Responses to Induced Passive Heat in Two Local Common Bean (Phaseolus vulgaris L.) Varieties Under Humid Tropical Field Conditions in Costa Rica

Climate change is a major constraint for common bean (<i>Phaseolus vulgaris</i> L.) cultivation in tropical regions, where elevated temperatures drastically affect reproductive efficiency and yield. This study aimed to evaluate the response of two local varieties, <i>Matambú</i> and <i>Tayní</i>, under passive induced heat using Open Top Chambers (OTC) in the humid tropics of Costa Rica. A factorial randomized block design with two genotypes and two environments (control and OTC) was applied to assess morphological, physiological, and yield-related traits. OTC increased daily maximum, minimum, and mean air temperatures by +2.29, +0.93, and +2.80 °C, respectively, and raised cumulative growing degree days by 325 °C·day^-1 compared with the control. Heat stress reduced grain yield by more than 80% (from 0.15 to 0.03 t·ha^-1) and significantly lowered the harvest index, confirming strong reproductive vulnerability. However, <i>Matambú</i> maintained higher nodulation and above-ground biomass under heat, whereas <i>Tayní</i> showed marked declines in pod set and nodule number. Correlation analyses revealed that pod number and harvest index were the strongest predictors of yield across environments. These results provide the first field evidence of local varietal responses to induced passive heat in Costa Rican common bean varieties and highlight <i>Matambú</i> as a valuable genetic resource for breeding climate resilient cultivars.