Navoneel Karmakar's answer is correct -- there are many plant species for which bees are crucial or necessary pollinators because of self-incompatibility. Indeed, the disappearance of bees would lead to the collapse of these plant populations if no other mechanism of pollination (e.g. wind) was available.
But there are many plants that do not rely on insect pollination, and can self-pollinate to reproduce as well as exchange gametes by insect cross-pollination. Would the disappearance of bees (or the preferred insect pollinator) lead to reduced survival of such a plant population, by a mechanism other than drastic population collapse?
From a population genetics perspective, variation is a good thing. If a geographically isolated population of plants reproduces exclusively by pollination between genetically homogenous neighbors, the genetic diversity of that population is limited to the genes of individuals in that population (plus mutagenesis, though beneficial mutations are generally a consideration only in the long term). If a pest or pathogen is introduced to this population, and no individuals in that population carry the genetic determinants of resistance, that population will be threatened, barring some rare mutagenesis leading to resilience. Even outside the threat of disease, any isolated population is subject to inbreeding depression by propagation of deleterious alleles.
Now, introduce an insect pollinator. This organism can ferry plant gametes between geographically isolated populations, expanding the existing genetic space of each population to include the other. This pollinator is an agent of outcrossing, which combats the effects of inbreeding depression by the introduction of new alleles.
Take, for example, Chamaecrista fasciculata, a species of bee-pollinated legume. Mannouris and Byers discuss the effect of habitat fragmentation on this plant, which has led to smaller populations resulting in reduced genetic variation and increased genetic drift load, defined here:
In small populations, genetic drift is likely to play a stronger role in changes in allele frequencies than natural selection. Random loss of alleles through drift is more likely to leave behind deleterious recessive alleles than selection would do. The result is an enhanced level of genetic load in small populations due to the effects of drift, known as ‘genetic drift load’.
This species can be selfed, but relies heavily on bee pollination to maintain genetic diversity.
Chamaecrista fasciculata is 80% outcrossed by bumblebee pollinators and the populations have been found to be very spatially structured due to limited seed and pollen dispersal.
Moreover, fragmentation of the prairie habitats has led to populations that are too isolated for bee cross-pollination.
All of the prairies listed as isolated on Table 1 were at least 30 km from another known prairie with C. fasciculata, which is beyond the distance bumblebee pollinators are expected to forage.
All of this to say, while C. fasciculata does not exclusively rely on bee pollination to reproduce, there is a deleterious effect of the loss of cross-population pollination evident in reduced population genetic diversity.
Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollinationbut it is unclear to me what it means. $\endgroup$