Question

What are recalcitrant seeds?

Answer 1

Plant seeds are divided into two categories: conventional seed and unorthodox seed (Recalcitrant seed).
Unorthodox seeds are recalcitrant seeds that are short-lived and unable to endure low temperatures and moisture (less than $10^\circ C$ and $20 - 30\%$ moisture, respectively); if they fall below these levels, they will perish.

Complete answer:
Unorthodox seeds are recalcitrant seeds that are short-lived and unable to endure low temperatures and moisture (less than $10^\circ C$ and $20 - 30\%$ moisture, respectively); if they fall below these levels, they will perish.
Soon after they mature and disseminate from the parent plant, these seeds germinate. It can't be dried since it would lose its vitality. Because the conditions in such a habitat are continually favourable for seed germination and establishment, most species from the humid, damp tropics have refractory seeds. Mango, jackfruit, and other plants with resistant seeds are common examples.
These seeds are also known as unconventional seeds since they are short-lived and cannot endure low temperatures or dampness. If they are exposed to these conditions, they will die. In ex-situ settings, they are difficult to store. This type of seed is found in shrubs and trees that thrive in hotter climates or in the tropics.

Additional information:
Orthodox seeds are long-lived seeds that can easily dry their moisture content to as low as $5\%$ and can withstand low temperatures for storage without losing viability. Desiccation-tolerant seeds are a type of seed that can withstand dry conditions. Orthodox seed is typically found in biennial, annual, and agroforestry plants. Cashew, guava, cereals, and legumes are examples of orthodox seeds. The vitality of such conventional seeds varies from one to hundreds of years, depending on the species and storage conditions.

Note:
Difference between orthodox, intermediate and recalcitrant seeds-
Orthodox seeds can be dried to less than $12\%$ moisture content and stored at freezing temperatures and yet thrive.
Recalcitrant seeds cannot be stored in a standard freezer because they will perish after drying and/or freezing at $- 20^\circ C$.
Intermediate seeds mature more quickly than conventional seeds, and when stored at $- 20^\circ C$, they may only last $5$ years. When dried between $45$ and $65$ percent relative humidity, they last the longest.