Gas exchange and productivity responses of Panicum maximum cultivars to increasing soil potassium levels in a greenhouse study.

Potassium (K) is essential for the productivity of tropical grasses, but its optimal supply remains unclear. This study evaluated the effects of K on gas exchange, photosynthetic rate (A), forage production (FP), and root development (RP) in Panicum maximum cultivars. The experiment was conducted using a randomized block design in a 6 × 4 factorial scheme (six cultivars: Tanzânia, Quênia, Mombaça, Zuri, Massai and Tamani; and four K rates: 0, 205, 410 and 820 mg dm-3). There was interaction between cultivars and K rates for A and stomatal conductance (gs), with linear increases in A for all cultivars. The gs response in the Zuri cultivar was quadratic, with a maximum of 5.32 mmol m-2 s-1 at the dose of 410 mg dm-3 K, and linear for the other cultivars. The CO2 concentration (Ci) and leaf temperature (Tleaf) were not influenced by the K dose or by the cultivars, maintaining an average of 129.28 ppm and 29.32 °C, respectively. Transpiration (E) increased by 0.018 mmol m-2 s-1 with increasing K doses. The chlorophyll content fitted a quadratic model, with a maximum of 35 SPAD at the dose of 530 mg dm-3. The FP increased linearly for all cultivars, with the highest FP in the cultivars Quênia, Mombaça and Zuri. Increasing K doses improved A and gs, reflecting in higher FP. The cultivars Quênia and Mombaça showed the greatest increases in FP up to 820 mg dm-3 K, while the maximum efficiency for RP occurred between 205 and 410 mg dm-3, suggesting that excessive K supply may not proportionally increase root development.