Without knowing your soil,fert,water and light data it is very difficult to make a accurate diagnosis but they could be showing a micro-nutrient deficiency. Typically upward turned leaves indicate a Manganese deficiency and can be corrected with a light dose of ordinary Epsom salts (as little as 1/4 teaspoon/gal of water.) If you attempt this, be sure to use a accurately calibrated pH meter and try to get the water to between 6.5-7.0. Abuse/overuse of Mg can cause "lockout" of macros and micros as well so be patient and wait 1-2 weeks for any improvement before attempting more amendment(s).