigments, wax, and leaf water content. The possibility of applying exogenous wax to leaf surfaces to augment naturally existing wax content to enhance the plants’ heat avoidance capacity was novel. Canopy temperature and VIs including NDVI, PRI, NPCI and WBI can be used to indicate the overall physiological and biochemical status of a plant. Finally, optimal soil water supply can moderate the impacts of heat stress by 2 °C