The term phytotoxic refers to substances that:

The term phytotoxic specifically refers to substances that are toxic to plants, meaning they can cause damage or harm to plant growth and development. When a substance is termed phytotoxic, it indicates that it has the potential to adversely affect plant health, potentially leading to stunted growth, leaf burn, or even plant death.

In considering the options, while it may seem that a phytotoxic substance would affect all plants universally, it generally refers to the toxic properties rather than implying that every single plant species will be affected equally or to the same degree. Some substances can be more harmful to certain types of plants compared to others; however, the key point here is the inherent toxicity of the substance.

The other options do not accurately align with this definition. For instance, a substance described as harmless to plants has no negative impact, thus contradicting the essence of phytotoxicity. Similarly, stating that a substance causes no effect on plant growth also suggests neutrality rather than toxicity. Lastly, being selectively toxic to certain plant species highlights variability in response rather than the universal toxicity implied in the correct interpretation.