EFFECT OF HIGH TEMPERATURE ON OVULE DEVELOPMENT IN FIELD PEA (Pisum sativum L.)

Abstract

Field pea (Pisum sativum L.) is a cool-season crop that is highly vulnerable to high temperature especially during flowering. Temperatures exceeding 28℃ cause abortion of flowers and young fruits in the field, leading to severe yield loss of the crop. In this research, I aimed to investigate the impact of high temperature on ovule development during the reproductive development of pea. Assessments of gynoecium, ovule development, ovule viability, seed set, and ovule abortion from several cultivars exhibiting a wide range in heat tolerance revealed that high temperature altered the normal progression of ovule development under both growth chamber and field conditions. Plants with open flowers at the first reproductive node, but with closed mature buds at the second reproductive node, were exposed to high temperature (35℃/18℃ day/night) for 4 days under growth chamber conditions. The gynoecium evaluated at the first four reproductive nodes of these plants showed contrasting effects of high temperature among nodes and cultivars. A larger size of gynoecium components, such as ovary, style, and stigma, was identified at the youngest reproductive nodes (Node 3) on some heat-treated cultivars compared to the controls, which was consistent with older flower stages found at those nodes. Assessments of embryo sac and ovule size on these nodes revealed that greater size and advanced ovule development were the main effects of high temperature on heat-tolerant cultivars. In turn, less advanced ovule development on diverse nodes of the plants appears to be the factor that separates medium and low heat-tolerant cultivars under heat stress, where medium heat-tolerant cultivars showed poor development at one node, and a low heat-tolerant cultivar at two nodes. Importantly, the occurrence of embryo development at its early stages (zygote to globular-stage embryo) was detected in > 90% of these ovules. A different level of embryo development suggested that high temperature compromised early embryo growth at affected nodes. Ovule viability, analyzed by the presence of callose deposition and reactive oxygen species (ROS), revealed that high temperature could disrupt ovule development in more than one way. An increase of callose accumulation found around the vascular bundle region of ovules suggested that high temperature could disrupt assimilate transport to the embryo sac. Moreover, a heavy presence of ROS was detected in the embryo sac, indicating possible oxidative damage of the embryo sac contents in young ovules, specifically in pods at the raceme’s distal position at young nodes (Node 4). Evaluation of abortion in mature pods confirmed a consistent failure of ovules right after fertilization and ovules containing embryos at early stages of embryo development in heat-treated plants. In the field, the assessment of young ovules and mature pods of 18 cultivars showed a more severe effect of high temperature on ovule development. Ovules collected at 4 days after flowering and a few days (2-3) of high temperature (>28℃) in the field displayed poor embryo sac development, embryo sac decline, and endosperm and embryo growth disruption. Similar to growth chamber conditions, > 90% of these young ovules showed embryos at early development (pro-embryo to globular stage). Finally, seed number reduction in the field occurred mainly because of high ovule abortion (20-57% per pod) at various stages of embryo growth (pro-embryo to late cotyledon stage). Cultivars that showed the least ovule abortion were 40-10, Naparnyk, and CDC Golden, whereas cultivars with the greatest ovule abortion were Carneval, CDC Centennial, and MFR043. Overall, these findings demonstrated that high temperature disrupted normal ovule development, specifically when embryo formation was taking place. Although a certain level of accelerated development was observed on some nodes, poor ovule development on other nodes could be related to a conflict of assimilate availability for an embryo in development. The outcomes from this research provide valuable insights that enlighten how high temperatures hinder the success of reproductive development in field pea. These findings can also be used to select and assess more proficient varieties with high yield performance under warmer environments.