3.1. HTA4 Is Tolerant to HS
As HTA4 is the result of a breeding program that aimed to incorporate the heat tolerance of tepary beans (Phaseolus acutifolius) into the Andean race of Phaseolus vulgaris, and as HTA4 had exhibited heat tolerance in the chronic stress field and pot trials at CIAT, Colombia (personal communication Steve Beebe, Milan O. Urban), we expected it to be tolerant to the HS applied in this chronic high temperature cabinet experiment. The data presented show that this is indeed the case, and HTA4 not only performed well under HS compared to the control during vegetative development (as measured by plant height and final dry weight), but it also successfully made the switch to reproductive growth, maintained high percentage pollen viability, and fertilized and filled more seeds than its heat-susceptible counterpart. HTA4 switched to reproductive growth two days earlier under HS than the control, and its leaves began to senesce around 20 days earlier than under the control conditions. It is thereby efficiently flowering, setting seed and (re)mobilizing resources to fill the developing seeds, and avoiding prolonged exposure to HS.
Calima, meanwhile, grows rapidly in the vegetative stage, and similarly makes the reproductive switch early, but is unable to translate that into a successful reproductive output. Calima plants continue vegetative growth, do not senesce earlier, and do not mobilize enough resources due to the absence of a strong sink. Where Calima did fill seeds under HS, they were often smaller and/or misshapen and discolored, traits that are actually much more related to their market value than yield. Due to seed/pod failure under HS, Calima continuously produced many pods, the majority of which were aborted or abscised early in development. This early pod abortion and extended reproductive phase has been observed in heat-sensitive
Phaseolus species previously [
5,
23].
3.2. The Reproductive Stage Is Sensitive to HS
The high rate of pod abortion in Calima under HS, coupled with the negative effect of HS on all pod and seed characteristics measured, indicates that it is the early reproductive stage that is sensitive to HS in Calima, and it is at this stage that HTA4 has a tolerance mechanism. We observed that, where Calima had successfully filled seeds, they were located at the end of the pod proximal to the stigma, suggesting that seed failure might be a consequence of failed fertilization. This phenomenon led us to consider the effects of HS on pollen viability and pollen–stigma interaction.
While the temperature ranges that plants can grow in without suffering heat stress are distinct to each plant species and genotype [
24], they are generally narrower for male gametophytic tissues [
25]. The morphology of anther tissue was previously shown to be impacted by HS, leading to the decreased or unsuccessful production of pollen grains, and/or anther indehiscence in bean [
5] and other crops [
26]. Anther indehiscence was not observed under the conditions applied in the current study; however, interestingly, the HT anthers from both cultivars appeared to be morphologically similar to the controls. This indicates that if male sterility causes the differences observed between Calima and HTA4 under HS, pollen viability and stigma receptivity rather than anther indehiscence could be responsible.
Common bean pollen viability has been shown to reduce under HS, particularly when HS is applied during meiocyte and microspore formation [
4,
13]. Overall, this leads to reduced pollen number, and percentage viability. In this study, pollen viability assays showed a greater drop in pollen viability and an increase in variability between flowers, under HS in Calima compared with HTA4, explaining, to an extent, the reduced reproductive success in this cultivar. In Calima, there was a large proportion of pollen that was small and was lacking cellular contents, including, in some cases, nuclei, indicating that development was arrested at an early stage. Where Calima pollen appeared viable, it showed morphological changes upon closer inspection by TEM, often being smaller and misshapen, with a thicker exine and fewer starch granules. Pollen starch granule size and number decreased in maize under HS [
8], and pollen malformations, such as disordered tecta, absent nexine, and uneven surface sporopollenin, were observed in HS rice pollen [
27], as well as a thicker exine in HS soybean pollen [
28]. Field pea pollen was also shown to be smaller, with reduced interior contents under HS [
29]. Non-viable HTA4 pollen was mono-nucleate, while viable pollen appeared structurally similar to control pollen.
High variability in pollen viability within a single anther locule under HS was previously attributed to the amplification of initially small developmental or metabolic differences between microspores by competition for limited nutrients [
30], resulting in a mixed population of viable and non-viable pollen within a single anther locule [
31]. This could explain the huge variability of data observed in Calima. While the plants in this study have sufficient resources (as photosynthesis and vegetative growth are unaffected or improved), developing pollen could be starved of nutrients because of a problem with nutrient mobilization in the anther, such as loss of integrity of the tapetum, which has been observed under HS previously [
28,
32]. Soltani et al. [
33] previously reported that moderate HS does not negatively affect photosynthesis in a heat-sensitive
Phaseolus vulgaris cultivar, Redhawk, but HS disrupts the source–sink balance, impacting pollen development and pod filling, with a four-fold reduction in free hexoses in HS beans. Wang et al. [
8] showed that the leaf assimilate was remobilized but was not converted to starch in HS maize pollen. Disrupted sugar metabolism was also reported in tomato anthers, leading to reduced pollen viability under HS [
16,
34].
Although pollen viability drops in HTA4, it is to a lesser extent, and the range is similar to that of the control pollen samples. This demonstrates that HTA4 is able to maintain pollen development under HS and its ability to retain similar levels of variability could be a key trait for tolerance in this genotype.
3.3. Calima Pollen Does Not Adhere to the Stigma under HS
Scanning electron microscopy showed an absence of pollen on the stigmas of heat-treated Calima plants, and any pollen present was easily dislodged from the stigma during sample processing. This fits with a previous observation that reduced yield under HS is associated with problems penetrating the stigma [
17]. Considering that Calima does have a proportion (c25%) of apparently viable pollen, it might be expected that it could maintain a higher yield than was observed, considering the necessity of only six viable grains to fill each pod. The altered morphology of apparently ‘viable’ HS Calima pollen could explain why this is not the case. The lack of pollen adherence to the stigma might be due to differences in the structure of the exine and/or impaired rehydration as a requirement for germination. So far, it is also unknown if some feedback effect of the pollen grain presence itself affects the stigma receptivity.
Previous studies found morphological and/or biochemical changes in the pistil under HS [
35,
36], which could be a factor in the failure of pollen to adhere to the stigma surface. Our TEM images, and observation during imaging for pollen viability, show that the stigma appears structurally normal in HS Calima plants but further analysis would be required to rule out the role of stigma damage in the failure of fertilization. However, Monterroso and Wien [
4] showed in reciprocal crosses that, in
Phaseolus vulgaris, applying control pollen to HS stigmas rescued yield to some extent, indicating that the problem is with the pollen and not the stigma, or to a lesser degree due to the stigma. A small pilot study to replicate this in our lab with Calima and HTA4 showed the heat sensitivity of both pollen and stigmas, and plants that had experienced HS to either the male or female reproductive organs showed higher pod abortion rate than controls.
Understanding how HTA4 pollen has avoided or overcome the effects of HS suffered by Calima could be a key to understanding the molecular and physiological mechanism of heat tolerance in common beans.